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Community and Q&A

Unvented Roof and Ice and Water Membrane

spoons22 | Posted in General Questions on

I built unvented roof assemblies for the cathedral ceilings in my home and now the roofer I hired wants to put membrane on my entire roof (Owens Corning WeatherLock Tile and Metal Underlayment). Huber has a technical tip regarding the installation of ice and water membranes over their Zip sheathing in bold writing at the bottom that states: “IF AN UNVENTED ATTIC ASSEMBLY IS DESIRED IN ADDITION TO COVERING THE ENTIRE ZIP SYSTEM SHEATHING WITH AN ICE AND WATER MEMBRANE, HUBER ENGINEERED WOODS HIGHLY RECOMMENDS CONSULTING A BUILDING SCIENCE OR INSULATION SPECIALIST PRIOR TO INSTALLATION.”

What would my concerns be with this installation? Should I avoid it? Seeing as the membrane is a water and vapor barrier I would guess that trapping vapor under the assembly could potentially be harmful since the only drying path for the assembly is out through the sheathing.

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  1. Jon_R | | #1

    Sounds like they understand that the .6 perms or so of outward drying you can get through tar paper and shingles/metal roof assemblies can perform significantly better than the 0.0 perms you get through ice and water membrane.

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