Effect of air leakage on the hygrothermal performance of highly insulated wood frame walls: Comparison of air leakage modelling methods

Air leakage is one of the important moisture sources that may increase the risk of moisture problems in wood frame walls. Two simplified air leakage modeling methods, i.e. air convection method and air infiltration method, can be implemented in commonly used hygrothermal models. This paper investigates the applicability of these two methods in evaluating the hygrothermal performance of three types of wood frame walls: one baseline 2 × 6 wood frame with fiberglass insulation, two highly insulated walls, i.e. deep cavity wall with I-joist framing and cellulose fiber insulation and 2 × 6 farming with exterior polyisocyanurate insulation and fiberglass stud cavity insulation. Firstly, the hygrothermal models are calibrated by comparison to measurements. Then the best matched models are used for two years' simulation to evaluate the mold growth risk of these walls at various airtightness levels. The results show that both methods can model the effect of air leakage reasonably well by adjusting the position of air layer in the air convection method and the amount of the indoor air reaching the condensation plane in the air infiltration method. The air infiltration method tends to overestimate the moisture content of OSB sheathing, while the air convection method tends to underestimate the moisture content of OSB sheathing. The baseline wall has higher mold growth risk than the I-joist wall with cellulose fiber, while the I-joist wall's mold growth risk becomes slightly higher than the baseline wall when the insulation materials are changed to fiberglass. The exterior insulated wall has no risk of condensation and mold growth.