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Roof foam – 2 or 4 layers?

ururk | Posted in Green Building Techniques on

I’m insulating the exterior of a gambrel barn. The roof needs 8″ of XPS insulation, and I’m wondering if two layers of 4″ foam will sufficiently block airflow to the roof deck, or if I would be better off doing four layers of 2″ foam. I’m doing this myself, so 2 layers will be easier to handle, I think


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  1. GBA Editor
    Martin Holladay | | #1

    I think that two layers will work well, especially if you tape the seams of each foam layer with a high-quality tape. Needless to say, it's important to stagger the seams of the second layer of foam with respect to the first layer of foam.

  2. ururk | | #2

    Ok - sounds good. I am going to stagger the seams - not too sure how I'll do that where the roof angle changes, I suspect I will just end up filling those transitions with spray foam.

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