Risk of Condensation and Mold Growth in Wood-Frame Wall Systems with Different Exterior Insulations

The National Research Council of Canada (NRC) has undertaken a research project to investigate the risk of condensation and mold growth in 2x6 wood-frame wall assemblies associated with increasing the thermal resistance (R-value) of cavity insulation for various scenarios of exterior insulation products. The project was requested by the Task Group (TG) on Properties and Position of Materials in the Building Envelope, acting on behalf of the Standing Committee on Housing and Small Buildings (SCHSB). The work originated from a Code Change Request “CCR-802” in which it was suggested that the Water Vapor Permeance (WVP) limit be raised from 60 to 300 ng/(Pa•s•m²) while leaving the limit for air leakage of building envelope materials unchanged at 0.1 L/(s•m²) at a pressure difference of 75 Pa. In response to the CCR-802, NRC undertook a research study by conducting hygrothermal modelling using hygIRC-C model. The risk of condensation and mold growth in wall assemblies with wall cavity insulation of R-19 and R-24, and with and without exterior insulation of wide ranges of R-value (4, 5 and 6 ft2•h•oF/BTU) and WVP (2 – 1800 ng/(Pa•s•m²)). The parametric study did not include values of WVP specific to product brands; rather, it included the values for WVP of most products currently available. This research project has led to a code change in Section 9.25 of the 2015 National Building Code of Canada (NBCC).
This paper focused on wall assemblies with stud cavity insulation of R-19, and with and without exterior insulation of R-4 and a range of WVP of 2 – 300 ng/(Pa•s•m²). The results of the hygrothermal performance was expressed using the mold index criteria, which allowed sufficient resolution to assess the risk of moisture condensation and related risk of mold growth in the wall assemblies. Also, the respective mold index criteria were selected so that those cases where assemblies comply with information provided in Table 9.25.5.2. of the 2010 NBCC would fall into an acceptable performance. The results showed that adding exterior insulation of different WVP has resulted in lower risk of condensation and mold growth than the reference wall systems (i.e. without exterior insulation). The risk of condensation and mold growth in wall assemblies with cavity insulation of R-24, and with and without exterior insulation of wide ranges of R-value and WVP will be published at a later date.

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