HYGROTHERMAL ANALYSES
UNDERSTANDING HEAT AND MOISTURE TRANSPORT
The movement of heat energy and moisture through materials and enclosure assemblies can under particular conditions result in performance concerns such as moisture-related issues (e.g., condensation, water damaged materials, reduced R-values, corrosion, freeze-thaw damage) and occupant discomfort. Hygrothermal analyses use the principles of psychrometrics and physics to evaluate the anticipated heat and moisture transport through building enclosure materials. The experience and knowledge gained from hygrothermal analyses enables FCG to provide expert interpretation of the modeled response of individual assembly components to heat and moisture. Our understanding of the model results can be used to identify potential performance risks and guide detailing and materials specifications for the building enclosure.
Steady State Analysis
While antiquated and does not account for time-dependent changes, FCG sometimes uses manual calculations of steady state one-dimensional hygrothermal analyses to highlight the physics and principals of psychrometrics involved in an exterior building enclosure assembly. Since manual calculations are steady state (i.e., do not account for time-dependent changes in ambient conditions), they usually represent worst-case design conditions.
Transient Analysis
FCG performs transient state hygrothermal analyses using WUFI Pro (WUFI). WUFI is a modeling tool that can simulate the transient thermal and moisture distributions through a multi-layered system, such as an exterior wall or roofing assembly, in terms of one-dimensional simultaneous heat and moisture transport. The software can be used to model trends in the moisture content and wetting and drying cycles of each component in the enclosure assembly over a period of multiple years using historic climatic conditions for the area of interest.
WUFI is a powerful tool for the relative comparison of various enclosure assemblies by analyzing moisture and heat transfer through the assemblies over time. While, WUFI only analyzes a one-dimensional “cut” through an assembly without any consideration for thermal bridging; it can be adapted to account for three-dimensional phenomenon, like air leakage, to simulate the more significant effects of moisture transfer via convection. While WUFI Pro does not inherently consider the effects of air movement, FCG has supplemented WUFI’s simplified approach to consider its effects in all climates. Using the data output from the WUFI Pro program
The experience and knowledge gained from thousands of hygrothermal analyses enables FCG to provide expert and practical interpretation of modeling that can guide detailing and materials specifications for the building enclosure. Using the data output from the WUFI program, FCG interprets and compares the results with the acceptance criteria published in ASHRAE 160 as well as commonly accepted maximum moisture contents of the materials being analyzed.
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