Impact of Perimeter Insulation on Heat Flow through Slabs in Residential Buildings

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The "F2 factors" method for calculating heat flow through slab-on-grade is currently required in the demonstration of residential energy code compliance in California. In this study we confirm that this method overestimates slab-on-grade heat gain and calculates significantly higher cooling loads than in reality. The "F2 factors" method of calculation (Standard Method) was simulated with an unmodified version of DOE-2.1E. The results were compared to those obtained with a more accurate method incorporated in a modified version of DOE-2.lE (called the Simplified Method because it is a simplification, using regression analysis, of a "detailed" two-dimensional finite-difference calculation of slab heat transfer).

The differences in the DOE-2 simulation results between the Standard and Simplified Methods can be clearly explained by the differences in modeling techniques between the two Methods. In cooling, the results from the more accurate Simplified Method are consistently lower than those from "F2 factors;" the differences are inversely proportional to the amount of slab perimeter insulation.

This report documents the differences in calculation between the two methods. It also reports and discusses the results of simulation for a one-story house with a relatively large footprint in all California Climate Zones, and those for a two-story house (with a much smaller footprint) in Fresno (Climate Zone 13). The subject houses used in all DOE-2 simulation are typical of residential construction in California and closely match test houses that are used by the California Energy Commission staff in their studies.

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