Cathedral ceiling venting?
Hello,
Just looking for a little help with my Ceiling. I built a new log home this summer and made a mistake with my cathedral roof. I was told by a relative to use a new method of 1″ of spray foam and roxul R30, the 1″ of spray foam would be enough to not need to vent the roof.
I am in northern Vermont and things get very cold here (not this winter knock on wood). As soon as we finished putting up the tongue and groove pine ceiling I had a gut feeling I was going to have problems. Not long into winter I was getting condensation running off my ceiling. I did some research, asked around and called the efficiency Vermont service and they confirmed my worst fear… they said I needed at least 2-3″ more spray foam.
We just finished the house and I cannot imagine taking the entire ceiling down again. Not to mention the cost of the spray foam.
Are there any other options I could consider? I did leave an air space between the roxul and the spray foam. Would adding a soffit vent and ridge vent help? I would be losing the R value of the spray foam, but I would rather eat that cost rather than pour 5K plus my labor back into my roof.
Thanks for your help!
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Replies
The vent space between the rock wool & foam is a mistake, but if you used it for soffit to ridge venting it would be OK.
When do foam+ fiber unvented roofs the fiber has to be in contact with the foam, and in your climate the foam has to be 50% of the total R.
If you re-roofed it with rigid foam above the roof deck of sufficient R value you could stop the condensation. Prior to then you could A: Air seal the ceiling as tight as possible and B: paint the ceiling with "vapor barrier latex" to limit the moisture diffusion, and that may be enough to stop the visible dripping, but not enough to fully mitigate the mold potential at the foam/fiber gap.
John,
You made an expensive mistake. There is no easy way to fix this problem.
First, here is a link to an article that you should have read before you built your house: How to Build an Insulated Cathedral Ceiling.
There are two ways to fix this: from the inside and from the outside.
To fix the problem on the interior, you would probably want to install soffit vents and ridge vents to convert this roof to a vented roof assembly. You would then want to beef up your roof framing to provide room for an additional R-19 of Roxul insulation on the interior. Alternatively, you could add about 4 inches of polyiso on the interior side of your ceiling -- that would give you the R-value you need to have a code-minimum roof. This solution would also need soffit vents and ridge vents.
To fix the problem from the exterior, you would need to remove your roofing and add about R-19 or R-20 of rigid foam above your roof sheathing, followed by another layer of plywood or OSB roof sheathing and new roofing. If your roof has metal roofing, the roofing could be temporarily removed and re-installed. For more information on this option, see How to Install Rigid Foam On Top of Roof Sheathing.
One factor that is contributing to your problem (I think) is that you have chosen to install a tongue-and-groove pine ceiling. Unless you installed a ceiling of gypsum wallboard with taped seams before you installed the pine boards, your ceiling has no interior air barrier. That means that the warm, interior air has free access to the cold surfaces in your roof assembly -- and that's why you are getting condensation. This type of roof definitely needs an interior air barrier.
Thank you for the input. From the above advice I am going to start by venting the roof. I also believe that not having an air barrier is causing a lot of problems. Upon removing some of the ceiling I noticed ice on top of the roxul, which explains why I get the condensation when it warms up.
John,
The fact that you found ice in your ceiling is no surprise. Sadly, your problem is common, and there is no mystery whatsoever as to the mechanism responsible for your dripping ceiling and the ice accumulation you witnessed.
Installing soffit vents and ridge vents is a good first step. Your next two steps are (a) to increase the R-value of your ceiling insulation, and (b) to install an air barrier on the interior side of your insulation.
Thanks again, Martin
I am planning on running a vent behind the Roxul (such as accuvent or something similiar) to prevent the wind from contacting the insulation. My rafters are 2x12's and I believe I might have enough room to add R15 into the existing rafter, following by drywall then my tongue and groove pine.
If I don't have room, between my rafters, I am planning on running foil board against the rafters, taping the joints, and attaching the tongue and gr0ove to strapping. Will this provide enough of an air barrier or would you recommend the polyiso?
John,
Q. "I am planning on running foil board against the rafters, taping the joints, and attaching the tongue and groove to strapping. Will this provide enough of an air barrier or would you recommend the polyiso?"
A. Your question is confusing. The two options -- "foil board" and "polyiso" -- sound similar to me.
If you are talking about foil-faced polysio: Assuming that the seams of the polyiso are carefully taped with a high-quality foil tape, and the perimeter of the ceiling (where it meets the wall) is sealed with caulk, canned spray foam, or high-quality tape, then the polyiso will be an adequate air barrier.
Taped foil is a very good air barrier, but it's also a vapor barrier, which may limit what you'd be able to do for future improvements. Detailing Intello Plus or MemBrian as the air barrier would leave more options.
You could also use a layer of 1/2" OSB as the air barrier, which is a more rugged as well as a smart vapor retarder, and may make for an easy nailing/stapling substrate for the t & g pine. Random grades of OSB can actually leak a bit of air, but if you paint it with a heavy primer it will probably tighten it up a lot, without affecting it's smart-vapor-retarder characteristics very much. Painted with primer the seams can be easily air-sealed with tapes.