Florida Solar Energy Center says:
Question: What is the best kind of insulation to use in my attic to reduce my air conditioning bills?
Answer: Instead of adding more insulation, you should consider adding a radiant barrier system. This will have a greater effect on your cooling bills because it reflects heat away from the attic before it can reach the insulation and ceiling below.
Without a radiant barrier, your roof radiates solar-generated heat to the insulation and, if they are mounted in the attic, your air conditioning ducts below it. The insulation absorbs the heat and gradually transfers it to the material it touches, principally the ceiling. This heat transfer makes your air conditioner run longer and consume more electricity.
If the outside air is 87 degrees, the roof can reach 148 degrees and the attic air can be more than 120. Heat from the hot attic roof radiates downward into the house. A radiant barrier will block 95 percent of the heat radiated down by the roof before it reaches the insulation.
A radiant barrier is a layer of aluminum foil placed in an attic airspace to block radiant heat transfer between a hot roof and conventional attic insulation. It can be stapled or nailed to the underside of the top chord of roof trusses or to the underside of the roof decking.
Your attic should have the proper level of insulation for the climate (at least R-19), adequate vents to allow hot air to escape and a radiant barrier to shield the house from heat. The foil radiant barrier and good ventilation can reduce heat gain through the ceiling by 40 percent. The home's cooling system should also operate more efficiently and last longer because the ductwork is cooler.
A Question and Answer Primer
(with revisions by P. Fairey)
Attic radiant barriers made of aluminum foil are becoming a popular way for homeowners to save energy and money in Southern states. They are increasing in popularity for two reasons. First, tests by the Florida Solar Energy Center and other groups show that they work. Second, manufacturers are improving the quality of radiant barrier materials.
To most homeowners, attic radiant barriers are a new energy conservation concept; many of them have questions about how radiant barriers work and how to use them. This Energy Note answers some of the most commonly asked questions. It also recommends ways to install radiant barriers in existing attics and new homes.
What is a radiant barrier?
A radiant barrier is a layer of aluminum foil placed in an airspace to block radiant heat transfer between a heat-radiating surface (such as a hot roof) and a heat-absorbing surface (such as conventional attic insulation). Figure 1 illustrates the locations in which a radiant barrier may be installed in an attic. Only locations 1 and 2 are recommended for sheet radiant barriers because dust will accumulate on the radiant barrier if it is installed at location 3.
What are the benefits of radiant barriers in attics?
In hot climates, benefits of attic radiant barriers include both dollar savings and increased comfort.
Without a radiant barrier, your roof radiates solar-generated heat to the insulation below it. The insulation absorbs the heat and gradually transfers it to the material it touches principally, the ceiling. This heat transfer makes your air conditioner run longer and consume more electricity.
An aluminum foil radiant barrier blocks 95 percent of the heat radiated down by the roof so it can't reach the insulation.
In summer, when your roof gets very hot, a radiant barrier cuts air-conditioning costs by blocking a sizable portion of the downward heat gain into the building.
In the warm spring and fall, radiant barriers may save even more energy and cooling dollars by increasing your personal comfort. During these milder seasons, outdoor air temperatures are comfortable much of the time. Yet solar energy still heats up your roof, insulation, attic air and ceiling to temperatures that can make you uncomfortably warm. An attic radiant barrier stops almost all of this downward heat transfer so that you can stay comfortable without air conditioning during mild weather.
You may also find that radiant barriers can expand the use of space in your home. For instance, uninsulated, unconditioned spaces such as garages, porches and workrooms can be more comfortable with radiant barriers. And because radiant barriers keep attics cooler, the space is more usable for storage.
One final benefit: a cooler attic transfers less heat into air conditioner ducts, so the cooling system operates more efficiently.
How do radiant barriers "block" heat transfer?
Aluminum foil the operative material in attic radiant barriers - has two physical properties of interest here. First, it reflects thermal radiation very well. Second, it emits (gives off) very little heat. In other words, aluminum is a good heat reflector and a bad heat radiator.
Your grandmother probably made use of these properties through "kitchen physics." She covered the Thanksgiving turkey with a loose "tent" of aluminum foil before she put it in the oven. The foil reflected the oven's thermal radiation, so the meat cooked as evenly on top as on the bottom. She removed the foil briefly to let the skin brown, but when she took the bird from the oven, she "tented" it with foil again. Since aluminum doesn't emit much heat, the turkey stayed hot until the rest of the meal was ready.
Cooking a turkey is a simple analogy, but the same principles of physics apply to an attic radiant barrier. Aluminum foil across the attic airspace reflects heat radiated by the roof. Even if the radiant barrier material has only one aluminum foil side and that side faces down, it still stops downward heat transfer because the foil will not emit it will not radiate the roof's heat to the insulation below it.
How much can radiant barriers save?
Since everyone's home and lifestyle are different, we can't precisely calculate your personal savings from attic radiant barriers. However, it's reasonable to expect that an attic radiant barrier can save 8-12 percent of your annual cooling costs in the Southeast.
Savings from an attic radiant barrier depend on the amount of heat the roof and attic contribute to your home's cooling load. ("Cooling load" is the total amount of heat your air conditioner must remove to maintain comfortable indoor temperatures.) In general, the more energy efficient the rest of your home is, the larger the percentage of energy you save from an attic radiant barrier because the roof and attic make up a larger portion of the cooling load.
Figure 2 shows a breakdown of cooling loads in a typical 1500-square-foot Central Florida home. The attic (including heat gains to the duct system) accounts for 22 percent of the total cooling load. In this house, an attic radiant barrier could save 8-12 percent on the annual air-conditioning costs.
Although not as significant, heating savings may also accrue from the use of radiant barriers.
Are claims of greater savings untrue?
As in most cases, claims for radiant barriers that sound too good to be true are too good to be true. If your roof accounts for less than 20 percent of your cooling load, then an attic radiant barrier can't possibly save more than 20 percent on your bills.
Claims of greater savings may simply be the results of misunderstanding. For instance, FSEC has measured and reported that radiant barriers can reduce heat gain through R-19 insulated ceilings by over 40 percent. If the ceiling portion of the total cooling load is 20 percent, that's a reduction of 40 percent of 20 percent, which amounts to 8 percent savings on the total cooling load.
If all the facts and figures tend to confuse you, just remember that an attic radiant barrier can save about 8-12 percent on your air-conditioning costs in the Southeast. Any Sunbelt homeowner knows that an 8-12 percent saving on air-conditioning bills can be significant.
What kinds of radiant barrier materials are available?
There are many types of radiant barrier materials on the market, and more are being developed as radiant barriers become more widely used. Five generic types are most common:
Some of these
products may have R-values, which may be claimed only if the product was tested
according to Federal Trade Commission regulations for insulation.
Although it is not by definition a radiant barrier, there is a low-emissivity paint available that can be applied directly to the underside of the roof decking.
Which material is best?
While the Florida Solar Energy Center strongly recommends radiant barrier systems in attics, it doesn't endorse any particular brand of radiant barrier material. However, we suggest that you look for a few common-sense characteristics:
What does a radiant barrier cost?
Costs for an attic radiant barrier depend on several factors, including the following:
condition greatly affects the cost of a radiant barrier system to the homeowner
- the individual's knowledge and willingness to do some comparison shopping. A
few phone calls and a little research can save you money on most purchases.
Radiant barriers are no exception.
Informal surveys show a wide range of material costs ($0.07-$1.00/square foot) and installation costs ($0.10-$1.00/square foot). The increases costs appear to be due more to marketing practices than to any inherent difference in thermal performance.
In some cases, radiant barriers are included in a package of energy-saving features sold to homeowners. When considering a "package deal," you may want to ask for an itemized list that includes material and installation costs for all measures included. Then shop around to see what each item would cost if you purchased them individually. You may see considerable savings. And you may decide that you want to install the items yourself, including the radiant barrier.
My material has only one foil side; should the foil face the roof?
No. In attics, single-sided radiant barrier material should be installed with the foil side facing down. This may run counter to our intuitive feel for "how things work," but it does work, and work well.
To understand how it works, remember the two properties of aluminum foil from our Thanksgiving turkey analogy; foil reflects radiant energy very well but does not radiate heat well. It does not emit heat to the cooler surfaces around it.
If you install a single-sided radiant barrier with the foil side facing up, the aluminum will (for a time) reflect the thermal energy radiated by the hot roof.
If you install a single-sided radiant barrier with the foil side facing down, the aluminum simply will not radiate the heat it gains from the roof to the cooler insulation it faces.
At first, a single-sided radiant barrier will work equally well with the foil facing up or down. But over time, dust may accumulate on the surface of foil facing up. The dust will reduce the radiant barrier effect by allowing the foil to absorb rather than reflect thermal radiation. However, a radiant barrier with the foil side facing down will not collect dust on the foil and will continue to stop radiant heat transfer from the hot roof to the insulation over the life of the installation.
Even if you use a double-sided radiant barrier material, it is best to install it at the rafter level so that the bottom side faces the attic airspace and will not collect dust.
How should I install a radiant barrier in my attic?
The most effective way to install a radiant barrier in an existing attic is simply to staple the foil material to the underside of the top chord of the roof trusses or to the underside of the roof decking.
It is not very easy to work in any attic, even one with a steep pitch. And always keep in mind that a misstep could be disastrous, since most attic "floors" are not floors at all, but rather 2x4s holding ceiling drywall topped by conventional insulation. You should consider safety first.
Take care to avoid compressing existing insulation in the attic.
Tools and materials needed to install a radiant barrier include the following:
Perhaps your most
important aid will be a partner. Working in pairs in the attic makes the work go
faster. Even more important, it adds to safety.
Begin by measuring the length of the attic roof from peak to soffit. Then, return to a stable, ground-or floor-level surface to measure and cut the radiant barrier material to size. The material usually comes in rolls of 50 to several hundred feet; it's easiest to cut and reroll all the lengths you'll need before returning to the attic.
At one end of the attic, place the plywood or 1x12 as a stable surface across two of the attic truss members. Try to minimize compression of existing insulation. Provide one surface at the peak and one at the soffit end so that both installers can work together.
Safety reminder: Be extremely careful at the sides of your support surface. If you step on an edge, the surface can tilt and drop you through the ceiling drywall below.
With your partner, unroll one length of the radiant barrier material from soffit to peak. Leaving one or two inches of free space at the roof peak, staple one corner of the material to the underside of the top chord of the first roof truss. Continue stapling the edge of the radiant barrier material down the truss at 6 to 12-inch intervals, stopping 2 to 3 inches from the ceiling insulation. Next, staple the other edge of the material to the underside of the adjacent roof truss. Continue the process at adjoining trusses until the underside of the roof is no longer visible except for a one- or two-inch strip at the roof peak.
As an alternative, you may staple the radiant barrier material to the underside of the roof decking, adjacent to the top chord of the truss. The weight of the material will allow it to drape naturally between trusses.
Safety tips for installing an attic radiant barrier system
Does the radiant barrier have to be airtight in order to work?
No; you're installing a barrier against radiated not convected heat, so you need not cut off air motion. In fact, ventilation from soffit to peak improves radiant barrier system performance.
Small tears and holes will not significantly lessen the performance of the radiant barrier, so don't worry if you must cut and patch around obstructions such as vent stacks and truss supports.
Can't I just roll the material out on top of the insulation?
It's not recommended to place the material directly on top of insulation. In this type of installation, dust will accumulate on the foil surface facing the roof. In time, the dust will negate the radiant barrier effect. In addition, problems could develop with moisture condensation.
Will foil-faced batt insulation work as a radiant barrier?
While some conventional batt-type insulations have an aluminum foil backing, it's probably not a good idea to simply flip your insulation over to use it as a radiant barrier. Not only will you encounter the eventual dust problem, you may also encounter a fire hazard in the glue that bonds the foil to the batt.
At least one batt insulation manufacturer has introduced a product with a foil face that is bonded to the insulation with a fire-retardant glue. This product meets fire codes, but it still has the potential for dust problems.
Will heat build up in the roof and damage my shingles?
It's extremely unlikely. The Florida Solar Energy Center has measured the temperatures of roof shingles above attic radiant barriers on hot, sunny summer days. Depending on the color of the shingles, their peak temperatures are only 2-50F higher than the temperature of shingles under the same conditions without a radiant barrier.
Roofing materials are manufactured to withstand the high temperatures to which they are frequently exposed. A 2-50F increase in peak temperatures that normally reach 160-1900F should have no adverse affect.
What about my shingle warranty?
Shingle warranties should not be subject to cancellation by the manufacturer on the basis of radiant barrier installation. However, it may be wise to review the warranty to be sure that work of this nature will not void it. You may want to inquire directly of the manufacturer. Any changes in warranty should be substantiated in writing.
Can I just roll the material out on the roof before reshingling?
This will not provide a radiant barrier. Remember, to be a radiant barrier, the aluminum foil must be installed facing an airspace. If there is no airspace, the foil acts as a conductor and quickly passes heat by conduction from a hot surface to a cooler one.
What's the easiest installation method in new construction?
There are two widely accepted methods of installing attic radiant barrier systems in new construction:
Note: When installing a single-sided radiant barrier material, remember to face the foil side down toward the attic floor/insulation.
What else can I do to decrease heat gain through my attic?
While a radiant barrier is one effective way of reducing heat gain through attics, it's not the only one. Other options include:
If you shop carefully, you will probably find that attic radiant barriers are one of the least costly and yet most effective of the attic conservation measures for Southern climates.
When will I see a payback from an attic radiant barrier?
Computer studies conducted in the development of the Florida Model Energy Code indicate that a typical attic radiant barrier installed in a Florida home will offer a six to seven year simple payback and a 15% to 19% return on investment.
Attic radiant barriers are an inexpensive but effective way for Sunbelt homeowners to save energy and money. While they are not a new concept, radiant barriers have only recently been proved effective for energy conservation.
Manufacturers are continuing to improve radiant barrier materials, which are becoming widely available throughout the southern states.
A radiant barrier may be installed in an existing attic or during construction of a new home. Both are relatively easy procedures.
Publication of this document was supported in part by Florida Power & Light Company.
Fairey, P., "Designing and Installing Radiant Barrier Systems," FSEC-DN-7, Florida Solar Energy Center, Cape Canaveral, FL, 1984.
Fairey, P., "Effects 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, Las Vegas, NV, December 1982.
Fairey, P., "The Measured Side-by-Side Performance of Attic Radiant Barrier Systems in Hot-Humid Climates," proceedings of the 19th International Thermal Conductivity Conference, Cookeville, TN, October 1985.
Fairey, P., "Radiant Energy Transfer and Radiant Barrier Systems in Buildings," FSEC-DN-6, Florida Solar Energy Center, Cape Canaveral, FL, 1984.
Joy, F.A., "Improving Attic Space Insulating Values," ASHAE Transactions, Vol.64, 1958.
Levins, W.P., and M.A. Karnitz, "Cooling-Energy Measurements of Unoccupied Single-Family Houses with
Attics Containing Radiant Barriers," Oak Ridge National Laboratory, Contract Report, DE-ACO5-840R21400, July 1986.
"Radiant Barriers: How They Work and How to Install Them," videotape, FSEC Producer, Cape Canaveral, FL, 1986.
Van Stratten, J.F., Thermal Performance of Buildings, New York: Elsevier Publishing, 1967.
Executive Summary: The FPC Monitoring project has evaluated radiant barrier systems (RBS) as a new potential DSM program. The objective was to examine how the retrofit of attic radiant barriers can be expected to alter FPC residential space conditioning loads. An RBS consists of a layer of aluminum foil fastened to roof decking or roof trusses to block radiant heat transfer between the hot roof surface and the attic below. The radiant barrier can significantly lower summer heat transfer to the attic insulation and to the cooling duct system. Both of these mechanisms have strong potential impacts on cooling energy use as illustrated in Figures E-1 and E-2.
project involved installation of RBS in nine homes which had been
extensively monitored over the preceding year. The houses varied in
conditioned floor area from 939 to 2,440 square feet; attic insulation
varied from R-9 to R-30. The homes had shingle roofs with varying
degrees of attic ventilation. The radiant barriers were installed during
the summer of 2000 (Figures E-3 and E-4). The first was installed on
June 22nd and the last was installed on September 16th. One home, the
last installation, was excluded from the analysis due to its late
application date which left too little summer data to perform a reliable
Data analysis on the pre and post cooling and heating consumption was used to determine impacts on energy use and peak demand for the utility. The pre and post periods were carefully matched so that time periods evaluated and outdoor air temperatures were nearly identical. Based on the performance at the individual sites, we performed an overall evaluation of the savings from the radiant barrier systems by preparing a composite load profile for the average of the eight sites.
The average cooling energy savings from the RBS retrofit was 3.6 kWh/day, or about 9%. The average reduction in summer afternoon peak demand was 420 watts (or about 16%). The aggregate load profile for all eight sites pre and post RBS installation is shown in Figure E-5. There was an average 8 degree drop in the average maximum daily summer attic temperature. Reductions on the hottest days were greater. Perhaps more importantly, the measured interior air temperature in the homes averaged over 2oF cooler post RBS retrofit, indicating the application had beneficial impacts to customer comfort.
For comparison, we analyzed the influence of higher levels of ceiling insulation from the summer data taken in the FPC monitoring project. We compared cooling energy use and demand profiles in a large sample of homes with R-19 or R-30 attic insulation. Customers with R-30 used 3.4 kWh/day less for cooling than did homes with R-19. However, the peak demand reduction due to the higher insulation was only 130 Watts (5%). Although the technologies produce roughly the same energy savings for the customer, the RBS resulted in a peak reduction three times greater than added insulation. We hypothesize this is partly due to reduced heat gains to the duct system in the attic and also because radiation potential between roof and attic is greatest during peak cooling demand periods.
Analysis of winter data collected in 2001 showed small benefits of the RBS to reducing space heat demand during the morning peak demand period. This occurs because the RBS do not allow the attic to become as cold during winter nights (lower radiative losses to the night sky).
Based on our findings, we recommend that radiant barrier systems be considered for homes with shingle roofs both in FPC's Home Energy Improvement program as well as emphasized in FPC's new homes program where installations would be most cost effective.
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