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Radiant Barrier Update

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 12-45¢/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 20¢/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.         
______________________________________________________________________________

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.)

         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.