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Questions regarding insulating a 1912 double-wythe brick building

5grams | Posted in Energy Efficiency and Durability on

We have a 1912 brick schoolhouse that we are renovating in Michigan. We just demoed the entire floor system including joists and main beam due to extreme powder post beetle and moisture damage.

We are planning to reuse the masonry joist pockets for the new floor joists and plan to encapsulate the crawl using closed cell foam on the interior foundation walls, cover the soil with a 27 mil polyethylene liner and spray joists where they enter masonry pockets. We plan to close off the (2) crawl space vents on the EXTERIOR and condition the crawl space. (heat & central air). Firstly, I am concerned about the end of the floor joists which will be in the cold airspace between the wythe. Should I be wrapping them with sill seal, spraying them with undercoating, etc?

On the main floor interior of the building we have exposed brick on 3 walls, and plan build a stud wall and insulate with closed cell foam on the remaining wall. We also plan to use 1-1/2″ EPS foam on the EXTERIOR and cover with house-wrap and vinyl siding. Our thought was that by insulation the exterior we will circumvent freeze thaw damage to the brick and provide some insulation for the building. I am wondering if I should leave the (2) crawl vents open on the INTERIOR so that the conditioned air would be able to circulate between the wythes which may protect the ends of the joists as well as provide some heat to the exterior bricks–or could this cause a problem with the conditioned air going up to the attic? I am not familiar with how the double wythe walls terminate in the attic, but assume they are open.

Any comments would be appreciated. There are so many schools of thought on insulating brick buildings and our local contractors also have differing ideas. Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    Five Grams,
    The good news is that you have decided to insulate the brick walls on the exterior. That approach is by far the best way to insulate an old multi-wythe brick wall.

    I advise you to read this article: Insulating Old Brick Buildings.

    For your plan to be fully effective, however, you want to be sure that most (or all) of the wall's R-value is on the exterior, not the interior, side of the bricks. Your current plan -- to install about R-6 of rigid foam on the exterior -- is the right idea, but the foam is too thin.

    Michigan has two climate zones: Zone 5 and Zone 6. Most codes require about R-20 for walls in Zone 5 and about R-25 for walls in Zone 6. You should strive to hit these R-values with your exterior rigid foam.

    If instead you put thin foam on the exterior, and spray foam with a higher R-value on the interior, your bricks will remain cold, and the beam pockets could be damp, putting your joists and beams at risk for rot.

    Ideally, you'll install 4 or 5 inches of exterior rigid foam, and you'll skip the interior spray foam. If instead you go ahead with your riskier plan, don't reuse the beam pockets. Instead, support your floor system on new load-bearing wood-framed walls.

  2. 5grams | | #2

    Thank you Martin. Our overhangs are quite small so I’m afraid the 5” EPS won’t be an option but if we can afford the cost 3” closed cell on the exterior could be an option. I will keep the closed cell on the one interior wall to a minimum. Thank you for referring us to the article, I have reviewed it several times before. Do you have any advice for the 2 vents in the crawl? I will completely brick up the exterior holes but the interior holes are open to the air space between the 2 wythes. Let’s
    Assume we will have an R-20/21 exterior envelope. The crawl will be conditioned and we will be using the masonry pockets for the new floor joists. Thanks again!!

  3. GBA Editor
    Martin Holladay | | #3

    Five Grams,
    A roofer can help you extend the overhang at the eaves and rakes with pre-painted drip-edge. You can order a custom drip-edge, extra stiff and extra wide, and install it at the perimeter of your roof without having to reconfigure your roofing very much.

    You don't want to be encouraging air flow from your crawl space to the air space in your walls, so you should seal the crawl space vents from both sides.

  4. 5grams | | #4

    Thank you so much for the expert advice!!

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