Exterior insulation envelope retrofits in cold climates: Implications for moisture control

Abstract

Retrofitting walls with foam insulation is a common practice in residential construction to reduce heating demand; however, the implications of this practice for moisture control are less straightforward. Typically structures in cold climates have a polyethylene vapor retarder on the interior framing surface, therefore adding relatively water vapor impermeable exterior insulation greatly reduces the drying potential for the wall system. Furthermore, while condensation potential is reduced by the addition of exterior insulation, wood framing can be subject to a temperature and humidity regime more conducive to fungal growth relative to pre-retrofit conditions. To investigate the potential for exterior insulation retrofit strategies in subarctic climates to cause moisture accumulation in wood-framed structures, nine test wall sections were constructed using varying ratios of stud-fill and exterior insulation. The wall sections were tested in Fairbanks, Alaska, over two winters and were monitored for temperature, humidity, and wood moisture content. Test walls with less than two-thirds of the nominal wall R-value exterior to the framing performed poorly in terms of wood moisture content and relative humidity at the sheathing interior surface whether or not the test walls were equipped with vapor retarders. The findings are used to examine conventional moisture control frameworks.