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Community and Q&A

Cathedral Ceiling Insulating Best Practices

deadmaulwalking | Posted in General Questions on

Hello All,

I’m sure the situation I’ve gotten myself into is a bit of a novice one, but before I make it worse I wanted to seek out some advice from this wonderful community (wish I had found it sooner). We bought a house and have been doing some serious renovating, the main element of which is vautling the great room. Our contractor and structural engineer completed the actual framing portion of that already, but I’ve opted to do the insulating and finish work for the actual ceiling. Here’s where my predicament has started.

I purchased a closed cell foam kit from Foam it Green to spray on the underside of the roof. I got a 1″ fill all around the original 2×4 truss, but there are low spots (never foamed before and it’s rather difficult to maintain clear eye sight when your goggles are getting doused in foam, so that’s completely on me) where some of the roofing nails are protruding through still.

I didn’t give this much thought as I’d run out of foam and figured the 12″ thick fiberglass bats I was supplementing with would be more than sufficient. I had the bats up on half of the roof for about a week when I pulled one down to check on some wiring I have in the ceiling and noticed it had (cold) wet spots on it. The areas where the 12″ bats were in direct contact with the foam were creating minor condensation, which I then narrowed down to the exposed roofing nails. In hind sight this all makes sense, but I obviously didn’t think about it at the time.

I found a great article on this website ( that effectively tells me what I need to do, but there are still some questions I’m not sure of the answers on.

1) My current mindset is to order more foam, fill in the 2×4 to at least 2.5 – 3″ deep and then put the bats back on it. However, I’m not sure if I need to leave an air gap in between for safety? If so, how much? The aforementioned article mentions being able to go either route, but I’m not sure what would be the best practice in my case. Not all of my bats were damp and it was only the ones touching the roof directly that had an issue. The ones higher up towards the peak (that had an air gap naturally because there was more height to work with) didn’t have an issue, even with only a 3/4 – 1″ gap.

2) The other variable is we were planning to finish the ceiling with bead board, but as I’m reading more about that it won’t create a vapor barrier like gypsum drywall would. Do I still need to do a drywall (tape and mud the seams) underneath the bead board, or if I use enough foam and the fiberglass bats (which I’ll have to reduce by a few inches to accommodate the extra foam) will the lack of drywall not matter?

Apologies for this post as I know all of my answers are really in that article I linked already, I just want to be sure before I move forward any further. I have tried contacting a few insulating companies locally, but since they didn’t start the project they won’t even give me any advice on where to go from here. I can’t blame them for that!

Thank you to any and all for any advice and guidance you can give me!

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  1. Steve Knapp CZ 3A Georgia | | #1

    Where are you located? Did you use open cell or closed cell foam? Can you post pictures of your roof assembly?

  2. deadmaulwalking | | #2

    Hello Steve,

    Thank you for taking the time to respond to this! I am located in Ohio. I used closed cell foam. I'm attaching a few photos of the current setup of the trusses. We were planning to leave some of the scissor truss exposed, so I'm working with a space anywhere from around 15" at the peak to about 7" at the lowest point where the trusses rest on the walls. We like the idea of the exposed truss, but if we need to just drywall/bead board on the bottom of the truss for insulating purposes, then we can go that route as well.

    I did get in touch with another insulating company today that was helpful and was told that I should do 3" of foam total and then a 6" thick bat. They said if the foam is done correctly I don't need the air gap, but the way the roof of my house is setup is it the roof changes direction towards the back of the house, so the last half of my vaulted ceiling is actually underneath another roof as it goes back towards the house, which means it's never going to be truly "sealed" off unless I spray foam that end of the house as well (which currently isn't feasible budget wise). Thus I'm thinking i should leave an air gap regardless so the air can channel to the back of the house if need be?

    Hopefully these photos help, but if need be I can take more.

    Thank you again for any advice / guidance you can give!

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