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ice and water membrane on a low slope unvented roof

I'm currently building my own house with an unvented cathedral roof in climate zone 4A. So far, I have 2 taped layers of polyiso (total of R-15) sandwiched between 2 layers of OSB above 2X12 rafters which will be insulated with rock wool. Most of the roof is a 12:12 gable but there is a small (8'x16') shed roof with a 2:12 pitch which abuts one of the gable ends.

I'm going to use asphalt shingles and the installation instructions insist on ice and water membrane for 2:12 roofs but I'm concerned about trapping moisture in the upper layer of OSB between the polyiso and the membrane. Any suggestions of how best to proceed?

Asked by Eric Matsuzawa
Posted Thu, 01/30/2014 - 11:12

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