Moisture monitoring of wood-frame walls with and without exterior insulation in a Midwestern U.S. cold climate

Syracuse, NY

Continuous exterior insulation is becoming more common in North American above-grade walls in both retrofit applications and new construction, as a means to improve the thermal performance of wall assemblies. Although moisture performance of wood-frame wall assemblies has been studied extensively, the drying capability of wall assemblies with exterior insulation and an interior vapor retarder in cold climates is not well characterized. This study monitored the hygrothermal performance of wall assemblies with and without exterior insulation under high and low interior humidity conditions and with intentional wetting of the wood structural panel sheathing. Moisture content and temperature of standard 38 mm × 140 mm wood framing and 11 mm thick oriented strand board (OSB) sheathing were measured over a two-year period in eight different wall assemblies, each with north or south orientation, in a conditioned test structure in Madison, Wisconsin. Wall configurations differed primarily in the interior vapor retarder (kraft paper or polyethylene film) and the exterior insulation (none, expanded polystyrene, extruded polystyrene, or mineral wool). OSB sheathing was wetted in a controlled manner at three different times of year to investigate drying response. Wintertime moisture accumulation in OSB in the tested climate zone was not a concern except in the wall with no exterior insulation and interior kraft vapor retarder, though rapid drying occurred in springtime. Drying of OSB after controlled wetting events was generally faster during warm weather than cold weather; faster with exterior insulation than without during cold weather; faster with vapor-open exterior insulation than low-permeance exterior insulation during cold weather; and faster with interior kraft vapor retarder than polyethylene.

https://surface.syr.edu/ibpc/2018/BE4/4