The validity of predictive models for the thermal conductivity of foam insulation is established based on the fundamental geometry of the closed-cell foam. The extinction coefficient is experimentally and theoretically determined; the theoretical prediction based on measured geometrical properties differed from the measured values by an average of 6% for ten different foams An approximate method uses measured geometrical values to adjust the measured diffusion coefficients of reference foams. The adjusted coefficients are used as inputs to a computer program which computes the effective thermal conductivity of the foam as a function of time. Values of effective thermal conductivity measured on laboratory and field samples are used as a standard for comparing the results of the physical models and the ageing program. Measured and predicted values differ by 11%, 13%, 1%, 5%, and 1% for the initial thermal conductivity of five foams tested. These errors decrease with time. The ageing program is used to simulate the time-averaged performance as a function of foam density, mean cell diameter, and fractional distribution of solid polymer. The results of the simulation indicate that for a 15 year service life, the optimal density is approximately 3 lb / ft3.
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