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Is it more energy efficient to insulate roof slopes or the floor of an attic?

Moya Mim | Posted in Energy Efficiency and Durability on

Ignoring all other things, from a pure energy efficiency perspective, is it more energy efficient to insulate the slopes or the floor of an attic? Assume in both cases a vented attic (even if insulating slopes, there is fiberglass down below that needs to be vented. If insulating slopes, would you also endeavor to plug up holes in the floor, or is that unnecessary once attic is brought within house envelope?

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Replies

  1. Mike Horgan | | #1

    Floor of the attic. Hands down.

  2. John Klingel | | #2

    Why would you even want to insulate the "slopes" and then vent, letting all that heat out? Did I miss something?

  3. GBA Editor
    Martin Holladay | | #3

    Moya,
    Assuming that there are no ducts or HVAC equipment in the attic, and assuming no plans to use the attic for living space, it is always preferable to install the insulation on the attic floor.

    If you decide to create a conditioned attic by insulating the sloped roof, it isn't necessary to seal air leaks in the attic floor.

    For more information on this issue, see:

    Creating a Conditioned Attic

    GBA Encyclopedia: Vented or Unvented Attic?

  4. Moya Mim | | #4

    The venting would take place behind the foam. It's for the fiberglass that's in the slopes below the attic level.

  5. John Zito | | #5

    If you keep your insulation at floor level, it's a smaller volume to condition than if you insulate and condition up to the roof line.

  6. Moya Mim | | #6

    What all of you are saying is logical, but I did find one online source that said for some reason insulating the slopes is better. One of the foam companies I got estimates from also said the same thing, reason being that you don't have a hot attic in the summer.

  7. Expert Member
    Dana Dorsett | | #7

    If you have air conditioning ducts and air handlers up there the hot attic is an issue. If it's just dead space and the attic floor is well sealed to mitigate/eliminate stack effect infiltration, the hot attic is of little to no consequence.

    It's sometimes easier to air seal at the roof deck than at the attic floor though, since there are likely to be at least 101 electrical/plumbing/flue penetrations of the attic floor, yet only a handful of penetrations to detail at the roof deck. It can be pretty expensive to insulate to a high-R at the roof deck compared to attic floors though.

  8. Moya Mim | | #8

    My attic is actually simple. The only thing going through the floor are exhausts from the bathroom. Hopefully they can seal around those pretty well.

    The only problem that will remain is that the attic is only a small portion of the house. There is a substantially amount of sagging fiberglass in the part of the roof that is below the attic level, which forms the barrier of much of the house.

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