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Attic insulation in 100-year-old house

jgmann | Posted in General Questions on

I have a 100 year old house in Northern Iowa.  We have a walk-up full height attic with cellulose blown in the floor of the attic, about 7 inches.  We have an AC unit in the attic that cools the upstairs.  Our heat doesn’t come from this attic unit.  Our roof is wood shingles over spaced sheeting, in good condition, replaced about 15 years ago.  I would like to place rigid foam on the floor of the attic and cover with plywood so that we can still use the attic for storage.  This would also help me seal all of the penetrations of the ceiling.  I am planning to build a box of foam around my air ducts to include them in the insulated space.  I would like to leave the roof deck as it is to preserve the life of the shingles by allowing them to breathe.  Does this seem like a reasonable plan? I’m planning on 4-6 inches of foam covered with plywood.  I’m estimating the current blown in cellulose at R-21. Thanks for your thoughts and experiences.  I’m in zone 6 according to the insulation values on the map from the dept of energy.

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    You can't put foam over the celing joist and plywood over that. The foam will deform from joists, there is just not enough surface area. If you want to go with foam, you need to put down a layer of plywood/osb on the joists, then the foam, finally the layer of plywood for the walking surface.

    Probably a cheaper assembly would be to install 2x6 on edge at 90deg to your joists, then blow in insulation or install batts between the 2x6 and finish with plywood. This would get you from R22 celing up to around R40.

    With the plywood/ 4" foam/plywood sandwich you get an R51 assembly. The 2x in cost is probably not worth it in energy savings.

  2. Deleted | | #2

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  3. jgmann | | #3

    Akos,

    Thank you for your reply. We have a ship-lap floor in the attic which is 3/4" solid wood. I was planning on just putting the plywood on top. Sorry I wasn't clear about the existing floor.
    Jonathan

    1. Expert Member
      AKOS TOTH | | #4

      If there is already a sub floor, going with foam would be easier. You can just put the foam over the existing sub floor and plywood/osb on top as your walking surface as per your initial plan.

      I'm not sure there much benefit for going with more than 3" to 4" of foam, getting screws through that much foam is already not easy. The extra R value also does not buy you much in energy savings.

      For the foam, I like to go with roofing polyiso (seems to be the cheapest here) plus lot of times you can get it reclaimed at a fraction of the cost.

    2. Expert Member
      BILL WICHERS | | #5

      Adding to Akos’s Comment, over 2” or so of foam can be difficult to fasten, BUT that’s usually due to the difficulty of driving a screw straight through relatively squishy foam to hit a stud that you can’t see behind the foam. If you are laying the foam over an existing wood floor, the screw will find wood anywhere it hits, so slight deviations from straight won’t cause you to miss the structure in the way it would with a studwall.

      That said, I agree that going over maybe 2-3” of foam won’t buy you much in this case. Reclaimed roofing polyiso would be great here, and you’d be putting in around R5.5-6 per inch of the stuff. A 2 inch thick sheet would give you around R11-13 or so, depending on the density of the foam. You’d also be able to tape the seams for a good air seal.

      Bill

  4. GBA Editor
    Martin Holladay | | #6

    Jonathan,
    The work you suggest is possible but relatively expensive and time-consuming. It would probably be simpler to convert your attic from a vented unconditioned attic to an unvented conditioned attic.

    Note as well that your solution leaves your air handler in unconditioned space.

    You might want to read these two articles:

    "Creating a Conditioned Attic"

    "Should I Build a Mechanical Room in My Attic?"

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