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Community and Q&A

Semi-controlled conditioned space?

greenhouse437 | Posted in Energy Efficiency and Durability on

As we move toward installing closed-cell foam in our attic rafters and gable walls, the choice has come up as to how much of the old fiberglass insulation under the attic floor we want to remove. Obviously it has to be taken out in the area where the roof meets the floor to enable an airtight fit by the foam. Ordinarily making this a conditioned space would entail keeping the temperature of the new space as close to the floors below as possible. (To make it occupiable we’d have to get a permit, install radiators, etc.) However our space will be used basically for our AC unit and storage, though we might occasionally hang out up there. Building science advisors are quick to point out that one of the negatives of converting the attic to a conditioned space is that you’re adding about 10% energy costs by expanding the building envelope. Of course the insulation makes this by far an energy saver since the heat is no longer going through the leaky fiberglass in the attic flooring and then out the rafters.

But our plan is to leave as much leaky fiberglass in as possible, hoping that the temps will stay within a few degrees of the downstairs and then, since we have a full-sized door and stairway leading to the attic, we can just leave the door open when we want to be up there for awhile. This should allow temps to equalize even more if we’re up there in winter or summer. (Perhaps the AC will operate in a slightly warmer environment in summer.) Anyone see a down side to this?

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  1. JC72 | | #1

    I would leave the fiberglass in place unless the amount of spray foam you intend to install meets current code, and I would make sure the attic will receive some supply of conditioned air in order to handle interior generated humidity.

    Your local codes will dictate whether you can install a small return/supply register in the now unvented attic.

  2. GBA Editor
    Martin Holladay | | #2

    It's always easier to provide guidance if we know your geographical location or climate zone.

    In most cases, it's perfectly acceptable to leave the insulation on the attic floor in cases like yours. Allison Bailes, a GBA blogger, makes the case for removing the insulation on the attic floor in one of his articles: Three Reasons to Remove Attic Floor Insulation in a Sealed Attic.

    I don't find Allison's case particularly compelling, but you should read his article and reach your own conclusion.

    -- Martin Holladay

  3. greenhouse437 | | #3

    Thanks for the link, sorry for not including location; this is Climate Zone 4A. And a very knowledgeable General Contractor just suggested to me adding a register in the attic to the ac system and then running just the ac fan in order to condition the attic and equalize house temps, say in winter, where upstairs may get warmer than down.

  4. charlie_sullivan | | #4

    A few cautions about spray foam.

    1. Closed cell spray foam traditionally comes with a hefty environmental footprint. Alternatives that avoid that problem have just become available:

    2. Getting sufficient R-value to meet code with spray foam is expensive, and some spray foam contractors fraudulently claim that the effective R-value of their insulation is higher than its actual R-value, in order to sell insulation jobs that don't even meet code minimum levels of insulation.

    3. Rarely, but often enough to be of some concern, spray foam insulation installation goes wrong and it results in lingering nastly smells and health impacts that sound like nightmares. Lots of jobs have no such problems, so I don't want to blow them of proportion, but it's worth considering whether you have confidence in your contractor and your contract to protect you from such a scenario. You can see examples in this thread:

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