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In numerical terms, the U-factor is the reciprocal value of the
total resistance (1/ΣR). The U-factor is only an indication of the
conduction rate of heat transfer; it is a quantitative measurement
of how well heat conducts through a roof or wall for a given
temperature difference.
For instance, if the total R-values (ΣR) of a roof's several
components add up to 1.892 (m2 K)/W in the metric system or 10.75 (h
ft2 °F)/Btu in the U.S. terminology, then the average thermal
transmittance (U-factor) is 0.529 in the metric expression and 0.093
in the U.S. language.
Although ASTEC has an excellent k-value, this does not translate
into a good R- value only because applied ASTEC has very little
thickness. Therefore, ASTEC does not have a direct impact on the
U-factor per se. However, ASTEC significantly enhances the U-factor
value in that it (a) prevents the absorption of 85% of the solar
radiation and (b) reduces heat transfer by as much as 70% by
lowering the temperature difference across the roof or wall which,
in turn, makes the total resistance much more effective.
