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Closed-cell foam for existing cathedral ceiling cavities?

berryjb | Posted in Energy Efficiency and Durability on

I’ve a 150 year old house with a partial cathedral ceiling covered by plaster lathe, and dry wall on top. There is no insulation at all in the cathedral ceiling bays. I can access the top of each bay from the attic, and the bottom of the bay via the knee wall – is there a closed cell foam solution I can use which would not require me to tear down the approximate 8 linear feet (each bay) of ceiling, without blowing out the plaster? Thank you – Jim

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  1. charlie_sullivan | | #1

    How deep is the cavity, and what climate zone are you in (or zip code)?

    Are you willing to consider alternatives other than closed cell spray foam? (I assume that's what you mean, although there are other types of closed cell foam)

    How and where are the attic and kneewall spaces insulated, or is that work you want to plan in conjunction with this?

  2. berryjb | | #2

    Charlie, the cavity is only 4 1/2 to 5 inches deep, and has a lot of nails coming into it from the roof - I'd thought about trying to slide in board board (then spray foam the boards in), but the nails, and other miscellaneous debris get in the way. We're in climate zone 5 - so even if I can truly fill the cavity with closed cell foam I won't reach recommended min R38 - but I'd at least get good sealing and much better than nothing R value. I'm figuring I need to turn the whole assembly into unvented space. I can access both attic space and knee wall to overcome the current poor use of fiberglass. Thanks! Jim

  3. berryjb | | #3

    Ooops - meant to say slide in foam board ...

  4. GBA Editor
    Martin Holladay | | #4

    You wrote that your ceiling has "plaster lathe and drywall on top." I'm guessing that the plaster is actually on top -- it's older -- and that the drywall was installed later, on the interior side of the plaster (putting the plaster on top and the drywall on the bottom).

    If my guess is correct, I would suggest that it's easy to drill holes in your drywall ceiling, as many holes as you need, because drywall is easy to patch.

    That said, I don't think you want to drill holes to install spray foam. Spray foam installers need excellent access to do a good job. They need open, exposed framing bays -- not a series of holes.

    You could ask if any local contractors install low-expansion pourable foam. This approach has mixed reviews -- pourable foams can pop ceilings loose.

    The best approach is to install rigid foam on top of the existing roof sheathing, followed by another layer of roof sheathing and new roofing. Keep that approach in mind when it's time for new roofing.

  5. berryjb | | #5

    Martin - sorry for my poor description - you're exactly correct. Is the primary concern about the pourable foam that it would pop the ceiling, or are there additional issues? Is pourable also closed cell?? I'd gotten some guidance from a foam contractor that one should not apply closed cell more than a couple of inches per applications, to ensure it does not combust during curing. Thank you - Jim

  6. GBA Editor
    Martin Holladay | | #6

    I don't have much experience with pourable foam. I know that most contractors stay away from it, so any speculation is worthless unless (a) you can find a contractor who will install this product, or (b) you want to do the work yourself.

    Pourable foams are used by boat-builders as well as insulation contractors. There are definitely closed-cell foams in this category. Here are some links -- not necessarily the most relevant links:

  7. berryjb | | #7

    Martin, thank you for the links - I'll likely have to DIY it, for cost. When it does come time to reroof I am hoping to apply a layer of foam board on top of the decking, but I won't be able to add more than maybe 1" without impacting the bottom of the dormer windows, and (hopefully) I won't have to reroof for a number of years, so I figure to start with the cavities. Another question please - will I cause harm to the building if for now I'm only able to partially fill some of these cavities (ie the middle third), such they they would block air venting?

  8. GBA Editor
    Martin Holladay | | #8

    Whenever you insulate a sloped roof assembly, you have to follow certain rules -- both building science rules (designed to keep your roof sheathing from rotting) and building code rules. Here is a link to an article that explains the rules: How to Build an Insulated Cathedral Ceiling.

    If an insulated sloped roof assembly is designed to be vented -- the usual method when air-permeable insulation like fiberglass or cellulose is used -- then these vent channels shouldn't be blocked. They need to be uninterrupted, from soffit to ridge.

    If you want to install closed-cell spray foam in the middle third of your roof assembly, you would ordinarily also need to install the same type of insulation in the bottom third of your roof assembly. If the top third of the roof is an attic, that attic can be insulated differently.

    One inch of rigid foam has an R-value of R-4 to perhaps R-6. If that's all you can install above your roof sheathing, I hope that you live in Climate Zone 1, 2, or 3. If you live anywhere colder, that approach won't work, for reasons explained in the article I linked to.

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