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Open vs Closed Cell on Rim / Band Joist

STCook | Posted in Energy Efficiency and Durability on

– Climate zone 4
– Brick sided home, with air vented air gap behind (1″ typ)
– Home is over an encapsulated crawlspace.
– Crawlspace floor sealed is with poly vapor barrier, the block walls are covered with R12 of closed cell foam.
– The HVAC system is in the crawlspace and “conditions” the space as required (two ports direct conditioned air into the crawlspace).
– The crawlspace also contains a heat pump water heater

The rim has r19 batts and I want to improve that so I get air sealing along with the insulation. The two easiest approaches, AFAIK, are to use spray foam either open or closed.

open cell would allow for inward drying potential of the band joist and mud sill, but since its open there is potential for moister air to condense on the band during the winter months.

closed cell prevents inward drying but would also prevent winter condensation. But it then needs to depend solely on outward drying and here is the brick, / mortar droppings and potential for the home owner to put a flower bed where it does not belong, etc.

So I’m looking for what others have done for this set of conditions. My current thinking is the open cell mainly due to the “conditioned” space it faces should have a low RH.

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  1. Expert Member
    Dana Dorsett | | #1

    Closed cell foam is about 0.5 perms @ 2", which is vapor retardent, but not so tight that it "...prevents inward drying...".

    A flash-inch is good enough for sealing up the seams, and would run about 1 perm, after which you could re-install carefully sculpted to fit higher density batt insulation (R15 rock wool) to beef up the R-value. It's more work than 2" of closed cell though.

    Open cell foam in contact with masonry or concrete can get pretty damp, not recommended, but if it's isolated from the top of the block wall by 2" of foam (?) it's fine.

  2. STCook | | #2

    Thanks Dana, that helps a lot.

    I should have mentioned there will be an inspection flashing for termites on the mudsill, so thiat will, in theory, keep the open cell in the band away from the top of the block. Below the flashing is the closed cell.

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