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Closed-cell foam on attic floor?

jakehuff | Posted in Energy Efficiency and Durability on

Does it make sense to use a couple of inches of closed cell/ or open cell foam to seal my attic floor before adding blown fiberglass? The home would have a vented attic and located in Zone 5. Just wondering if it is overkill. Any thoughts would be appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    It doesn't take a full covering of foam to air-seal the floor, but it SHOULD be air-sealed at all penetrations prior to blowing insulation- it's essential in fact. But you'd likely be able to air-seal the attic floor with less than 20 board-feet of foam and a few tubes of acoustic sealant caulk, with maybe some sheet-metal & fire rated mastic to air seal around flue penetrations. What it takes depends on what you really have, but sealing with a full cover of foam only makes sense for sealing planked surfaces. It's extreme overkill for air-sealing sheet gypsum/plywood/OSB, even if you had a hundred electrical & plumbing penetrations to deal with.

    Blown fiberglass significantly under-performs cellulose in open blown applications, at the wintertime temperature extremes you'd see in a climate zone 5 location, unless a top-side air-barrier is added (which isn't always practical). Low density cellulose is an order of magnitude more air-retardent than low-density fiberglass, and effectively blocks air convection between the cold attic and the entrained air in the insulation layer, at least from a thermal performance point of view (but not from a moisture-transport point of view), whereas without top side air barriers fiberglass loses performance with increased temperature difference in a cold-side-up configuration.

  2. jakehuff | | #2

    Thanks Dana!

  3. srivenkat | | #3

    As tangential question to OP's:

    I live in zone 5 as well. About 3/4 of the ranch level of the home has a 13-ft ceiling with the rest 1/4 9-ft. So, there's a knee-wall in the attic where the ceiling height drops down. Would Open-cell Spray-foam be the best way to insulate it? I am considering a local Icynene contractor for the work. I am not considering closed-cell foam because of the high GWP. Or are there better ways to insulate the knee-wall? Thanks in advance.

  4. GBA Editor
    Martin Holladay | | #4

    Dana gave you good advice. Here is a link to an article that describes the necessary air-sealing work that needs to happen before the cellulose is installed: Air Sealing an Attic.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Are there better ways to insulate the kneewall?"

    A. It doesn't really matter what type of insulation you use, as long as the work is done with close attention to air sealing. If a fluffy insulation is chosen, there must be an attic-side air barrier. For more information on reducing air leaks through kneewalls and insulating kneewalls, see:

    Two Ways to Insulate Attic Kneewalls

    Solving Comfort Problems Caused by Attic Kneewalls

    Video: How to Air-Seal an Attic Kneewall

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