Critical Plane Analysis of Wall Assembly in a Hot, Humid Climate

Abstract

Condensation plane analysis for determining critical planes at which condensation may occur can be performed for building assemblies in any climate. Procedures for doing so in heating climates where buildings dry to the outside of envelope assemblies are given in 1997 ASHRAE Fundamentals Handbook, Chapter 22 "Thermal and Moisture Control in Insulated Assemblies - Fundamentals." Little original work is available elsewhere in the literature to guide analysis for buildings in hot and humid climates. Example 1 in Chapter 22 of the Fundamentals Handbook gives step-by-step calculations, for a heating climate. To analyze envelope assemblies in hot and humid climates where drying predominately occurs to the indoors, no direct discussion or examples are available. This paper presents this detail for a typical light commercial wall assembly, and provides the basis for analysis of any envelope assembly in hot and humid climates. Analysis of an envelope assembly in hot and humid climates seeks to determine if there is a critical plane in the wall towards which water vapor flows more rapidly from the outdoors than it flows to the indoors. (In heating climates, the analysis is reversed). In order to do this, weather data must be examined to yield outdoor conditions, and indoor conditions must be identified. Water vapor and thermal resistance of the materials in the wall assembly must also be established. These data are then used to perform calculations using the basic diffusion equation and methods described in the Fundamentals Handbook.' Each potentially critical plane is analyzed to determine if water vapor can accumulate more rapidly than it dissipates. This potential accumulation would signify a heightened risk of equilibrium relative humidity sufficient to amplify microbial growth, or to promote the deterioration of building materials.