(OK, it is really, but my plan is to make it sound like it’s not by giving you just what you need to know.) Let’s dive in. If you’ve read Holladay”s article and looked at the 77 comments, you may end up hopelessly confused. As promised in the title of this section, I’m going to condense this issue, so to speak, down to its essence. In the second paper, A Hygrothermal Risk Analysis Applied to Residential Unvented Attics, the authors used MATLAB and WUFI.Īnd that’s as far down that path as I’m going. In the first paper, Roof and Attic Design Guidelines for New and Retrofit Construction of Homes in Hot and Cold Climates, the authors used HERS BESTEST and AtticSim. The two papers were based on computer simulations. In fact, the way Holladay reported the remarks of William Miller, the author who presented one of the papers, it sounds like he has no doubt: “The roof sheathing is humid when open-cell spray foam is used,” is how Holladay quoted him. Open-cell spray foam has become popular in warmer climates, and this is where the two papers that Holladay reported on could cast the most doubt. But open-cell spray foam with a vapor retarder can work, too. Many builders in cold climates use closed-cell spray foam instead because of its lower water vapor permeability, which means it doesn’t need the extra step of installing a vapor retarder. As a result, in IECC climate zones 5 and higher, building codes require the use of a vapor retarder if you install open-cell spray foam. Moisture from indoors can diffuse through the foam and find the cold roof sheathing, where it accumulates and eventually rots the roof. We already know open-cell spray foam is risky in cold climates. Both papers basically came to the same conclusion: “Open-cell spray foam insulation is risky in all climate zones.” In it, he reported on two papers presented at the Conference on Thermal Performance of the Exterior Envelopes of Whole Buildings XII last December. Martin Holladay, the Energy Nerd who muses at Green Building Advisor, stirred up a hornets’ nest at the beginning of this year by writing an article titled Open-Cell Spray Foam and Damp Roof Sheathing. Open-cell spray foam is getting a bad reputation among some people in the construction industry. Not long ago, someone even told me that in Florida, roofing companies won’t let their workers go up on roofs with open-cell spray foam because the roofs are so spongy, the guys fall right through. Murmurs and hearsay about open-cell spray foam insulation have been gaining traction for a while.
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