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Replacing a flat roof in Wisconsin

tina81 | Posted in General Questions on


I’m in zone 6A and I’m having my flat roof replaced. It will be replaced with fibertite membrane with tappered ISO insulation underneath to correct for ponding. The insulation will start at 1/2″ on the edges and go up to 4.75″ in the middle. I believe I need to do an unvented roof with this set up. Can I do a layer of closed cell spray foam on the underside of the roof decking? I’m looking for any type of solution.


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  1. GBA Editor
    Martin Holladay | | #1

    As a first step, I suggest that you read this article: Insulating Low-Slope Residential Roofs.

    When a project includes tapered insulation, the R-value of the tapered insulation is assumed to be equal to the R-value of the thinnest part of the insulation. So your tapered insulation (which sounds like polyisocyanurate insulation) should be assumed to have an R-value of about R-3.

    In your Climate Zone, you are aiming to install rigid foam insulation above the roof sheathing with a minimum R-value of R-25. Since your tapered polyiso has an R-value of R-3, you still need an additional R-22 of insulation above the roof sheathing. You can accomplish this by installing about 4 more inches of polyiso or about 5.5 inches of EPS. In most cases, you would install the tapered insulation above the insulation I just described.

    After you have completed installing this rigid foam insulation above the roof sheathing, you still need to complete the insulation job by installing an additional R-24 of fluffy insulation (fiberglass, mineral wool, or cellulose) under the roof sheathing (and in direct contact with the underside of roof sheathing).

    If for some reason you are unable or unwilling to install an adequate thickness of rigid foam above the roof sheathing, an alternative approach is to install closed-cell spray foam on the underside of the roof sheathing, as explained in the article I linked to.

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