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Spraying foam into stud bays without any kind of barrier

Roger Steinbrink | Posted in General Questions on

One of my clients has a pre-war home, (Civil War, that is), and would like to spray foam into the stud bays, although there is no barrier of any kind. Just old pine siding nailed to the studs.
I told him first that he should re-naill the siding using splitless ring shank nails to secure the siding.
Without any kind of barrier, I envision foam squirting out between the gaps.
Short of removing all the siding, and installing a vapor barrier, I suggested lining the stud bays from the inside using roofing felt stapled to the studs encapsulating the stud bays.
Any better ideas?

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  1. GBA Editor
    Martin Holladay | | #1

    In a home without any wall sheathing, it's a bad idea to install spray foam directly on the back side of the siding. Your instincts are correct:
    (a) the spray foam may ooze out of cracks on the exterior;
    (b) the spray foam will make the siding dry unevenly, promoting checking, splitting, and curling; and
    (c) the spray foam will make siding repairs difficult or impossible.

    There are two methods for doing this type of work:
    1. You can install 1"x1" vertical "sticks" in the corners of each stud bay -- two sticks per stud bay, up against the siding. Then install a layer of rigid foam (perhaps 3/4" or 1" thick) the full width of the stud bay. This method provides an air gap of 1 inch between the rigid foam and the back of the siding. Once these rigid foam pieces are installed in each stud bay, the rest of the wall cavity can be filled with spray foam.

    2. You can install asphalt felt strips in the same location in each stud bay, to keep the spray foam from contacting the siding. The asphalt felt is cut wide, to create tabs on each side. The tabs are stapled to the sides of the studs. For a photo and more details of this technique, see Sticking With Spray Foam for My Renovation.

  2. Roger Steinbrink | | #2

    Thank you, Martin.
    I like the 1"x1" stick idea with the rigid foam.

  3. D Dorsett | | #3

    Cut the spacer sticks out of the rigid foam board, since it uses up some of the high scrap rates of cut'n'cobbled foam board.

    Even 1/2" of air gap is more than adequate back-ventilation for the siding. If you're in US climate zone 3 or lower using half-inch foil-faced polyiso would be more than adequate dew point control against winter moisture accumulation in the open cell spray foam, and would provide a tiny bit of thermal performance to the air space due to the low-E aluminum facer, which is also a vapor barrier against summertime moisture drives from the exterior. If using foam with facers it's important to avoid highly vapor impermeable interior finishes such as foil or vinyl wallpapers (latex paints are just fine.)

    If the stud spacing is wide (more than 24") and the siding or foam board is bowed to where they would touch, a third stick in the middle of the spans between studs may be in order.

    If the siding is not painted on the back it is more susceptible to warping, cupping & splitting, and has more rapid paint failure on the front side. While you have the stud bays open it may be worth spraying the back sides with a latex primer paint, which will slow the rate at which moisture moves in & out of the siding, mitigating those issues.

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