Today’s conventional construction paradigm unintentionally, yet significantly, increases the risk of mold growth within wood framed wall cavities. The increase of risk is not related to the stud framing but derives from the nearly universal use of OSB or plywood shear membranes on the outside and gypsum board (sheetrock) on the inside.
In many climate zones, it is natural for the level of water vapor (humidity) to rise and fall in a wall cavity. Even without a direct leak of liquid water, water vapor moves through walls during different seasons and climate conditions. The key is that the cavity needs to dry out. When it doesn’t dry out it can support enough moisture for mold to grow (usually on the cavity side of the gypsum). The challenge for conventional building is that the outside OSB/plywood surface of a wall has very low water vapor permeability (vapor perm rating ~ 1-2), while the inside gypsum surface has extremely high vapor permeability (vapor perm rating ~ 40+). This means that if it is very difficult for a wall cavity to dry out by passing vapor through the OSB/plywood surface. The retained vapor, in conjunction with the cellulose food source from the paper wrapping the gypsum, then allows mold to grow on the back or cavity side of the gypsum.
BamCore’s dual-panel Prime Wall panels enjoy a vapor perm rating of approximately 7.9. This means that water vapor can pass through either inside or outside panel far more easily than it can pass through the OSB/plywood. Moreover, since the vapor perm rating is identical for both sides of the wall, the likelihood of water vapor getting trapped in any season is significantly lowered.
Hygrothermal analysis reveals the risk or likelihood of mold growth by analyzing the interaction of thermal, wind and humidity factors within a specific location and climate. The resultant risk of mold is reflected in the 0 to 6 scale of the Mold Growth Index (MGI). The general interpretation is the mold risk is low when the MGI is below 2. A leading building performance consultant, Passiv Science, completed a 12-city MGI simulation. Across the 12 cities, the MGI for the Prime Wall never exceeded 2, suggesting in all situations a low risk of mold. The conventional wall, in contrast, had an MGI exceeding 2 in six cities. These were all in the colder climates where BamCore’s reduced mold risk and superior thermal performance can combine to provide a safe and economical building envelope. Across six cities with an MGI greater than 2, the BamCore Prime Wall reduced the MGI or mold risk by an average of 78%.