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Will spray foam at a rim joist affect the wood?

mtnsailor | Posted in General Questions on

I live in a 150-year-old balloon-frame, stone/brick exterior house in Philadelphia. I have a quote for getting the attic spaces filled with blown insulation (for the first time!) and for air sealing in the basement. Having worked on wooden ships in my past, I’m leery of having closed cell foam applied directly to the rim joist, fearing that it will increase rot potential.

do you share my concern?

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  1. GBA Editor
    Martin Holladay | | #1

    The danger doesn't really come from the foam being applied to the rim joist. If you install closed-cell spray foam, it will prevent any interior moisture from condensing on the rim joist.

    The problem comes from the fact that most older buildings lack a capillary break (for example, sill seal) between the stone foundation wall and the sill. The foundation wall can wick ground moisture. For years, the only thing protecting the rim joist from rot has been the heat flowing through the rim joist from the heated interior. When insulation is installed, the rim joist becomes colder, lowering its drying potential.

    In most homes, the trade-off is worth it, and the rim joist doesn't rot. If you have a particularly damp foundation wall, it might be beneficial to jack up the house half an inch -- just enough to slip in some metal flashing or rubber roofing as a capillary break. It sounds complicated, but it isn't as hard as it sounds.

  2. Expert Member
    Dana Dorsett | | #2

    In most 150 year old houses the foundation is either field stone, quarried stone, or brick, with much lower wicking properties than poured concrete. Don't worry about it unless there is a lot of efflorescence on the mortar within 6" of the foundation sill. Most of the time these foundations can dry toward the exterior with as little as 8" of above-grade exposure, but if the grading is pulling surface water toward the foundation (a condition that needs to be corrected, if you can) masonry or stone foundations are usually going to be dry enough.

    Fortunately you don't have to take your home out to sea, with all of that brick & stone ballast on the hull! ;-)

    Is there any cavity insulation in the balloon framing? Are the studs tight up against the brick, or is there wood sheathing and an air gap between the framing & brick?

  3. mtnsailor | | #3

    The original 1865 stone house was stuccoed and the plaster inside is right on the block (no voids). I'll have to check where the sill actually is on this part. The addition built in 1880s is brick with plaster/lath interior. There's a 1" void between. The addition's basement is partially below grade, with a mortared stone foundation. The addition is much colder than the rest of the house, with the basement being very chilly. I've ripped the plaster down in that basement and have been putting foam board in the 1" air gap. The house was built as a summer residence and had no insulation. It's a great looking house...

  4. mtnsailor | | #4


  5. Expert Member
    Dana Dorsett | | #5

    With a vented 1" void between plank sheathing and the balloon framing it may be possible to insulate those cavities, but you have to tread carefully. If there is continuous rosin-paper, kraft, felt, or similar on the exterior of the planking it's a bit safer than if it's bare planking, since knot-holes or seams are less likely to let blown or poured insulation into that 1" gap. Dense-packing fiberglass or cellulose is NOT an option, since it will guarantee to find the air leaks in the planking and spit a plug of fiber into the void, then wick moisture back into the wall from the masonry. The gap is an essential capillary break.

    Poured insulation (eg Perlite) or blown EPS beads can work. Blown EPS bead has very low wicking potential. EPS beads coated with adhesive & fire retardent are commonly used for retrofit insulation of cavity walls and framed brick veneer walls in the UK, but I've yet to find a source in the US. Perlite is normally poured, and less likely to end up spilling into the cavity, but has some potential for wicking.

    There doesn't appear to be any issues of concern for using closed cell polyurethane on band joist and stone in that basement picture. The foundation sill appears to be WELL above grade, and putting even an inch of spray polyurethane from the subfloor to the slab (or dirt?) would lower, not raise the risk of moisture problems in the timbers. The roof overhangs appear to be plenty for keeping the splash-back under the eaves well controlled too, and even if the surface grading is sub-optimal, you're unlikely to create a problem by insulating the basement and band joists with closed cell foam.

    On the all brick section there is likely to be a ledger board, or mortise holes in the masonry, or iron joist hangers for supporting the joists. That too can be insulated, but a picture of what you have might be useful.

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