In the past, some practitioners and designers have resisted the use of low permeability exterior insulation as part of a high R-value wall system because of the concern that it will reduce drying and result in moisture accumulation within the wall system. An alternative approach to achieving high R-values is to use thick (20–50 cm [8–20 in.]) cavity spaces filled with insulation. A field study was conducted of four full-scale high-R (approximately R-30 effective) wall assemblies and one standard wall (for comparison), on both the north and south orientation, using both exterior insulation construction and thick insulation-filled wall systems. The study was performed in Waterloo, Ontario, which is on the border of IECC Climate Zones 5 and 6. The measured moisture performance data from the full-scale test walls along with interior and exterior boundary conditions was used to validate a hygrothermal model for the testing location. The model was then used to predict performance in Climate Zones 4 through 8. Both the field measurements and hygrothermal modeling demonstrated that exterior-insulated walls were much more tolerant to cold weather moisture accumulation as a result of interior air leakage into the assembly than thick cavity-insulated walls. There was also correlation between measurements and hygrothermal modeling on the slow drying of the wood sheathing following a rain leak with low permeance exterior insulation and interior vapor barrier. However, when the interior vapor barrier layer was removed, the assembly dried much more quickly. Both the field measurements and hygrothermal analysis from this study showed that the least risky wall with respect to moisture-related durability issues was an exterior-insulated wall with sufficient exterior insulation to allow a Class II or Class III vapor control layer to be used on the interior.
Note: This article was published in Proceedings of Buildings XIII, 2016. Copyright 2016 ASHRAE. Reprinted by permission at rdh.com. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about the Buildings XIII Conference Proceedings, visit http://web.ornl.gov/sci/buildings/2016/proceedings/.