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Community and Q&A

Best foam for site-built ventilation baffles?

Tom Sloss | Posted in General Questions on

My question is about site built vents for my non-insulated attic space. If I go with 1 1/2″ space (dimensional lumber spacers) under the roof deck, with 1″ foam board attached to the spacers, and add 3″ closed cell spray foam, then 2 layers of polyiso attached to the face of the rafters, would I be OK as far as moisture within all the layers? Is there a better type of board to use on the underside of the roof deck (XPS, EPS, Polyios / coated or non-coated)? Is there a preferred type of board to use for the foam board between the closed cell and the living space? I’m not sure about all the types, concerning what is better for shrinking, R-value in hot or cold weather, thermal drift, and moisture control. Do I want some amount of absorption in my vent, or none? With the closed cell spray foam, would it matter? Would a 2″ space with 1/2″ board under roof deck be better, or not really much difference (more air gap, but less R-value in available space)? This is all in zone 5, and the roof is gambrel (12/12 and 4/12), with 20″ cellulose on the attic floor. Thanks.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Tom,
    Q. "If I go with 1 1/2 inch space (dimensional lumber spacers) under the roof deck, with 1 inch foam board attached to the spacers, and add 3 inches of closed cell spray foam, then 2 layers of polyiso attached to the face of the rafters, would I be OK as far as moisture within all the layers?"

    A. Yes, assuming the ventilation space connects with soffit vents and a ridge vent.

    Q. "Is there a better type of board to use on the underside of the roof deck (XPS, EPS, Polyios / coated or non-coated)?"

    A. Green builders avoid the use of XPS. For more information, see these two articles:

    "Site-Built Ventilation Baffles for Roofs"

    "Choosing Rigid Foam"

    Q. " Is there a preferred type of board to use for the foam board between the closed cell and the living space?"

    A. Again, green builders avoid the use of XPS.

    Q. "Do I want some amount of absorption in my vent, or none?"

    A. I'm not sure what you mean in this question.

    Q. "Would a 2 inch space with 1/2 inch board under roof deck be better, or not really much difference (more air gap, but less R-value in available space)?"

    A. Not much difference -- except that your baffle needs to be sturdy enough to withstand the forces exerted by the expanding spray foam, which means that a thicker baffle is usually better.

  2. User avatar GBA Editor
    Peter Yost | | #2

    Hi Tom -

    I don't think there is enough difference between any of the rigid insulations in terms of contraction/expansion to matter. They all need a perimeter air seal regardless.

    Peter

  3. Nick Welch | | #3

    As mentioned, XPS should be avoided.

    Between the two remaining options, I'd pick polyiso, as it's much neater to work with. EPS leaves a crumbly fluffy mess everywhere.

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