Links

Reflective Insulation,
Radiant Barriers
and
Raidant Control Coatings
Text Box: Understanding and Using






























Compiled by

 

REFLECTIVE INSULATION MANUFACTURERS ASSOCIATION

April 1999
TABLE OF CONTENTS

1.       ABOUT RIMA............................................................................................................................................................................

2.       INTRODUCTION......................................................................................................................................................................

3.       OBJECTIVES.............................................................................................................................................................................

4.       COMMENTS ON HEAT TRANSFER...................................................................................................................................

4.1          Conduction

4.2          Convection

4.3          Radiation

5.       THE NEED FOR INSULATION.............................................................................................................................................

6.       REFLECTIVE INSULATION..................................................................................................................................................

6.1      Concept of Reflective Insulation.................................................................................................................................

6.2      Understanding a Reflective Insulation System.........................................................................................................

6.3      Types of Reflective Insulation Materials....................................................................................................................

6.4      Applications for Reflective Insulation Materials.......................................................................................................

6.5      Installing Reflective Insulation Systems.....................................................................................................................

7.     RADIANT BARRIERS.............................................................................................................................................................

        7.1      Physics of Radiant Barriers..........................................................................................................................................

        7.2      Radiant Barrier Systems (RBS)...................................................................................................................................

        7.3      Types of Radiant Barrier Material...............................................................................................................................

        7.4      Installing Radiant Barriers............................................................................................................................................

7.4.1           Attics.................................................................................................................................................................

7.4.2           Walls.................................................................................................................................................................

7.4.3           Floors...............................................................................................................................................................

8.     INTERIOR RADIATION CONTROL COATINGS (IRCC)...............................................................................................

        8.1      Definition of an IRCC.....................................................................................................................................................

        8.2      Physics on an IRCC.......................................................................................................................................................

        8.3      Definition on an Interior Radiation Control System (IRCC)....................................................................................

        8.4      Advantages of an IRCC.................................................................................................................................................

        8.5      Installation Methods for an IRCC................................................................................................................................

        8.6      Typical Installations of Andiation Control System (IRCC)......................................................................................

                    8.6.1       Under Roof......................................................................................................................................................

                    8.6.2       Interior Side Walls..........................................................................................................................................

                    8.6.3       Exterior Side Walls........................................................................................................................................

8.6.4           Other Possible Uses – Construction..........................................................................................................

        8.7      Other Possible Uses of an IRCC.................................................................................................................................

9.       GLOSSARY OF TERMS.........................................................................................................................................................

10.    REFERENCES...........................................................................................................................................................................................                

10.1    Reviews

10.2    Technical Papers..........................................................................................................................................................

10.3    Documents......................................................................................................................................................................

            10.3.1     ASHRAE Handbook......................................................................................................................................

            10.3.2     Federal Trade Commission.........................................................................................................................

            10.3.3     International Conference of Building Officials (ICBO)............................................................................

            10.3.4     U.S. Department of Energy...........................................................................................................................

10.4    ASTM Standards............................................................................................................................................................

11.    APPENDIX – INTRODUCTORY COMMENTS ON ESTIMATING THERMAL RESISTANCES FOR

      REFLECTIVE INSULATION SYSTEMS..............................................................................................................................


About RIMA

 

The Reflective Insulation Manufacturers Association (RIMA) is the only trade association representing the reflective insulation, radiant barrier and radiant control coatings industries.  RIMA activities are guided by an active board of industry members that participate on national and local levels of building code organizations and governmental agencies.

 

RIMA’s objective is to further the understanding and acceptance of reflective insulation, radiant control coatings, and radiant barriers.  Toward this, RIMA members have contributed many articles that have appeared in magazines and newsletters such as:

 

Builders Magazine, Journal of Light Construction, Popular Mechanics, Popular Science, Architecture Magazine, RSI, Energy Design Update, Contractor’s Guide, Practical Homeowner, Rural Builder, Frame Builder Professional, Metal Construction News, Metal Architecture.

 

RIMA has also contributed technical papers to various conferences and workshops sponsored by the Department of Energy, ASHRAE, TVA, ASTM, and Oak Ridge National Laboratory.  RIMA members meet twice a year at the ASTM C-16 Committee meetings to discuss current technical issues and establish standards that promote the best use of reflective insulation, radiant control coatings, and radiant barrier products.  RIMA’s members come from a variety of backgrounds including engineers, scientists, manufacturers, marketers, and academicians.                 

 

The RIMA Handbook aims to provide a simple yet comprehensive guide elaborating on the fundamentals of heat transfer and the concept of reflective insulation and radiant barriers.

 

 

INTRODUCTION

The key to maintaining a comfortable temperature in a building is to reduce the heat transfer out of the building in the winter and reduce heat transfer into the building in the summer.

 

Heat is transmitted across confined air spaces by radiation, convection, and conduction.  The goal of all insulation and barriers is to reduce heating and cooling loads.  Reflective insulation, radiant control coatings, and radiant barriers are products that perform this function by reducing radiant heat transfer thereby reducing the heating and cooling requirements.

 

 

OBJECTIVES

 

·         Discuss heat transfer, with an emphasis on radiant heat transfer.

 

·         Explain the underlying principles of reflective insulation and radiant barriers.

 

·         Clarify the differences between these two reflective technologies and illustrate applications best suited to each product.

 

·         Provide a working knowledge of the effective use of reflective insulation and radiant barriers.

 

The handbook does not intend to be a definitive source, but will cover some basic information.  There are a large number of excellent authoritative publications about reflective technologies and products.  They are listed in section 10, References, and are recommended for additional information and guidance.  Our purpose in this section is to inform in an easily understandable way, the virtues of the reflective products represented by RIMA members.

 

 

FUNDAMENTALS OF HEAT TRANSFER

 

Heat flows from a hot or warm medium to a cold medium in three ways:

 

·         By radiation from a warm surface to a cooler surface through an air space

·         By conduction through solid or fluid materials

·         By convection, which involves the physical movement of air

 

Conduction

 


Conduction is the direct flow of heat through a material resulting from physical contact.  The transfer of heat by conduction is caused by molecular motion in which molecules transfer their energy to adjoining molecules and increase their temperature.

                                   

A typical example of conduction would be the heat transferred from hot coffee, through the cup, to the hand holding the cup.  Another example, as shown above, the contents of the kettle boils from heat transferred from the burner to the kettle.  Also, the poker becomes hot from contact with the hot coals.

                       

Heat transfer by conduction is governed by the fundamental equation described by Fouier’s law:

 

(Rate of heat flow) = k x (Area) x (Temperature Gradient)

 

The factor k is called the thermal conductivity and is a characteristic of the material through which heat is flowing, and it varies with temperature and the degree of compaction or its density.

 


The thermal conductivity of typical building and insulation materials is listed below1:

 

 

 

 

 

Material

 

k (Btu/(h.ft2) (°F/ft)

 

Btu *in/ft2*h*°F

Sawdust

0.034

0.408

Wood Shavings

0.034

0.408

Mineral Wool

0.0217

0.260

INSULATION

Std. Fiberglass Batt

0.313

3.2

High Performance Fiberglass Batt

0.263

3.8

Loose-Fill Fiberglass

0.400

2.5

Loose-Fill Rock Wool

0.357

2.8

Loose-Fill Cellulose

0.270

3.7

Expanded Polystyrene

0.263

3.8

Extruded Polystyrene

0.208

4.8

GASES

Air

0.181

5.52

Carbon Dioxide

0.113

8.85

Helium

1.031

0.97

Methane

0.234

4.27

LIQUIDS

Ethylene Glycol

1.80

0.56

Gasoline

0.94

1.06

Water

4.19

0.24

METALS

Aluminum

1404

0.0007

Copper

2636

0.0004

Iron

468

0.0021

Lead

241

0.0041

MISCELLANEOUS BUILDING MATERIALS

Acoustical Tile

0.40

2.5

Asphalt

0.43

2.3

Concrete (D=140 pcf)

9.7

0.1

Cotton (D=6 pcf)

0.42

2.4

Glass

9.7

0.1

Soil (D=130 pcf)

3.6

0.3

Fir Lumber

0.76

1.3

Oak Lumber

1.18

0.8

Yellow Pine Lumber

1.04

1.0

Plywood

0.83

1.2

 

 

 

 

 

 

Convection

 

Convection is the transfer of heat in fluid, such as air, caused by the movement of the heated air or fluid.  In a building space, warm air rises and cold air settles to create a convection loop and is termed free convection.  Convection can also be caused mechanically, (termed forced convection), by a fan or by wind.

 

Text Box: Typical examples of heat transfer through convection:

1.	Warm air rising from register. (forced  convection)

2.	Warm air rising from all surfaces of radiator, (after air in contact with radiator has been heated by conduction).

3.	Warm air rising from chimney. (free convection)

 

In the flow of heat through a solid body to air, it was observed that the passage of heat into the air was not accomplished solely through conduction.  Instead, it occurred partly by radiation and partly by free convection.  A temperature difference existed between the hot solid and the average temperature of the air.  In this case, the resistance to heat transfer cannot be computed using the thermal conductivity of air alone.  Instead, the resistance has to be determined experimentally by measuring the surface temperature of the solid, the temperature of air, and the heat transferred from the solid to air.  The resistance computed is the combined resistance of conduction, free convection, and radiation.  This resistance, denoted by the letter “R”, has the units of (hr ft2 °F/Btu) and is commonly used to indicate the thermal characteristics of insulation materials.

 

 

 

 

 

 

 

 

 

 

 

 


Radiation

Radiation is the transfer of heat or energy from a hot surface to a cold surface through air or vacuum.  All surfaces including a radiator, stove, a ceiling or roof and ordinary insulation radiate to different degrees.  The radiant heat is invisible and has no temperature, just energy.  When this energy strikes another surface, it is absorbed and increases the temperature of that surface.  This concept can be understood with the following example: On a bright sunny day, radiant heat from the sun travels through a car’s window, strikes the steering wheel and is absorbed, causing it to rise in temperature.  

In summer, radiation from the sun strikes the outer surfaces of walls and ceilings and is absorbed causing the surface to heat up.  This heat flows from the outer wall to the inner wall through conduction which is then radiated again, through the air spaces in the building, to other surfaces within the building.  Radiation between surfaces is through invisible, infra-red heat rays

 
 

 


There are two terms commonly encountered while discussing radiant heat transfer:

 

1.      Emittance (or emissivity), refers to the ability of a material’s surface to emit radiant energy.  All materials have emissivities ranging from zero to one.  The lower the emittance of a material, the lower the heat radiated from its surface.  Aluminum foil has a very low emittance which explains its use in reflective insulation. 

 

2.      Reflectance (or reflectivity) refers to the fraction of incoming radiant energy that is reflected from the surface.  Reflectivity and emissivity are related and a low emittance is indicative of a highly reflective surface.  For example, aluminum with an emissivity of 0.03 has a reflectance of 0.97. 

 

 

The emittance of various surfaces is listed in the following table2.


 



 

Material Surface

 

Emittance

Asphalt

0.90-0.98

*Aluminum foil

0.03-0.05

Brick

0.93

Concrete

0.85-0.95

Glass

0.95

Fiberglass / Cellulose

0.8-1.0

Iron (polished)

0.06

Iron (rusty)

0.85

Limestone

0.36-0.90

Marble

0.93

Paint: white lacquer

0.80

Paint: white enamel

0.91

Paint: black lacquer

0.80

Paint: black enamel

0.91

Paper

0.92

Plaster

0.91

Silver

0.02

Steel (mild)

0.12

Wood

0.90

 

 

THE NEED FOR INSULATION

 

When installed correctly, insulation reduces the heat transfer through the envelope of a building.  Whenever there is a temperature difference, heat flows naturally from a warmer space to a cooler space.  To maintain comfort in winter, the heat lost must be replaced by the heating system; and in summer, the heat gained must be removed by the cooling system.  Statistics show that 50% to 70% of the energy used in the average home in the United States and Canada is for heating and cooling.  It makes sense to use thermal insulation to reduce this energy consumption, while increasing comfort and saving money.  Naturally, less consumption of fossil fuels and the energy produced from them relieves the burden our ecosystem must bear.

 

To summarize, insulating the envelope of a building’s conditioned space yields these key benefits:

 

1.      Provides a much more comfortable, productive and livable structure.  In addition, the effects of moisture condensation and air movement are minimized in well-insulated buildings.  This results in lower maintenance costs and increased longevity of the building structure. 

 

2.      Reduces energy requirements, which lowers utility bills.

 

3.      Supports economic, environmental and energy conservation goals.  This is evidenced by the numerous studies sponsored by the Department of Energy.

 

Heat moves through wall cavities or between roofs and attic floors by radiation, conduction, and convection with radiation the dominant method of heat transfer.  A reflective insulation is an effective barrier against radiant heat transfer because it reflects almost all of the infrared radiation striking its surface and emits very little of the heat conducted through it.  By virtue of its impermeable surface, reflective insulation also reduces convective heat transfer.  Mass insulation like fiberglass or foam board primarily slows conductive heat transfer, and to a smaller extent, convective heat transfer.  However, mass insulation is not as effective against infrared radiation, actually absorbing it rather than reflecting or blocking it.

 

REFLECTIVE INSULATION

 

Concept of Reflective Insulation

 

Different types of insulation products reduce the heat transferred by conduction, convection and radiation to varying degrees.  As a result, each provides different thermal performance and corresponding “R” values.  The primary function of reflective insulation is to reduce radiant heat transfer across open spaces, which is a significant contributor to heat gain in summer and heat loss in winter.  The low emittance metal foil (usually aluminum) surface of the product blocks up to 97% of the radiation and therefore a significant part of the heat transfer.

 

There are many types of materials that reduce heat gain and heat loss.  Some materials provide greater resistance than others, depending on the mode of heat transfer: convection, conduction, or radiation.  Most insulation materials work on the principle of trapped air gas being a good insulator.  Mass insulation like fiberglass, foam, and cellulose use layers of glass fibers, plastic, and wood fiber respectively to reduce convection thereby decreasing the transfer of heat.  These materials also reduce heat transfer by conduction due to the presence of trapped air.  (However, these products, like most building materials, have very high radiant transfer rates.)  Heat flow by radiation has been brought to the public’s attention with high efficiency windows which commonly use the term “Low E” to advertise the higher performance ratings.  This value is measured in emittance or “E” values ranging from 0 to 1 (lower “E” value indicates better performance).  Most building materials, including fiberglass, foam and cellulose have “E” values in excess of 0.70.  Reflective insulation typically have “E” values of 0.03 (again, the lower the better).  Therefore, reflective insulation is superior to other types of insulating materials in reducing heat flow by radiation.  The term reflective, in reflective insulation, is in some ways a misnomer because aluminum either works by reflecting heat (reflectance of 0.97) or by not radiating heat (emittance of 0.03).  Whether stated as reflectivity or emissivity, the performance (heat transfer) is the same.  When reflective insulation is installed in building cavities, it traps air  (like other insulation materials) and therefore reduces heat flow by convection thus addressing all three modes of heat transfer.  In all cases, the reflective material must be adjacent to an air space.  Aluminum, when sandwiched between two pieces of plywood for example, will conduct heat at a high rate.

 

Understanding a Reflective Insulation System (RIS)

 

A reflective insulation system is typically formed by layers of aluminum or a low emittance material and enclosed air spaces which in turn provide highly reflective or low emittance cavities adjacent to a heated region.  Some reflective insulation systems also use other layers of materials such as paper or plastic to form additional enclosed air spaces.  The performance of the system is determined by the emittance of the material(s), the lower the better, and the size of the enclosed air spaces.  The smaller the air space, the less heat will transfer by convection.  Therefore, to lessen heat flow by convection, a reflective insulation, with its multiple layers of aluminum and enclosed air space, is positioned in a building cavity (stud wall, furred-out masonry wall, floor joist, ceiling joist, etc.) to divide the larger cavity (3/4” furring, 2” x 4”, 2” x 6”, etc.) into smaller air spaces.  These smaller trapped air spaces reduce convective heat flow.  Like other insulation, reflective insulation is labeled with R-values which provide a measure of thermal performance.

 

Reflective insulation differs from conventional mass insulation in the following:

 

1.      Reflective insulation has very low emittance values “E-values” (typically 0.03 compared to 0.90 for most insulation) thus significantly reduces heat transfer by radiation;

2.      A reflective insulation does not have significant mass to absorb and retain heat;

3.      Reflective insulation has lower moisture transfer and absorption rates, in most cases;

4.      Reflective insulation traps air with layers of aluminum, paper and/or plastic as opposed to mass insulation which uses fibers of glass, particles of foam, or ground up paper;

5.      Reflective insulation does not irritate the skin, eyes, or throat and contain no substances which will out-gas;

6.      The change in thermal performance due to compaction or moisture absorption, a common concern with mass insulation, is not an issue with reflective insulation.

 

Types of Reflective Insulation Materials

 

Reflective insulation has been used effectively for decades and is available throughout the world.  The following are the major types of reflective insulation currently available:

 

1.      Layer or layers of aluminum foil separated by a layer or layers of plastic bubbles or a foam material;

2.      Multiple layers of aluminum, kraft paper, and/or plastic with internal expanders an flanges at the edge for easy installation;

3.      Single layer of aluminum foil laminated to a kraft paper or plastic material.

 

Applications for Reflective Insulation Materials

 

Reflective insulation materials are designed for installation between or over framing members and as a result, are applicable to unfinished walls, floors, and ceilings.  Applications for reflective insulation extend to many commercial, agricultural and industrial uses, such as panelized wood roofs, pre-engineered buildings, pole barns and other wood framed structures.  A few representative applications are listed below:

 

·         Residential Construction, New and Retrofit

            Walls, basements, floors, ceilings, roofs, and crawl spaces.

 

·         Commercial Construction, New and Retrofit

            Walls, floors, basements, ceilings, roofs, and crawl spaces.

 

·         Manufactured Housing Construction, New and Retrofit

            Walls, floors, roofs, and crawl spaces.

 

·         Other Uses, New and Retrofit

Water heater covers, cold storage units, poultry, and livestock buildings, equipment sheds, pipe insulation and recreational vehicles.


 

Typical applications (new and retrofit) for reflective insulation


 

 

 


Reflective insulation in typical basement installation

 

 

 

 

Installing Reflective Insulation Systems

 

Reflective insulation products incorporate trapped air spaces as part of the system.  These air spaces, which may be layered or closed-cell, can be included in the system either when the product is manufactured or while it is being installed.  In either case, the advertised performance of the insulation requires that these air spaces be present after the product is installed.  The labeled R-values will not be achieved if the product is not installed according to the instructions of the manufacturer. 

 


The thermal performance of the reflective system varies with the size and number of enclosed reflective spaces within the building cavity.  Most reflective systems range from one to five enclosed air spaces, as shown in the figure and schematic below.

 



Schematic of reflective insulation installed between framing members

 

 


Air spaces in typical Reflective Insulation System

 

There are other beneficial considerations for using reflective insulation.  Generally, these products have a very low water vapor and air permeance.  When installed properly, with joints taped securely, reflective insulation materials are efficient vapor retarders and an effective barrier to air and radon gas. 

 

Since reflective insulation materials are effective vapor retarders, care should be taken to ensure that they are installed correctly within the structure.  Correct installation depends on the climatic conditions and moisture sources involved.  An appropriate installation ensures that all joints and seams are butted against each other and taped, or overlapped and taped.  This will reduce the possibility of moisture condensation within the cavity and improve performance.

 

 

RADIANT BARRIERS

 

Physics of Radiant Barriers

 

A “radiant barrier” is a reflective/low-emittance surface, on or near a building component, that intercepts the flow of radiant energy to and from the building component.  It is, as the name suggests, a barrier to radiant heat movement, the same as a vapor barrier blocks water vapor migration and an air barrier stops air flow.

 

A radiant barrier can be aluminum foil laminate, aluminized plastic film or a low emittance coating.  The only requirement is that its surface must have low emittance and high reflectivity in the infrared band of the spectrum.

 

The aluminum foil shields that are commonly inserted behind radiators in older houses are radiant barriers, blocking radiant heat transfer from the radiator to the exterior wall.  The invisible glass coating in low-E windows is also a radiant barrier. 

 

It should be clearly understood that although a radiant barrier reduces heat loss and gain through the building envelope, it is not an insulation material per se and has no inherent R-value.

 

Radiant Barrier Systems (RBS)

 

A “radiant barrier system” (RBS) is a building section that includes a radiant barrier facing an air space.  An attic with a radiant barrier on top of the mass insulation on the floor, or under the roof is an RBS.  A vent skin wall with a radiant barrier facing the vented air space is also an RBS.

 

The distinction between a radiant barrier “material” and radiant barrier “system” is not merely academic.  In an attic, the effectiveness of a radiant barrier is significantly affected by the amount of attic ventilation.  A vented attic with a radiant barrier is a very different system from an unvented attic with the same radiant barrier.

 

TECHNICAL NOTE:  The generally accepted definition of a radiant barrier system specifies that the reflective material face an open air space.  The idea is that a radiant barrier facing an enclosed air space is a “reflective insulation” with a measurable R- value. 

 

Types of Radiant Barrier Material

 

Several types of radiant barrier materials are available.  Although they all have similar surface properties (and consequently similar performance), variations in materials and construction result in significant differences with respect to strength, durability, flammability and water vapor permeability.

 

Most products available commercially fall into three major categories:

 

1.   Aluminum Foil Laminates - foil laminated to kraft paper, plastic films, or to OSB/plywood     roof sheathing

2.   Aluminized Plastic Films  -  a thin layer of aluminum particles deposited on film through a vacuum process

3.   Reflective Paints/Coatings - liquids that reduce the emissivity of the surface to which they are applied

 

Installing Radiant Barriers

 

Attics

 

The most common location for a radiant barrier system is in attics.  Three basic configurations are used:

 

1.      Rafter/truss installation

 


 

 

 

 

 

 

 

 

 

 

 


2.      Under, or pre-laminated to, roof sheathing


 

3.      Horizontal installation (directly above ceiling and/or ceiling insulation)

 

 

 

As noted before, a vented attic with a radiant barrier is a very different system from an unvented attic with the same radiant barrier.  Common types of attic ventilation are:

 

·         Soffit to ridge

·         Soffit to gable

·         Soffit to soffit

·         Gable to gable

 

Most codes require at least a 1 to 150 ventilation rate.  What this means is that for every 150 square feet of floor space, there should be one square foot of free vent area.

 

 

Walls

 

Several types of radiant barriers are used in walls, as shown in the figure below.  An example is foil faced fiberglass batts stapled to the sides of the wall studs, leaving an air space between the foil facing and interior sheathing.  Another less common technique is to use foil faced drywall over furring strips on the interior stud faces.  The furring strips create an air space between the foil facing and cavity insulation.  The technique used commonly in Florida is to apply a radiant barrier to the exterior of the wall, followed by furring strips and sheathing.  In this construction, commonly referred to as “vent skin” construction, the air space created by the furring strips is typically vented top and bottom so that outdoor air can circulate into and through the space.

 

 

 

 

TECHNICAL NOTE: When an interior barrier is used, all seams should be taped to avoid possible moisture migration.  When an exterior barrier is used, it should be perforated unless a vapor retarder is used on the interior side, otherwise it may trap moisture.  Application techniques will vary depending on the climate in which radiant barriers are used.

 

Floors

 

Radiant barriers can also be used in floor systems above unheated basements and crawl spaces.  The radiant barrier is either stapled to the underside of floor joists, creating a single reflective air space, or between the joists, followed by some type of sheathing, creating two separate reflective air spaces as shown below.

 

Laboratory experiments and computer modeling suggest that floor radiant barrier systems may exhibit R-values as high as R-7.5 to R-8.0 for reducing heat loss to basements and crawl spaces.  Radiant barriers make an ideal choice for this application because, in addition to their excellent thermal properties, they are also vapor barriers that prevent ground moisture from migrating into the living space above.

 

 

 

 

 

INTERIOR RADIATION CONTROL COATINGS (IRCC)

 

Definition of an IRCC

 

An Interior Radiation Control Coating is a non-thickness dependent, low emittance coating.  When applied to non-porous building materials such as plywood, OSB, metal siding or plasterboard, it lowers the normal surface emittance of these materials to 0.24 or lower and may be effectively used as an interior radiant barrier.

 

Physics of an IRCC

 

An IRCC works by changing the emittance of the surface where it is applied.  Building products, such as wood, brick, painted surfaces and plasterboard exhibit high emissivities (0.7 - 0.95).  When heated above the temperature of adjacent surfaces, they radiate most of their heat energy to cooler surfaces.  An IRCC works by lowering their surface emittance to 0.24 or lower, lessening their ability to radiate heat.

 

Definition of an Interior Radiation Control System (IRCCS)

 

A building construction consisting of a low emittance (normally 0.25 or less) surface bounded by an open air space.  An IRCCS is used for the sole purpose of limiting heat transfer by radiation and is not specifically intended to reduce heat transfer by convection or conduction.  (ASTM C 1321, section 3.2.3)

 

Thus, an IRCCS is similar to a Radiant Barrier System (RBS) but is somewhat less efficient due to its higher emissivity and is comprised of a coating on a building surface, not a foil or film product.

 

Advantages of an IRCC

 

An IRCC is normally applied using airless spray equipment, resulting in very low labor costs and greatly reduced installation times.  Also, a water based IRCC can be safely installed in existing structures where the costs of installing foil or film products may be prohibitive or impractical.  An IRCC may also be used in many manufactured products (such as infrared heat reflectors of automotive parts) where it is impractical to adhere foil or film radiant barriers.

 

Installation methods for an IRCC

 

Since an IRCC is a paint product, spray painting, either air atomization or airless is the most effective method of installation.  Where spray painting is not practical.  An IRCC may be applied using a low nap roller.  Brush painting is usually impractical since these coatings are very low viscosity and not formulated for brush application.

 

The IRCC may be applied to a building surface already in place (such as the underside of an installed roof deck or the inside of a wall) or it may be applied to a building component before it is installed (such as roof decking painted while laying on the ground before it is lifted into place.  Regardless when a building component is painted with and IRCC, it is imperative that after installation the surface painted with the IRCC face a minimum of a 2” air space.

 


Typical installations of an IRCC


Under Roof

Interior Side Walls

 

Exterior Side Walls

 

Other Possible Uses - Construction

 

An IRCC is a paint product therefore it can be used on almost any solid surface where paint can be applied and where radiant heat transfer is a problem.  An example would be painting the inside of a boiler room to retain heat that might make adjacent areas uncomfortable.  Even painting the boiler, itself, might make it operate more efficiently.  Freestanding heat shield in welding bays or at foundries can be painted with an IRCC.  Exterior roof surfaces may also be painted with an IRCC to repel summer heat and lower radiation losses in the winter.

 

Other Possible Uses of an IRCC

 

IRCC technology has many applications in manufacturing and industry.  It is used in the automotive industry to keep temperature sensitive parts and automotive interiors cool.  It is used in the lighting industry to make plastic reflectors for heat lamps and radiant heating devices.  It is used as a heat reflecting surface in industrial ovens.  It is used on high temperature process piping and storage tanks in chemical plants to lessen heat loss.  Any process or device that is temperature sensitive to infrared heat problems or uses reflected heat in its operation may be a candidate for IRCC technology.

 

GLOSSARY OF TERMS

 

Conduction:  Conduction is the direct flow of heat through a material resulting from physical contact.  The transfer of heat by conduction is caused by molecular motion in which molecules transfer their energy to adjoining molecules and increase their temperature.

 

Convection: Convection is the transfer of heat in fluid or air, caused by the movement of the heated air or fluid itself.  In a building space, warm air rises and cold air settles to create a convection loop and is termed free convection.  Convection can also be caused mechanically by a fan and is termed forced convection.

 

Emittance: Emittance refers to the ability of the surface to emit radiant energy.  Emissivity ranges from 0 to 1 and a lower value indicates a reflective surface with a low level of radiation.

 

“R” value:  Property of an insulation material used to characterize the effectiveness of the insulation in reducing heat transfer by conduction.  The higher the “R” value, the better the insulation’s ability to reduce this heat transfer.

 

Radiation: Radiation is the transfer of heat or energy from a hot surface to a cold surface through air or through a vacuum.

 

Radiant Barrier: A radiant barrier is a reflective surface, on or near a building component, that intercepts the flow of radiant energy to and from the building component.

 

Radiant Barrier System: A Radiant Barrier System (RBS) is a building section that includes a radiant barrier facing an air space.

 

Reflectance: Reflectance refers to the fraction of incoming radiant energy that is reflected from the surface.

 

Reflective Insulation System: Reflective Insulation System is formed by a combination of low emittance surfaces and air spaces that provide reflective cavities which have low levels of radiant energy transmission.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REFERENCES

 

The following list of references is selective rather than exhaustive.  Technical papers, reports, sections of books, and important compliance documents have been included.  Many of the papers and reports contain references that broaden the list and provide additional insight into the performance of reflective insulation and radiant barriers.

 

Reviews

 

1.      Gross and R.G. Miller, “Literature Review of Measurement and Predictions of Reflective Building Insulation System Performance: 1900-1989”, ASHRAE Transactions 95 (2) 651-664 (1989).

 

2.      Ned Nisson: Radiant Barriers, Principles, Practice, and Product Directory”, Energy Design Update, Cutter Information Corporation, Arlington, MA (1990).

 

Technical Papers

 

1.      Ludwig, Adams, “Thermal Conductance of Air Spaces”, ASHRAE Journal (March, 1976) pp. 37-38

 

2.      Cook, D.W. Yarbrough, and K.E. Wilkes, “Contamination of Reflective Foils in Horizontal Applications and the Effect on Thermal Performance”, ASHRAE Transactions 95 (1) (1989).

 

3.      Andre O. Desjarlais and David W. Yarbrough, ‘“Prediction of the Thermal Performance of Single and Multi-Airspace Reflective Insulation Materials”, Insulation Materials:  Testing and Applications, 2nd Volume, ASTM STP 1116, R.S. Graves and D.C. Wysocki, Editors, American Society for Testing and Materials, Philadelphia (1991).

 

4.      Fairey, “Effect of Infrared Radiation Barriers on the Effective Thermal Resistance of Building Envelopes”, Proceedings of the ASHRAE/DOE Conference on Thermal Performance of the Exterior Envelopes of Buildings II, ASHRAE Special Publication 38 (1983).

 

5.      Philip Fairey, “The Measured, Side-by-Side Performance of Attic Radiant Barrier Systems in Hot-Humid Climates”, Thermal Conductivity 19, David W. Yarbrough, Editor, Plenum Press (1988) pp. 481-496.

 

6.      Robert Hageman and Mark P. Medera, “Energy Savings and HVAC Capacity Implications of a Low-Emissivity Interior Surface for Roof Sheathing”.

 

7.      Joy, “Improving Attic Space Insulating Values”, ASHRAE Transactions 64 251 (1959).

 

8.      Levins and M.A. Karnitz, “Cooling Energy Measurements of Unoccupied Single-Family Houses with Attics Containing Radiant Barriers”, ORNL/CON-200 (1986), Oak Ridge National Laboratory, Oak Ridge, TN.

 

 

9.      Levins and M.A. Karnitz, “Cooling Energy Measurements of Unoccupied Single-Family Houses with Attics Containing Radiant Barriers, ORNL/CON-213 (1987), Oak Ridge National Laboratory, Oak Ridge, TN.

 

10.  Levins and M.A. Karnitz, “Cooling Energy Measurements of Unoccupied Single-Family Houses with Attics Containing Radiant Barriers, ORNL/CON-226 (1987), Oak Ridge National Laboratory, Oak Ridge, TN.

 

11.  Levins and M.A. Karnitz, “Cooling Energy Measurements of Unoccupied Single-Family Houses with Attics Containing Radiant Barriers”, ORNL/CON-239 (1988), Oak Ridge National Laboratory, Oak Ridge, TN.

 

12.  Levins, M.A. Karnitz, and J.A. Hall, “Moisture Measurements in Single-Family Houses Containing Radiant Barriers”, ORNL/CON-255 (1989), Oak Ridge National Laboratory, Oak Ridge, TN.

 

13.  McQuiston, S.L. Der, and S.B. Sandoval, “Thermal Simulation of Attic and Ceiling Spaces”, ASHRAE Transactions 90 739-163 (1984).

 

14.  Pratt, “Heat Transmission in Buildings, John Wiley and Sons, Chapter 3, “The Thermal Resistance of Airspaces in Cavity Building Structures”, (181) pgs. 66-98.

 

15.  Robinson and F.J. Powell, “The Thermal Insulating Value of Airspaces”, Housing Research Paper No. 32, National Bureau of Standards Project NE-12, National Bureau of Standards, Washington, DC (1954).

 

16.  Robinson, L.A. Cosgrove and F.J. Powell, “Thermal Resistance of Airspaces and Fibrous Insulation Bounded by Reflective Surfaces”, Building Materials and Structures Report 151, National Bureau of Standards, Washington, DC (1957).

 

17.  St. Regis, “Reflective Insulation and the Control of Thermal Environments”, St. Regis-ACI, Diethelm & Co., LTD, Bangkok, Thailand (1969).

 

18.  Wilkes, “Thermal Modeling of Residential Attics with Radiant Barriers: Comparison with Laboratory and Field Data”, Thermal Performance of the Exterior Envelopes of Buildings IV, ASHRAE (1989) pp. 294-311.

 

19.  Wilkes, “Thermal Model of Attic Systems with Radiant Barriers”, ORNL/CON-262 91991) Oak Ridge National Laboratory, Oak Ridge, TN/

 

20.  Kenneth E. Wilkes, “Analysis of Annual Thermal and Moisture Performance of Radiant Barrier Systems”, ORNL/CON-319 (1991), Oak Ridge National Laboratory, Oak Ridge, TN.

 

21.  Wu, “The Effect of Various Attic Venting Devices on the Performance of Radiant Barrier Systems in Hot Arid Climates”, Thermal Performance of the Exterior Envelopes of Buildings IV”, ASHRAE (1989) pp. 261-270.

 

22.  Yarbrough, “Assessments of Reflective Insulation for Residential and Commercial Applications”, Oak Ridge National Laboratory Report ORNL/TM 8819, Oak Ridge, TN (1983).

 

23.  Yarbrough, “Estimation of the Thermal Resistance of a Series of Reflective Air Spaces Bounded by Parallel Low Emittance Surfaces”, Proceedings of the Conference on Fire Safety and Thermal Insulation, S.A. Siddiqui, Editor, (1990) pp. 214-231.

 

24.  Yarbrough, “Thermal Resistance of Air Ducts with Bubblepack Reflective Insulation”, Journal of Thermal Insulation 15 137-152 (1991).

 

25.  Queer, “Importance of Radiation and Heat Transfer Through Air Spaces”, American Society of Heating and Air Conditioning Engineers.

 

 

 

 

 

 

Documents

 

 

1993 ASHRAE Handbook Fundamentals - IP Edition, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullie Circle, NE, Atlanta, GA.

 

·        “Surface Conductances and Resistances for Air”                       Table 1 - page 22.1

 

·        “Thermal Resistances of Plane Air Spaces”                    Table 2 - page 22.2

 

·        “Emittance Values of Various Surfaces and Effective     Table 3 - page 22.3

 

·        Emittances of Air Spaces”

 

·        “Effective Thermal Resistance of Ventilated Attics”         Table 5 - page 22.11

 

Federal Trade Commission, Part 460, “Labeling and Advertising of Home Insulation”

 

Para. 460.5   R-value Tests    2(b)      Aluminum Foil systems

2(c)      Single sheet systems

2(d)      Foil facings

 

International Conference of Building Officials (ICBO), “Acceptance Criteria for Reflective Insulation,” (1987, revised 1997).

 

U.S. Department of Energy, “Attic Radiant Barrier Fact Sheet,” (1991).

 

 

 

 

 

 

 

 

 

 

ASTM Standards

 

 

C 236-89         “Standard Test Method for Steady-State Thermal Performance of Building Assemblies by Means of a Guarded Hot Box.”  1995 Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 52-62.

 

C 727-90         “Standard Practice for Use and Installation of Reflective Insulation in Building Constructions.”  1995 Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 339-341.

 

C 976-90         “Standard Test Method for Thermal Performance of Building Assemblies by Means of a Calibrated Hot Box.”  1995 Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 463-481.

 

C 1158-90       “Standard Practice for Use and Installation of Radiant Barrier Systems (RBS) in Building Construction.”  1995 Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 655-657.

 

C 1224-93       “Standard Specification for Reflective Insulation for Building Applications.”  1995 Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 670-673.

 

C 1313-95       “Standard Specification for Sheet Radiant Barriers for Building Construction Applications.”  1996 Annual Book of ASTM Standards Vol. 04.06 (1996) pp.

 

C 1340-96       “Standard Practice for Estimation of Heat Gain or Loss Through Ceilings Under Attics Containing Radiant Barriers by Use of a Computer Program.”  1997 Annual Book of ASTM Standards Vol. 04.06 (1997) to be published.

 

C 1371-96       “Standard Test Method for Determination of Emittance of Materials Near Room Temperature Using Portable Emissometers.”  1997 Annual Book of ASTM Standards Vol. 04.06 (1997) to be published.

 

E 84-95b         “Standard Test Method for Surface Burning Characteristics of Building Materials.”  1995 Annual Book of ASTM Standards Vol. 04.07 (1995).

 

E 96-95           “Standard Test Method for Water Vapor Transmission of Materials.”  1995 Annual Book of ASTM Standards Vol. 04.06 (1995) pp. 697-704.

 

 


APPENDIX A

 

INTRODUCTORY COMMENTS ON THERMAL

RESISTANCES FOR REFLECTIVE INSULATION SYSTEMS

________________________________________________________________

 

Reflective insulation materials (RIMs) are available in a variety of forms that includes one or more low emittance (emissivity) surfaces.  The low emittance surfaces are generally provided by aluminum foils or deposited aluminum surfaces which exhibit very low emittances and high reflectances for long wavelength radiation.  The foils are attached to other materials for mechanical strength or support.  In some cases, supporting materials add to the thermal resistance of the reflective insulation system that is created upon installation of a reflective insulation in a building or vehicle cavity.  The following discussion of thermal resistances will be limited to one-dimensional heat flow across reflective air spaces.

 

A reflective insulation system (RIS) is formed by a RIM positioned to form one or more enclosed air spaces.  A good RIS design will have at least one low-emittance major surface bounding each air space.  The purpose of the low-emittance high-reflectance surfaces is to significantly reduce the radiative heat transfer across the enclosed air space.  The enclosed air spaces that make up a RIS are not ventilated.  There should be no air movement in or out of the enclosed space.  The reflective air spaces (enclosed spaces) are positioned so that the major surfaces are perpendicular to the anticipated heat flow direction.  When this is done, the thermal resistances of the air spaces in series are additive.  If the reflective insulation material has thermal resistance, then this resistance is added to that provided by the reflective air spaces.

 

The thermal resistance for one-dimensional heat-flow through a series of n reflective air spaces is:

 

RTOTAL = RAIRSPACE ONE + RAIRSPACE TWO + ... RAIRSPACE “n” + RREFLECTIVE MATERIAL

 

Heat is transferred across air spaces by conduction and convection as well as radiation.  Convective heat transfer within the air space is related to the movement of air caused by temperature differences.  The density of air at constant pressure decreases as the temperature increases.  A temperature difference between two regions will result in air density differences which will result in buoyant forces and air movement or natural convection.  The magnitude of the buoyant forces increases as the temperature increases and the induced movement of  air depends on the buoyant force magnitude and its direction relative to gravity.  Since heat flow is in the direction of decreasing temperature, the direction of the buoyant force will depend on the orientation and temperatures of the bounding surfaces.  As a result, the convective contribution to the overall heat transfer depends on heat flow direction.  Convective heat flow upward is the greatest, and convective heat flow down is the least and can be zero in an idealized system with stagnant air.

 

Estimates of the thermal resistance of a single reflective air space that has parallel bounding surfaces perpendicular to the direction of heat flow can be made using the following equations.

 

 

                                                                                     (1)

 

 

                                             (2)

 

 

                                        (3)

 

 

                               (4)

                                                                                                                                        

 

 

Îi         IR emittance for surface “i”, i = 1 or 2

E          Effective emittance for an air space

hc         Convective heat transfer coefficient, Btu/ft2·hr·°F

hr         Radiative heat transfer coefficient, Btu/ft2·hr·°F

l           Thickness of air space, inches

Q         Heat flux, Btu/hr·ft2

R         Thermal resistance, ft2·hr·°F/Btu

Tm         Average of hot and cold surface temperatures, °F

DT        Difference between hot and cold surface temperatures, °F

 

Equation (1) expresses mathematically the fact that R-value depends on heat transfer by radiation, E·hr, and heat transfer by conduction-convection, hc.  The multiplying factor, E, is often called an effective emittance and takes on values between 0 and 1.  Its value depends on the emittances of the two major bounding surfaces, Îi and Î2, as shown by Equation (2).  The “E” value for an air space with one low-emittance aluminum boundary is very low, usually in the range 0.03 to 0.05.

 

Equation (3) is the heat transfer coefficient for radiation, h r, between two parallel surfaces.  The hr is multiplied by “E” to introduce the effect of surface emittances.  Equation (2) has been derived for infinite parallel planes and discussed in most texts dealing with radiative heat transfer.

 

The equation for hc is the complication in the R-value calculation.  Equation (4) indicates that hc depends (is a function of) four variables for one-dimensional heat flow between parallel surfaces.  Values for hc are developed from experimental data for total heat flow such as that obtained  with a hot-box facility  such as that described in ASTM C 236.  The terms R, E, and h r are obtained from emittance and hot-box measurements.  Values for hc are derived from sets of hot box measurements done for a specific heat-flow direction.  Robinson and Powell (see references) have provided hc in graphical form and Yarbrough (see references) has provided hc in analytical form.

 

 

 

 

 

One-dimensional heat flow and R-values between large parallel surfaces held at different temperatures and separated by distance “l” are established by the above equations and discussion.  The procedure has been used to generate the following three tables for single air space R-values for Tm = 50°F and DT = 30°F.   These temperatures match the requirements of the FTC labeling rule for “single-sheet” products.

 

Tables 1, 2, or 3 can be used to estimate the R-value for a RIS provided that the overall temperature difference across each element in the RIS is known.  The steady-state temperature difference (DT) across each element is related to the R-values of the RIS elements, Ri, by Equation (5).

 

                                                     (5)

 

                                                       (6)

(7)

 

Unfortunately, Ri values are related to DTi.  The only known quantity in Equation (5) is the overall temperature difference DT.  An approach to solving for R is to first estimate the DTi values.  This should be done in such a way that Equation (6) is satisfied.  Given a trial set of DTi, the average temperature T in each element can be calculated and R i can then be estimated from Tables 1, 2, and 3.  This, of course, limits the accuracy since the tables are for 50°F.   The total R is calculated by adding the Ri as indicated by Equation (7).  The calculated Ri are used to recalculate DTi by means of Equation (5).  This iterative procedure is continued until constant values for DTi and Ri are obtained.

 

The calculational procedure can be improved by using the iterative procedure and Equation (1) to calculate Ri values.  Table 4 has been prepared to expedite the calculation for a mean air space temperature of 75°F.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 1.  Calculated R-Values for an Enclosed Air Space at 50°F

and DT = 30°F -- Heat Flow Down

-----------------------------------------------------------------------------------------------------------------------------------

         l                E/    0.030        0.050        0.100      0.150        0.250      0.500        0.750      0.820

-----------------------------------------------------------------------------------------------------------------------------------

 

      0.50                      2.63          2.51          2.25        2.04          1.72        1.24          0.97        0.91

      0.75                      3.72          3.48          3.01        2.64          2.13        1.44          1.08        1.01

      1.00                      4.69          4.32          3.61        3.10          2.42        1.56          1.15        1.07

      1.25                      5.57          5.06          4.11        3.46          2.63        1.65          1.20        1.11

      1.50                      6.36          5.70          4.53        3.75          2.80        1.71          1.23        1.14

      1.75                      7.03          6.23          4.86        3.98          2.92        1.76          1.25        1.16

      2.00                      7.60          6.68          5.12        4.15          3.01        1.79          1.27        1.18

      2.25                      8.08          7.04          5.34        4.29          3.09        1.81          1.28        1.19

      2.50                      8.49          7.36          5.51        4.41          3.15        1.83          1.29        1.20

      3.00                      9.15          7.84          5.78        4.58          3.23        1.86          1.31        1.21

-----------------------------------------------------------------------------------------------------------------------------------

 

 

 

 

 

 

Table 2.  Calculated R-Values for an Enclosed Air Space at 50°F

and DT = 30°F -- Heat Flow Horizontal

-----------------------------------------------------------------------------------------------------------------------------------

         l                E/    0.030        0.050        0.100      0.150        0.250      0.500        0.750      0.820

-----------------------------------------------------------------------------------------------------------------------------------

 

      0.50                      2.41          2.31          2.09        1.91          1.63        1.19          0.93        0.88

      0.75                      2.88          2.74          2.43        2.19          1.83        1.29          1.00        0.94

      1.00                      2.76          2.63          2.35        2.12          1.78        1.27          0.98        0.93

      1.25                      2.67          2.55          2.28        2.07          1.74        1.25          0.97        0.92

      1.50                      2.62          2.50          2.25        2.04          1.72        1.24          0.97        0.91

      1.75                      2.60          2.48          2.23        2.02          1.71        1.23          0.96        0.91

      2.00                      2.59          2.47          2.22        2.02          1.70        1.23          0.96        0.90

      2.25                      2.58          2.47          2.22        2.02          1.70        1.23          0.96        0.90

      2.50                      2.59          2.47          2.22        2.02          1.71        1.23          0.96        0.91

      3.00                      2.61          2.49          2.24        2.03          1.72        1.23          0.96        0.91

-----------------------------------------------------------------------------------------------------------------------------------

 

 

 

 

 

 

Table 3.  Calculated R-Values for an Enclosed Air Space at 50°F

and DT = 30°F -- Heat Flow Up

-----------------------------------------------------------------------------------------------------------------------------------

         l                E/    0.030        0.050        0.100      0.150        0.250      0.500        0.750      0.820

-----------------------------------------------------------------------------------------------------------------------------------

 

      0.50                      1.61          1.56          1.46        1.37          1.22        0.95          0.78        0.75

      0.75                      1.69          1.64          1.53        1.43          1.27        0.98          0.80        0.76

      1.00                      1.76          1.70          1.58        1.47          1.30        1.00          0.82        0.78

      1.25                      1.81          1.75          1.62        1.51          1.33        1.02          0.83        0.79

      1.50                      1.85          1.79          1.66        1.54          1.35        1.03          0.84        0.79

      1.75                      1.89          1.83          1.69        1.57          1.37        1.05          0.84        0.80

      2.00                      1.92          1.86          1.71        1.59          1.39        1.06          0.85        0.81

      2.25                      1.95          1.88          1.74        1.61          1.40        1.06          0.86        0.81

      2.50                      1.98          1.91          1.76        1.63          1.42        1.07          0.86        0.82

      3.00                      2.02          1.95          1.79        1.66          1.44        1.09          0.87        0.82

-----------------------------------------------------------------------------------------------------------------------------------

 

 

 

Table 4.  Conduction-Convection Coefficients, hc, for use in Equation (1)

-----------------------------------------------------------------------------------------------------------------------------------

Heat Flow Down                                Width of Air Space (l, in.)

-----------------------------------------------------------------------------------------------------------------------------------

DT                     0.5                   1.0                   1.5                   2.0                   2.5                   3.0

-----------------------------------------------------------------------------------------------------------------------------------

5                      0.359               0.184               0.126               0.097               0.080               0.068

10                    0.361               0.187               0.129               0.100               0.082               0.072

15                    0.363               0.189               0.131               0.101               0.085               0.075

20                    0.364               0.190               0.132               0.103               0.087               0.078

25                    0.365               0.191               0.133               0.105               0.090               0.081

30                    0.366               0.192               0.134               0.106               0.092               0.082

-----------------------------------------------------------------------------------------------------------------------------------

 

Heat Flow Horizontal                         Width of Air Space (l, in.)

-----------------------------------------------------------------------------------------------------------------------------------

DT                     0.5                   1.0                   1.5                   2.0                   2.5                   3.0

-----------------------------------------------------------------------------------------------------------------------------------

5                      0.360               0.204               0.169               0.179               0.185               0.189

10                    0.366               0.267               0.223               0.233               0.238               0.241

15                    0.373               0.247               0.261               0.271               0.275               0.276

20                    0.380               0.270               0.292               0.301               0.303               0.303

25                    0.387               0.296               0.317               0.325               0.327               0.326

30                    0.394               0.319               0.339               0.347               0.347               0.345

-----------------------------------------------------------------------------------------------------------------------------------

 

 

Heat Flow Up                                     Width of Air Space (l, in.)

-----------------------------------------------------------------------------------------------------------------------------------

DT                     0.5                   1.0                   1.5                   2.0                   2.5                   3.0

-----------------------------------------------------------------------------------------------------------------------------------

5                      0.381               0.312               0.295               0.284               0.275               0.268

10                    0.429               0.381               0.360               0.346               0.336               0.328

15                    0.472               0.428               0.405               0.389               0.377               0.368

20                    0.511               0.465               0.440               0.423               0.410               0.400

25                    0.545               0.496               0.469               0.451               0.437               0.426

30                    0.574               0.523               0.494               0.475               0.460               0.449

-----------------------------------------------------------------------------------------------------------------------------------

 

Example 1.      Calculation of Thermal Resistance for a Single Air Space.

 

Specifications             Surface One:  T = 70°F, e1 = 0.03

Surface Two:  T = 80°F, e2 = 0.80

Space between surfaces, l,    2.0 inches

Heat flow down

 

Equation 2 for E          E = (1/0.03 + 1/0.8 - 1)-1 = 0.0298

Tm = (70 + 80)/2 = 75

DT = 80 - 70 = 10

 

hc from Table 4           hc = 0.100

hr from Equation 3       hr = 1.049

R from Equation 1       R = (0.0298 x 1.049 + 0.100)-1 = 7.6 (ft2·h·°F/Btu)

 

Example 2.      Estimation of Thermal Resistance for Two One-inch Reflective Air Spaces in Series.

 

Specifications:            Air space 1:     1.0 inch wide

Side one          e1 = 0.80

Side two          e2 = 0.03

 

Air space 2:     1.0 inch wide

Side one          e1 = 0.03

Side two          e2 = 0.80

 

Cold side temperature            70°F

Warm side temperature         80°F

 

First Approximation for DT

 

DT across air space 1:           DT1 = 5°F

DT across air space 2:           DT2 = 5°F

 

Use hc at mean temperature 75°F as an approximation.

 

Tm for air space 1:                   72.5°F

Tm for air space 2:                   77.5°F

E1 = E2 = 0.0298

 

From Table 4              hc1 = 0.184

hc2 = 0.184

 

From Equation 3         hr1 = 1.034

hr2 = 1.064

 

From Equation 1         R1 = 4.66

R2 = 4.64

R = R1 + R2 = 9.3

 

Check approximation for DT

DT1 = 10 x 4.66/9.3 = 5.01

DT2 = 10 x 4.64/9.3 = 4.99

 

These DT values agree with the assumed values.  If the agreement is not satisfactory then the calculation should be repeated using the calculated DT values.

 

Examples 1 and 2 show the approach used to calculate thermal resistances for an idealized system.  A more precise calculation can be carried out with a mathematical expression for hc rather than a table.  In most cases measured R values are less than those calculated for an idealized system.

 

 

 

 

 

 

Return to Home Page