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To vent or not to vent?

Eric Chandler | Posted in General Questions on

First, I apologize for a question asked many times on here. After just asking about access to an unvented attic, I started to question the whole unvented idea. I live in zone 3, plan to use Zip R (R-6.6) with cellulose inside 2×6 walls.

I originally planned on unvented with 5 inch nailbase with around 3 inches of OC spray foam in the bottom of roof.

I’ve read every article I can find on the subject on here and other sites. If my ducts and air handler will not be in the attic, would it be better to go unvented? I know with unvented you gain a lot of volume of air to condition.

If I go unvented, should I spray a few inches of spray foam to air seal and then go with cellulose over that? If so, which spray foam: open or closed?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Eric,
    Only you can decide whether you want a vented unconditioned attic or an unvented conditioned attic.

    If you have no ducts, appliances, or air handler up there, there is no compelling reason to choose an unvented conditioned attic -- unless you think that the attic space may be useful for some future purpose.

    In almost all cases, a vented unconditioned attic is less expensive to build than an unvented conditioned attic.

    Q. "If I go unvented, should I spray a few inches of spray foam to air seal and then go with cellulose over that?"

    A. If you plan to install cellulose insulation on your attic floor -- and that's a good approach, in my opinion -- there is no reason to install spray foam first. However, it is certainly necessary to seal any air leaks in your ceiling before you insulate. This is usually fairly easy to do in a new-construction project, using canned spray foam, caulk, weatherstripping, and tape. If you take this advice (instead of installing a layer of spray foam over the entire ceiling), you should save money.

    Here is a link to an article that describes the necessary work: Air Sealing an Attic.

  2. Expert Member
    Malcolm Taylor | | #2

    Eric,
    Apart from recent reports of small amounts of mold on the underside of north facing roof sheathing in rainy Vancouver, there is no building science showing any problems with properly air-sealed and vented attics. The same can't be said for unvented ones.

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