Numerical Investigation of Mold Growth Risk in Vapor-Permeable Building Envelopes with Bio-Based Insulation in Cold Climates

Abstract

It is generally accepted that a vapor barrier is necessary to ensure the durability of building envelopes, especially in cold climates where the risk of winter condensation is high. However, the water vapor resistance of the typical vapor barrier —polyethylene foil— is considered by many to be overly high because it also reduces the drying capacity of the envelope. However, installing an excessively permeable vapor barrier could trigger condensation and mold growth. Organic insulation materials could allow reducing the vapor resistance of the vapor barrier in a safer way, as these are more hygroscopic than petroleum-based insulations. This study numerically investigated the risk of condensation and mold growth of three wall assemblies, from which two were insulated with bio-based materials and the other one was a reference standard wall. Hygrothermal simulations considered different levels of permeability of the vapor barrier and ventilation rates in the air gap behind the exterior cladding. Results indicated that walls with bio-insulation materials have the potential to function in a cold climate without triggering mold growth, especially when using a vapor retarder. Ventilation of the exterior air layer proved to be effective when in contact with organic materials, but practically useless otherwise. It was found that the polyethylene foil performed similarly to a smart vapor barrier in all assemblies, except for the highly hygroscopic wall.