This article was originally published in the July/August 1992 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online July/August 1992
TRENDS IN ENERGY
Trends in Energy is a bulletin of residential energy conservation issues. It covers items ranging from the latest policy issues to the newest energy technologies. If you have items that would be of interest, please send them to: Trends Department, Home Energy, 2124 Kittredge St., No. 95, Berkeley, CA 94704.
Radiant Barrier UpdateWhen Home Energy last covered radiant barriers (May/June '89 and Nov/Dec '89), the level of confusion and outright misstatements associated with radiant barriers were at an all-time high. Some of the misinformation came from manufacturers who extrapolated limited test information to validate erroneous claims. The research by that point was too scanty to support any claims to speak of. Major funding by the radiant barrier industry for an independent study evaporated shortly afterward.
To this day, much less is known about the effectiveness of radiant barriers-the thin sheets or coatings of reflective materials that virtually stop transfer of infrared energy-than about any other type of insulation. But since 1989, the U.S. Department of Energy has sponsored more research into some of the nagging questions about dust accumulation and moisture, and has finally released the long- awaited Radiant Barrier Fact Sheet to help consumers and contractors understand the knowledge accumulated to date. Still, no simple ratings (such as R-values for conventional insulation) have yet been developed to aid the public in decision-making about radiant barriers.
Until recently, part of the problem with studying dust and moisture has been that most radiant barrier testing has been field testing. Now, controlled environments such as the Large Scale Climate Simulator at Oak Ridge National Laboratory (see Convective Loss in Loose-Fill Attic Insulation, HE, May/June '92, p. 27-30) are available and are being effectively used to analyze the complex nature of building systems and thermal envelopes.
When the radiant barrier is placed on the attic floor, dust does become a problem. As dust builds up, insulating value is reduced by as much as half, over a period of one to ten years. For reflective barriers attached to rafters-shiny side down-dust appears to be little problem. These findings are integrated into the Fact Sheet in the form of tables distinguishing attic floor and rafter installations. In cold climates, vapor can become a significant problem when radiant barriers are placed over conventional insulation, on an attic floor. In this case, the foil must be perforated to allow for ventilation. When a radiant barrier is attached to the rafters, it may increase the roofing temperatures by 2 degrees F to 10 degrees F; and it is not presently known if the lifetime of the roof is affected.
Through computer modeling and confirmation by field checks, researchers devised formulas for predicting possible energy savings for many locations in the country. Cost-effectiveness calculations, called present value savings, using these formulas were put into tables in the Radiant Barrier Fact Sheet. They take into account many variables, including local weather, energy costs, efficiencies of equipment, building configuration, and factors that affect differences in future and present values, such as inflation, fuel price increases, etc.
To exemplify how the tables work, three cities have been selected from the Fact Sheet (See Table 1). In order to use the tables, one must know the cost of installing the radiant barrier. If the figure given for present value savings for the whole ceiling is greater than the cost of the attic radiant barrier, then the radiant barrier will be cost-effective. The calculations are based on a 25-year life of material, normal inflation, and 7% depreciation. (Both installations assume that the radiant barrier has an emissivity of 0.5 or less when clean.)
According to the Reflective Insulation Manufacturers Assoc., the range of costs for contractor-installed radiant barriers in 1991 was 12-45Ã‚Â¢/ft2 (range is for new versus existing construction, rafter versus attic floor installations and local variations). If one assumes the cost to install radiant barrier is 20Ã‚Â¢/ft2, the only installations that would be cost-effective are:
Rafter installation Attic floor installation ______________________________________________________________________________ Bismark, N.D. R-11 insulation Only one possibility. with central air conditioning Miami, Fla. R-11 insulation R-19 insulation Three possibilities. with central air R-11 insulation conditioning with central air conditioning Topeka, Kans. R-11 insulation Only one possibility. ______________________________________________________________________________ These three examples, in three distinct climate locations, clearly show that radiant barriers are not especially cost-effective. In fact, in the three cities selected, only five of the 48 possible insulating systems were cost-effective. Obviously, very careful evaluation is required before deciding whether or not to install a radiant barrier. For contractors and retrofitters, though, the evaluation is made easier by the Fact Sheet.
Reference Radiant Barrier Fact Sheet, Department of Energy, June 1991, in cooperation with the Electric Power Research Institute, the National Assoc. of Home Builders' National Research Center, and the Reflective Insulation Manufacturers Assoc. (For more tables, formulas, and work sheets write: DOE, Office of Scientific and Technical Information, P. O. Box 62 Oak Ridge, TN 37830.)
Table 1. Present Value Savings of Radiant Barriers (cents/ft2) used with different levels of conventional attic floor insulation. Radiant Barrier Attached to Rafter Bottoms* R-11 R-19 R-30 R-38 _____________________________ Bismark, N.D. 18-20 9-10 5-6 4-5 Miami, Fla. 28-36 15-20 9-13 7-10 Topeka, Kan. 22-26 11-13 7-9 5-7 Dusty Radiant Barrier on Attic Floor* R-11 R-19 R-30 R-38 _____________________________ Bismark, N.D. 5-14 2-6 1-4 1-3 Miami, Fla. 6-23 3-12 2-7 1-6 Topeka, Kan. 5-17 2-9 2-5 1-4 ________________________________________________________________________________ * The first number represents potential savings for radiant barrier attic installation without central air-conditioning ducts in attic. The second number is for attics with central air-conditioning ducts in attic. With rafter installations, radiant barriers must also be installed at the gables.
-David W. Conover
David W. Conover is an architect concerned with affordable housing and energy conservation.
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