Energy $aver$ of America

* Helping to $ave the Earth's Resources and Protecting the Environment *^tm

$aving Energy - $ave$ Money

It's the Really Smart Thing to do! ^tm

       

Reprinted by permission of the American Society of Heating, Refrigerating and Air-Conditioning

Engineers, Inc., from Proceedings of the ASHRAE/DOE Conference, Thermal Performance of the

Exterior Envelopes of Buildings (Atlanta: ASHRAE, 1983)

Proceedings of the ASHRAE/DOE Conference

Thermal Performance of the Exterior

Envelopes of Buildings II

December 6-9,1982

Dunes Hotel

Las Vegas, Nevada

In addition, it appears that foil products behave differently than the traditional insulation Products to which we assign fixed resistance values. They "reflect" rather than "resist" heat flow and may produce significantly different R-values depending upon the remaining components of the composite section and the direction of heat flow. Effective resistances presented in this paper are not universal and are pertinent only to the composite sections that have been examined.

Another reason these results appear surprising is because of the relative manner in which they are presented. We do not "expect" one layer of foil to out-perform a 6in. (152.4m) Batt. However, in real life situations attics do not behave in the same manner as guarded hot boxes. The radiation absorption characteristics of fiberglass are NOT incorporated in resistances calculated from guarded hotbox or guarded hot-plate data.

In a composite roof section, the introduction of an air-bounded radiation barrier changes the entire process of heat transfer rather than simply adding a constant resistance value. Of particular interest is the inability to derive a consistent effective resistance for the foil from the data set. Two sets of resistances are calculated with the foil inside the composite being evaluated.

One set uses roof surface temperatures and one set uses the underside temperature of the roof decking to calculate the DELTA T term. By logic, one would expect the difference in resistance between the foiled and non-foiled sides of the attic to be constant. This would allow a simple subtractive technique to define a specific additive value for the effective resistance of the foil. (We were relatively successful with this approach in walls.) This was not the case in roofs.

It appears that the radiant barrier does something to the remainder of the composite resistance (in this case it appears to double it) rather than provide a simple additive resistance.

If further investigation substantiates this tendency, it will make the definition of radiant barrier thermal performance parameters a very difficult task.