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Hip Roof Cathedral Ceiling – Insulation Help

cpop | Posted in General Questions on

Hello everyone,

I’ve scoured the internet and these forums looking for the answer(s) to my question(s), but am having some trouble and would appreciate any help you can offer.

I’ve just purchased 2 small cabins by the lake (500 sq ft)- they are in need of a total rehab. I’m starting on the first one which is completely gutted to the framing inside. The outside walls are 7′ and I have a hip roof. I don’t want to install a flat 7′ ceiling so I’ve decided on a cathedral ceiling – using the existing 2×4  roof. I’m lucky in that I don’t have to meet any codes or restrictions – not that I shouldn’t try, but I’m willing to compromise some for anything that greatly affects cost. With all that said – here are my questions.

1. It being a hip roof I don’t see any simple way for ventilation. Therefore, I’ve decided on closed-cell spray foam insulation and no ventilation. Is this the right choice?

2. On that same note, the house was built in the 40’s – so I’m working with 2×4 rafters. is 3.5″ of CC spray foam sufficient for air and vapor barrier? I’m in zone 4 – South West Missouri. I understand I would be under the recommended R-Value, but do you see this as an issue given I don’t have to meet code? If so, would flash-and-batt be a good idea? I would have to scab on 2×6 or bigger lumber to the existing rafters.

3. In terms of vapor/air barrier, how does this relate to the walls? I had planned on using regular insulation in the walls, but seeing how the ceiling is inherently connected to the walls, this is going to cause an issue with air leakage into the ceiling from the walls? Or is this a non-issue?

I have about 1000 sq ft of roof and as you know spray foam is extremely expensive. I’m trying to find the best way to work out this specific project. Thanks for any insight you may have!

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Replies

  1. user-2310254 | | #1

    Where are you located? The simplest solution would be to install rigid foam on top of the sheathing. (See https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing.) You might be able to source reclaimed foam, which would significantly reduce your materials cost.

    You also could consider a flash-and-batt approach, but you would need to do something about the 2x4s. That might mean furring down the joists or sistering larger material to the existing framing. Doing that on a hip roof might get complicated.

    1. cpop | | #2

      Thanks for your response! I had actually considered that option too. The thing I wasn't sure of is whether or not the roof could support the extra load. The current rafters are spaced (by my memory) about 24" - 30". Do you think I would have to add structure on the underside of the roof to support this option?

  2. user-2310254 | | #3

    Cpop,

    I'm not a professional, but I don't think even a professional would want to venture an opinion via an online discussion. If it were me and I was seriously considered exterior foam, I'd involve a structural engineer. FWIW, I wouldn't be surprised to learn that your roof needs some beefing up. Or it may be that it would be more cost-effective to tear off the existing roof an reframe it as a simple gable. Redoing the roof also would give you the chance to use parallel cords that are deep enough to support a ventilated cathedral ceiling.

    But let's see if one of the experts has any thoughts on your project.

  3. Expert Member
    PETER G ENGLE PE | | #4

    24 rafters don't meet current code for pretty much any roof. If you ask an engineer, "Is such-and-such OK?" what you're really asking is, "Does it meet code?" Even though there is no code enforcement in your area, the building codes still inform everything we do in construction. Chances are, you're already starting with a weak roof system. How weak is a matter of degree, but even on a very small house like that, a 2x4 hip roof is going to be pretty far below code minimums. That means your house is at greater risk for damage over time. actual collapse is unlikely except in extreme weather events (mostly big, heavy snowfall), but sagging, cracking rafters and/or spreading walls is certainly a possibility. Adding any additional weight to the roof, of course increases the risk. Exterior foam on top of the roof doesn't add any more weight than spray foam in the rafters, except for the additional layer of sheathing, which isn't much. Since nobody is making you do otherwise, the decision is yours - do you insulate the existing roof and hope for the best, or do you take it off and build a proper roof? Clearly, tearing it off costs more, but it would also make it easier to insulate and more durable. How aer ryou using these buildings? How often? What seasons? How long do you plan to own them? All of these questions will help you to decide on the best course.

  4. cpop | | #5

    Thank you for the info!

    I complete redo of the roof is probably out of the question for me due to time and budget. However, I don't mind strengthening it up if I need to. There currently doesn't appear to be any significant wear on the roof rafters and it's all original. I do know that the wood used is rock hard and even hard to cut... unlike the soft pine we use these days. Anyway, that's a completely different topic.

    Do you have an opinion on spray foam vs rigid foam on top of the roof? Like most, I'm simply looking for the most cost efficient way of getting this project done. I would just install a flat ceiling, but that would make them 7', which I want to avoid if at all possible.

    1. user-2310254 | | #6

      Cpop,

      You still haven't given us your climate zone. A lot of advice is specific to where the structure is located.

      1. cpop | | #7

        I did actually mention it in my original post, but there was a lot there so it may be easy to miss.

        I'm in Southwest Missouri - Zone 4

  5. user-2310254 | | #8

    If combining exterior rigid foam with interior air permeable foam in CZ4A, the rigid foam would need to represent at least 31% of the total r-value (R15 if shooting for r-49). There's more here on this topic: https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation.

    If the existing roof is solid, maybe you could beef it up by sistering bigger material to the existing 2x4 rafters. But this may be overkill for how you intend to use the cabins.

    Sorry I missed your location info.

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