Agepan: A Vapor-Permeable, Wood-Based Insulation Board

Advantages over polystyrene

High-performing wall assemblies are increasingly incorporating continuous insulation outside of the structural assembly to reduce thermal bridging and provide continuous air and water barriers. Wrapping the exterior in rigid polystyrene foam is common, but polystyrene poses problems of its own—from use of a non-renewable resource to toxic flame retardant content to global warming potential. The relatively low vapor-permeability of polystyrene (along with many structural sheathing products) can be a benefit, though, such as in installations where sunshine can drive water vapor from masonry cladding inward through an exterior assembly.

However, in most applications using polystyrene, its low vapor-permeability is not necessarily beneficial. As long as such an assembly allows drying to the interior, it’s fine, and prominent building scientists have lauded this overall approach—but some designers and builders argue that we should open the exterior to vapor movement to maximize drying potential, especially in colder climates where interior water vapor can condense inside a wall and cause rot.