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Community and Q&A

Rigid foam directly under rafters

Ryan Lewis - Zone 4A | Posted in Building Code Questions on

Hi,

In considering the answers to my last Q&A I wanted to evaluate a few other options for insulating my unvented cathedral attic in zone 4a. 

I wanted to understand one thing that is not fully spelled out to me. If you install rigid foam directly underneath the rafters on the ceiling, i.e not on the sheathing, and there is nothing but air there:

1) is this allowed by code? (It’s not really cut and cobble which isn’t allowed ?)

2) does this actually do anything ?

im interested in air sealing and insulating the floors very well, but I want to make the attic not horribly unpleasant.

So I was thinking of adding Air barriers to the ceiling to try and lower the RH in the attic in the summer, but I’m wondering if I can use rigid foam under the ceiling joists safely in this scenario.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    As long as the rafter bays are vented there aren't any moisture issues with this approach. The R-value of the rigid foam doesn't count toward your attic R-value from a thermal code compliance point of view unless it's in contact with the other insulation layers, but it still does something.

    Also cut'n'cobble IS allowed, as long as you have at least 1" of space above the foam for soffit to ridge venting.

    It's fine to put a layer of foam board between the ceiling gypsum & joists too, with the fluff piled on top of the foam board. In that instance the R-value of the foam board counts, and it's providing a thermal break on the joists.

  2. Ryan Lewis - Zone 4A | | #2

    Thanks! lets make sure we both are defining 'vented' correctly. My roof has no ridge vent or soffit vents (and my house has no soffits!). or are you just referring to the fact that air inside the attic can get behind the rigid foam?

  3. Expert Member
    Dana Dorsett | | #3

    Vented = soffit to ridge venting. Putting foam on an unvented rafter bay creates a potential moisture trap between the low-permeance foam and low-permeance roofing materials.

    Venting the rafter bay at the bottom & top of the foam layer to the attic interior allows the roof to dry, but is a thermal bypass, severely reducing any thermal benefit that would otherwise be gotten from the foam's R-value. As the temperature difference between the roof deck and attic increases, the convection current drive increases. It doesn't take a lot of cfm to move a lot of heat quickly from one side of the foam to the other side.

  4. Ryan Lewis - Zone 4A | | #4

    right, so your saying you would not recommend rigid foam for me, since there is no venting going on?

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