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Insulating a 1930s home with foam and fiberglass

Jason A | Posted in GBA Pro Help on


I have gutted my bathroom of my 1930 cape cod and am looking to insulate from the inside.

I live in zone 5a, upstate NY
The house exterior is brick, with a gap between the brick and the exterior sheathing ( which is 8 inch boards)

My thoughts are this

If I just installed fiberglass the high amount of air leaking in from the outside shiplap will compromise the fiberglass. Soy idea was to install 1/2 inch foam between the studs , spray foam all edges to make an air seal, then install fiberglass. I understand compressing the fiberglass is not a good idea, but this is a small bathroom and building out the studs is not in the cards. Im afraid this method will create 2 vapor vapor barriers and trap moisture. I thought about using roxul or unfaced fiberglass, thoughts? Also filling the entire cavity with foam would be alottle expensive, I’m working with a tight budget. Thanks. Jason

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  1. GBA Editor
    Martin Holladay | | #1

    The rigid foam is a good idea, and won't cause any "double vapor barrier" problems. Compressing fiberglass is fine.

    You might want to read this article (which describes a different type of house than the one you are insulating, but which presents a similar solution): Insulating Walls in an Old House With No Sheathing.

  2. Jason A | | #2

    Thanks Martin

    I had read that article but the part I was confused about was the gap between the shiplap (siding) and the foam. I don't have room to do this, and why is that nessecary? So the wood can dry? Since I have brick I assume this is not a issue.

  3. GBA Editor
    Martin Holladay | | #3

    In your case, you don't need the gap.

  4. Expert Member
    Dana Dorsett | | #4

    This house has ship lap sheathing, and even though it can dry into the masonry cavity, half inch foam board isn't necessarily the lowest-risk air barrier to install between the fluff and the ship lap. With only half-inch foam the fluff is going to accumulate moisture on the cold side over the winter, and the studs could become moldy. The problem is that half inch foam is vapor retardent, but does not have sufficient R for dew point control on 3-3.5" of fluff in a zone 5A climate.

    If full-dimension 2x4s you'll need at least 1" of polyiso or 1.5" of EPS to have sufficient margin. Unfaced 1.5" EPS would still allow some drying toward the interior, and would be the preferred material if available, but plastic or foil faced cheap Type-I goods are still OK.

    If the framing is milled 1.5" x 3.5" nominal 2x4s that drops to 3/4" polyiso or 1" EPS.

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