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Airsealing at joist plates and along fascia/soffit

1910duplex | Posted in Energy Efficiency and Durability on

So I think I’m actually getting close to figuring out who can do my attic insulation job. (original question and photos here:

Here’s my question. One company is the low bidder on closed cell foam at roofline, but is completely uninterested with dealing with cracks in the wood siding in the fascia, and one place, where there is a breadbox (?) sized hole in the fascia/soffit. Not sure what, if anything, they would do to airseal around the chimney at roofline, since when I talked to the ‘senior estimator’ about airsealing (a service on his site), he said the closed cell foam takes care of air sealing and wouldn’t discuss anything further.

Another company, which has been a subcontractor for a ‘green design’ company listed on this site, said they would install polyiso boards at the rear and side joist ends, and use mastic to airseal them, as well as reparing the large hole. That, along with installing one second-story storm window, plus the low-bid closed cell foam install quote, is about just $200 less than

Third company, which would install closed-cell foam and pay attention to air sealing details — cutting away flooring at the edges of the attic so that they can reach cracks at bottom of fascia, would air seal at top plates (there are plates over the balloon framing), would air seal around chimney, would put a board against large hole before doing closed cell foam against it.

In all cases, I am following Dana’s advice of a flash and batt approach, two inches of closed cell on slanted roof, one inch in gable walls, and mineral wool batts in the 2 x 8 rafters. We are installing that ourselves rather than paying ($3500 for professional installer to do it. There is 1,000 sq feet of rafters. (And yes, I know it’s going to be a pain, because most of the batts will have to be trimmed to fit the irregular rafter bays).

(The third company’s contract says they do not advise closed cell at roofline, and includes language about them not being responsible for damage at roofline, or any issues of moisture or growth, for what that’s worth).

So here’s my question — is the polyiso board/mastic solution as described above as good as foam sealing over the top plates and foam sealing the cracks at the fascia?

So far, I haven’t been able to find any company that uses HFO blowing agent, but I have asked a follow-up of one other company that wasn’t clear about it.



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  1. Expert Member
    BILL WICHERS | | #1

    Ask the company for the MSDS (materials safety data sheet) for whatever foam they will be installing for your project. They are legally obligated to provide you with this info on request. The MSDS will list the EXACT product they will be spraying, from which you can find out what blowing agent will be used. Problem solved, no more question dodging from your contractor :-)

    Usually the spray foam contractors will stuff something into any voids to provide a surface to spray the foam against. The spray foam DOES take care of any air sealing this way, as long as the installers don’t leave any voids.

    I’m not sure from your pics where exactly you want to install the spray foam. If you have any holes to the outside, you probably want to patch those prior to installing spray foam. If you leave open holes, the spray foam will bubble out through the hole and leave a ball on the outside that you’ll need to saw off flush to get a flat surface again. Small cracks usually aren’t a problem, the spray foam will just seal them is all.


  2. 1910duplex | | #2

    Most of the closed cell foam will be against gable walls and on the roof decking/skip sheathing. But my question in particular is how best to address air sealing the top of the plates on the balloon framing (also pictured) and the cracks open to the air in the fascia that is very near the floor or even below the floor. As well as the one more substantial hole at fascia/soffit.

  3. vap0rtranz | | #3

    >he said the closed cell foam takes care of air sealing and wouldn’t discuss anything further.

    Well their loss. One contractor, who I eventually went with, was happy to air seal other areas, like window sills, attic hatches, etc. even though we opted to DIY that to save on labor. If a business isn't interesting in doing what you want, then why keep at them?

    >We are installing that ourselves rather than paying

    That's what we did as well. Got the professionals to spray, and are doing the batt part of flash-n-batt ourselves.

    Is the basic question: whose best to plug holes? I guess I'm not following what's left to answer. You've got 3 contractors giving bids and some won't do all that you want.

  4. 1910duplex | | #4

    My question was whether the rigid foam and mastic was a better approach than spray foam to deal with the top of the balloon framing or cracks that are difficult to reach from the attic floor.

    Are you also doing rockwool batts, or fiberglass? If the former, do you have any lessons learned?

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