Rigid board insulation on the exterior of sheathing
I have read and continue to read the articles and blogs posted here about how and what to place on the exterior of the sheathing. As soon as I think I have the details worked out, another article points a different direction. My remodel project is a home framed with 2X4 walls and CDX plywood sheathing. Before I replace the windows, I must determine how i will insulate. Living in the Seattle, WA area brings in the issue of high humidity. I spoke with a CEO of a company that builds energy efficient homes in Portland, OR area (his house was featured in Fine Homebuilding) and he recommend 1 1/2 inches of poyiso on the exterior of the sheathing followed by housewrap on top of that followed with the furring strips and siding.
My question is to place the housewrap directly on the sheathing or outside of the rigid board insulation? Some say no housewrap as it is not impermeable to vapor. Others say use EPS or XPS, not polyiso.
Will there be issues in my climate for mold adjacent to the sheathing with insulation on the outside?
Thanks for the input.
Steve
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Replies
Steve,
Q. "My question is, to place the housewrap directly on the sheathing or outside of the rigid board insulation?"
A. Here are two articles to help you answer that question: Where Does the Housewrap Go? and How to Install Rigid Foam Sheathing.
Q. "Some say no housewrap as it is not impermeable to vapor."
A. If you are installing rigid foam on the exterior of your wall, the wall should be designed to dry inward, not outward. The vapor permeance of the housewrap is irrelevant, because your rigid foam is close to vapor-impermeable.
Q. "Others say use EPS or XPS, not polyiso."
A. Any of these three types of foam will work, but polyiso is the most environmentally friendly. Those who hope that EPS will allow a measure of outward drying are delusional. For more information, see Avoiding the Global Warming Impact of Insulation and Calculating the Global Warming Impact of Insulation.
Q. "Will there be issues in my climate for mold adjacent to the sheathing with insulation on the outside?"
A. No -- not if you do a good job with your flashing and water-manangement details, and as long as your follow the guidelines in the articles I've referred you to.