|| 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
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.
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This page updated on October 10, 2006, by Teresa Williams.