DA07 Criteria for Moisture Control Design Analysis in Buildings

This guide formulates design assumptions for moisture design analysis and criteria for acceptable performance. Ideally, a design analysis involves the determination of the probability of failure and treats all design parameters and loads as stochastic variables. However, sufficient data is often not available to make a full statistical treatment practical. Instead, where only limited data exist, a moisture design protocol must be based on a combination of statistical data and...
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This guide formulates design assumptions for moisture design analysis and criteria for acceptable performance. Ideally, a design analysis involves the determination of the probability of failure and treats all design parameters and loads as stochastic variables. However, sufficient data is often not available to make a full statistical treatment practical. Instead, where only limited data exist, a moisture design protocol must be based on a combination of statistical data and professional judgment. Another judgment involves the choice of an acceptable probability of the occurrence of damage. Although it is common to impose very stringent criteria for structural design because of safety concerns, moisture damage usually occurs over a long period of time and usually has less catastrophic, although sometimes costly, consequences. An international consensus has emerged that the analysis should be predicated on loads that will not be exceeded 90% of the time. This guide adopts this approach. In a moisture analysis for building envelope design, the choice of indoor environmental conditions is extremely important, especially for buildings in cold climates. This guide opts for a design indoor climate definition that is based on engineering principles, is independent of construction, and reflects the influence of ventilation and air-conditioning equipment and controls that may or may not be part of the building design. In buildings where indoor humidity and temperature are explicitly controlled, the building envelope performance should be evaluated with the intended indoor design conditions. In residential buildings, indoor humidity is rarely explicitly controlled, so default design assumptions are needed for these buildings. In general, the methodology outlined in this guide encourages designers to use their own design parameter values if they are known and part of the design. If they are unknown or not included in the design, the methodology outlined in this guide provides default values for those loads and parameters.