Inward Vapor Drives in Adhered Veneer Wall Assemblies with Continuous Exterior Insulation

Typical adhered veneer applications apply thin masonry units over a bed of lath-reinforced mortar over a sheathing membrane layer (often a single layer of building paper, felt, or housewrap). When used over wood- or steel-framed walls, numerous moisture problems and failures have been reported, in part due to inward vapor drives. One solution proven during field testing of full-scale wall specimens is the use of a vapor-impermeable membrane and air gap behind the adhered veneer (Straube et al. 2009). This paper describes an alternate solution using continuous exterior foam plastic insulation. For adhered veneer wall assemblies, the addition of continuous extruded polystyrene (XPS) on the exterior of the framing may prevent inward vapor drives, while meeting current and future codes, without the additional water management detailing of an air gap membrane. A set of full-scale wall assemblies were tested over a period of three years using a natural exposure facility in Midland, Michigan (International Energy Conservation Code climate zone 5). Results showed that the walls with continuous exterior insulation had fewer signs of moisture durability risk resulting from inward vapor drives compared to walls with direct applied veneer and (1) building paper (2) lower permeance sheathing membranes.

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