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Risky assembly

A friend called me today seeking advice for a new room he is adding to his brownstone. He has an unused space on the top floor and he wants to put a wine cellar there. A wine cellar on the top floor? Not an ideal location for sure. But just for kicks, let’s assume he has no floor load issues and he can adequately insulate and seal the walls and floor. What about the roof? Did I mention this space is below and unvented flat roof? So drying to the outside is out of the question. I think the biggest concern is the sheathing. He can’t have a cold roof come in contact with mid-50’s temp air that has an RH of 50% or higher. So first step is to put ccSP on the sheathing, let’s say 5 inches. I believe he has 2×10 rafters, so say he puts 4.25 of dense pack in to fill the bays and then adds a smart vapor retarder. Not ideal, and a few R short of code, but this is not an ideal situation to start with. While this assembly may be vapor open to the inside, it really isn’t because the humidity levels will be high in the cellar. Another option is to make a vapor sandwich, yes I said vapor sandwich, and hope for the best. Same stack-up, except foil-faced polyiso inboard which will kill the bridging through the studs and hopefully keep all the vapor in the cellar. He could add some wireless humidity sensors to see when he is going to have to replace the roof, maybe just a few years down the road. A third option would be to add a dropped ceiling in the cellar. So in the rafter bays he would have ccSP, DP, smart vapor retarder, and maybe an inch of Comfortboard to meet code. Then drop the cellar ceiling (he has 10′ of headroom to work with). Vent the area between the roof and the dropped ceiling into the adjacent areas of top floor. He could use 2×4’s for the dropped ceiling, add dense pack, 3 inches of foil faced Polyiso and he has an air and vapor tight wine cellar ceiling with an r-value of around 30. Assuming he can actually vent the space between the top of the cellar ceiling and the bottom of the roof into the rest of the house, he would have a way for both assemblies to dry if needed. Do you think this would work.

Thanks,

Jonathan

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jonathan,
    You are overthinking this. Insulate the room with closed-cell spray polyurethane foam.

    Closed-cell spray foam is an air barrier and a vapor barrier. The wine room will stay cool and humid, and all of the other surfaces on the exterior side of the spray foam will be unaffected by the conditions in the wine room.

  2. Jonathan Lawrence CZ 4A New Jersey | | #2

    Martin,

    Thanks and your are correct on my overthinking. I figured I would try to give him an option that minimizes ccSP, but I don't think the space is that big anyways and he will be glad to hear he can just use foam.

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