Modeling an air barrier system for a building is similar to calculating the conductive heat loss of a building enclosure. Heat loss is the sums of the assembly areas times their R-values, or conductive heat loss, plus the heat required to heat outdoor air coming into the building (ventilation plus air leakage). One-thousand square feet of roof, times the R-value of the roof assembly, times the temperature difference from inside to outside, equals the modeled heat loss of the roof. The same method is used for modeling the air leakage through the walls, floor, windows, etc. By identifying the leakage rate of each of the air barrier components times its area, the total air leakage of the building materials can be calculated. The leakage rates for each air barrier material are as tested and reported by the manufacturers in their product specifications. To this air flow total is added the leakage through the transitions between the air barrier components included in the building envelope design. Modeling programs like Contam can also be used to “model” the air-tightness for any set of building components and assemblies.
Modeling and testing typical air barrier component installations during construction allows us to guarantee that the components are properly installed and “connected.” Combined, modeling and QA procedures make it possible to specify a better-than-ASHRAE air-tightness performance standard in advance within a very narrow margin of error.