A computational and experimental study of conjugate heat transfer through composite thermal envelopes in post-frame buildings

The thermal performance of eight post-frame thermal envelopes was studied and optimized using a computational fluid dynamics model validated experimentally with a rotatable guarded hot box. In addition to providing thermal performance values for typical wall designs, this study proposed a new wall design that greatly increased thermal performance without sacrificing material efficiency. Study variables included structural geometry, level of insulation, and the presence and placement of radiant barriers. To reduce computational demand, modeling was primarily conducted using an area-weighted average of two-dimensional slices to represent three-dimensional assemblies. After modeling a portion of the assembly in three dimensions and comparing it with its two-dimensional counterpart, this simplification was found to result in less than a 6.7% error. Significant error (up to 57%); however, was determined to be integral to the simplifying assumptions commonly used by building designers, especially in envelopes common to the agricultural industry. This error was estimated to underpredict energy costs for a 2200 m2 cold-storage warehouse in Wisconsin by approximately $1700 a year.