UPDATED on July 8, 2015
Now that insulation contractors have been installing spray foam insulation on the underside of roof sheathing for several years, we’re beginning to accumulate anecdotes and data on successful installations and failed installations. The anecdotes and data are enough to provide a few rules of thumb for designers and builders who want to install spray foam on the underside of roof sheathing.
Increasingly, building scientists are investigating why OSB roof sheathing on many spray-foam-insulated roofs stays damp for months at a time. Most of these damp-sheathing problems involve open-cell foam rather than closed-cell foam.
I’ve been reporting on wet-sheathing problems arising from the use of open-cell spray foam since 2005, when I wrote two articles on the topic for Energy Design Update (“Vapor Retarders and Icynene,” April 2005, and “Every Failure Holds a Lesson,” July 2005). As originally understood, the problem with open-cell foam was that it is vapor-permeable, and therefore allows moisture in the interior air to diffuse through the insulation and reach the cold roof sheathing during the winter.
Five years later, Mark Parlee, an Iowa builder, wrote a seminal article on an Icynene-insulated roof with rotten roof sheathing. His article, “Repairing a Rotting Roof,” was published in the June 2010 issue of the Journal of Light Construction. One of the factors that contributed to the failure described by Parlee was high indoor humidity.
At a recent building science conference in Florida (Conference on Thermal Performance of the Exterior Envelopes of Whole Buildings XII, December 1-4, 2013), two academic papers were presented that shed light on questions surrounding the moisture content of roof sheathing that has been insulated on the underside with spray polyurethane foam.
One paper discussed a field study that found that even in a relatively warm climate (South Carolina), roof sheathing can accumulate moisture when open-cell…