This paper outlines a methodology to obtain design values for indoor boundary conditions for moisture design calculations for residences. This is part of a larger effort by ASHRAE Standard Project Committee 160P, Design Criteria for Moisture Control in Buildings, to formulate criteria for moisture design loads, analysis techniques, and material and building performance. The standard is being developed to provide a consistent framework for moisture analysis and design. The assumptions for boundary conditions can have a large influence on the results of the moisture design analysis of a building, and the choice of boundary conditions may be the most important determinant for design recommendations based on the analysis. This paper focuses on interior moisture design loads for residences and proposes a procedure to estimate the design indoor humidity for both winter and summer conditions. The interior humidity is a function of moisture release, ventilation, dehumidification, and moisture storage in the materials in the building. If the home is not air conditioned or dehumidified, the weekly or monthly average design indoor humidity can be calculated from design ventilation and moisture release. In an air-conditioned home, the situation is more complex. It is difficult to quantify the dehumidification of an air-conditioning system typically controlled by indoor temperature rather than moisture because the cycling frequency of the air-conditioning equipment affects its ability to remove moisture. Although the specific data required for sophisticated calculations that account for changes in moisture removal due to system cycling are not available, this paper will discuss the issues involved and describe simplified alternative approaches.

Authors: Anton TenWolde; Iain S. Walker, Ph. D.
Citation: Thermal Performance of the Exterior Envelopes of Buildings VIII
Keywords: December, Florida, 2001