Field moisture performance of wood-framed walls with exterior insulation in a cold climate
2019
Boardman, Charles R. | Glass, Samuel V. | Munson, Robert (Robert A.) | Yeh, Borjen | Chow, Kingston
Continuous exterior insulation is becoming more common in North America in above-grade exterior walls in both retrofit applications and new construction. It is used to improve the overall thermal performance of wall assemblies. The drying capability of wall assemblies with exterior insulation and an interior vapor retarder in cold climates is not well characterized. The moisture performance of wood-framed wall assemblies with and without exterior insulation was monitored during a 2-year period in the cold climate of Madison, Wisconsin, USA, under low and high interior humidity conditions and with intentional wetting of the wood structural panel sheathing. Moisture content and temperature of standard 38- by 140-mm wood framing and 11-mm-thick oriented strandboard (OSB) sheathing were measured in eight different wall assemblies, each with north and south orientation, in a conditioned test structure. Either a kraft paper or a polyethylene vapor retarder was used on the interior in combination with fiberglass cavity insulation. Exterior insulation was mineral wool, expanded polystyrene, or extruded polystyrene. The OSB sheathing was wetted in a controlled manner at three different times of year to investigate drying response. Wintertime moisture accumulation in OSB under the tested conditions was not a concern except in the wall with no exterior insulation and interior kraft vapor retarder, although rapid drying occurred in springtime. Exterior insulation had a predictable effect on wall cavity temperature. All 16 test walls were able to dry out quickly enough to keep moisture content below dangerous levels when challenged by modest water injection onto the interior OSB surface. The observed decrease in OSB moisture content after controlled wetting events was generally more rapid during warm weather than cold weather, more rapid with exterior insulation than without during cold weather, more rapid with vapor-open exterior insulation than vapor-tight exterior insulation during cold weather, and more rapid with interior kraft vapor retarder than with polyethylene.
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