Go with the Flows: The Promise and Peril of Hygrothermal Modeling

Insulation Can Damage Masonry

Quick: what’s the worst design mistake you’ve ever made?

Chances are it had something to do with heat and moisture transfer: cavity insulation that molded because it couldn’t dry; a roof damaged by moisture-laden air leakage; a cold steel stud on which water condensed, wicking into surrounding materials.

Heat and moisture dynamically interact in every nook and cranny of our buildings, every minute of every day. Hygrothermal movement (hygro for moisture, thermal for heat, put together because they move together) is driven by the laws of physics and is fairly predictable in uninsulated building assemblies. But as we air-seal, flash, vapor-retard and super-insulate—making it very difficult, but never impossible, for heat and moisture to flow through the building envelope—we lose that predictability and can sometimes actually increase the risk of a failure (see “The Hidden Science of High-Performance Building Assemblies”).

Enter hygrothermal modeling—simulation software meant to help project teams assess and avoid such risks when designing complex building assemblies.

With code requirements and customer demand pushing envelope performance to its utmost, more project teams have been seeking out this type of modeling. But detractors say the exercise is worthless, possibly outright dangerous. And even advocates warn that proper hygrothermal modeling requires highly specialized expertise—not only software skills but also a solid building-science background rooted in experience of actual buildings and how they can fail.