Go to Research Programs  Radiative Heat Transfer in Attic Insulation
Radiative heat transfer is a very significant fraction of the total heat transfer through typical fibrous attic insulations, amounting to about 40 percent of the total heat flow for low-density fiberglass batt insulation and about 60 percent of the heat flow for low-density loose-fill fiberglass insulation (in the absence of convection heat transfer).

Thermal insulations are rated by thermal resistance as measured in a heat flow meter apparatus (HFMA) or a guarded hot plate.  In either apparatus, the insulation is sandwiched between two flat, isothermal, impermeable metal plates that are held at different temperatures.  Measurements of the temperature difference between the plates and the heat flux through the plates gives enough information to determine the thermal resistance (R-value) of the insulation.  However, previous research has shown that this approach does not always provide a good indicator of the in-service performance of some attic insulations under cold winter conditions where natural convection may be significant.  Likewise, this approach may not adequately simulate the radiative conditions in attics under peak summer conditions, where the roof may be significantly hotter than the attic air and also hotter than the top surface of the insulation.

Assess the effect of radiative conditions on the thermal performance of various types of attic insulations and develop means to account for the effect

A combined experimental and modeling approach is being used to evaluate the effects of thermal radiation conditions on the thermal performance of attic insulation.  Initial experiments are being conducted using a large heat flow meter apparatus (HFMA) modified to include an air gap between the insulation and the top plate.  The top plate is held at a high temperature to simulate a hot roof.  Since tests in the HFMA are relatively inexpensive, tests can be run on many types of insulation.

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Pressure plate apparatus for measuring sorption in capillary range   Full-scale testing in the LSCS may be performed if necessary

A parallel effort is aimed at acquiring mathematical models to assist in interpretation of data and extrapolation to other conditions.  The models need to be adapted to represent the true attic environment, and should account for scattering, absorption, and emission of radiation through the insulation, coupling of radiative transport with heat transfer by conduction within the insulation, and convective heat transfer from the insulation surface to the attic air.  A means to estimate scattering and absorption coefficients, either theoretically or experimentally, also needs to be developed.
Depending upon the outcome of the heat flow meter apparatus tests and initial modeling efforts, large scale tests may be performed in an attic test module in the Large Scale Climate Simulator.  If effects are significant, then efforts would be made within the American Society for Testing and Materials to develop standardized methods to account for them.

FOR  MORE  INFORMATION  CONTACT:   Dr. Achilles Karagiozis
E-mail Dr. Achilles Karagiozis
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This page updated on October 10, 2006, by Teresa Williams.