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Does moisture get trapped in the roof insulation?

cbedgar | Posted in Energy Efficiency and Durability on

I read the building science insight “Leiningen Versus The Ants Redux” by Joseph W. Lstiburek with great interest. I plan to build a workshop in upstate NY (zone 5+) using a similar approach. This seems very similar to the PERSIST method discussed in Martin Holladay’s article “Getting Insulation Out of Your Walls and Ceilings”.

But the details are tricky, as Joe learned with his barn. In Figure 1, Joe shows fully adhered membrane on top of the under-roof sheathing, on top of the roof insulation, and on top of the over-roof sheathing.

Won’t moisture be trapped between the layers of membrane? Especially in the roof insulation which seems to be completely covered with fully adhered membrane?

http://www.buildingscience.com/documents/insights/bsi-056-leiningen-versus-the-ants-redux/
https://www.greenbuildingadvisor.com/blogs/dept/musings/getting-insulation-out-your-walls-and-ceilings

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Clement,
    I don't like Joe's decision to install a second layer of peel-and-stick membrane on top of the roof insulation. I would have used a vapor-permeable waterproof membrane instead -- maybe good old asphalt felt, or maybe one of those high-tech membranes from Europe if I made as much money as Joe.

    One problem is construction moisture -- dew and rain that gets into those foam layers while the roofers are installing the sandwich.

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