The building envelope is arguably the single most important determinant of building durability, energy efficiency, and occupant comfort and health. We deliver these qualities to our buildings by carefully managing heat, air, and moisture through envelope assemblies. Put simply, we want to limit the movement of air and heat through our assemblies, limit the intrusion of moisture, and promote drying of any moisture that does intrude. The interplay between heat, air, and moisture is extremely dynamic, however, and a full understanding of that interplay in any given assembly can require extensive analysis. Nonetheless, a couple core principles can guide much of our work:
1. Start by making it airtight. If you control for air, you also make major strides in controlling for heat and moisture. This is because much of the movement of heat and moisture into and through building assemblies is carried by air. When we stop air movement through our assemblies we also stop this air-borne problem.
2. Avoid condensation where it hurts. Condensation forms where relative humidity – a function of moisture concentration and air temperature – hits 100%, aka the “dew point.” Relative humidity can reach the dew point when (1) moisture concentration increases in a given volume of air to the point where it surpasses the air’s capacity to hold that moisture in suspension, or (2) when air temperature decreases, reducing the air’s capacity to hold moisture in suspension. Major problems, including building failure, arise when the dew point occurs inside building envelope assemblies where moisture can accumulate. The most likely place for this to occur is on the surface of a building component in an assembly. Warm concentrations of moisture, carried by air or moved via vapor drive (the diffusion of moisture from areas of higher concentration to areas of lower concentration), hit a cold surface and the moisture drops out of suspension and condenses on the surface. Therefore, our assemblies must be designed and built to prevent concentrations of moisture from hitting surfaces that are cold enough to cause this condensation inside our assemblies. Furthermore, when working with materials that are subject to mold growth we need to include an additional safety factor in assembly design, as mold grows at relative humidity levels even lower than 100%.