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How to fix my unvented cathedral ceiling roof assembly?

user-6758344 | Posted in General Questions on

We tried to build a house with a fairly tight envelope based on reading I could find. Well, apparently I messed up, so really want some advise before I mess things up more.
House is in central Missouri( zone 4, but real close to 5). Small, 750 sf but with an open loft making it about 940sf. Sealed Concrete slab as the finished floor, and new construction ( under roof in may but moved in The next January). Red metal roof and one side of that faces north..
Walls are 2×4 and we used open cell foam with 1/4″ foam inside the studs as an air ( not vapor) barrier.
The builder used 2×8 for the ceiling joists but we firred that down on the inside with 1 1/2″ foam board and then 1 1/2″ wood stud. We used 2″ closed cell foam under the roof deck. Then r 30 rock wool under that. And, no drywall or air barrier and a tounge and groove pine ceiling attached to those studs we added. Yeah. I know( now)- bunch of mistakes in there.
So- we move in during an unusually cold snap (1 degree on one night). And notice a puddle on the north side. Turns out to be condensation in many places of the ceiling. We put a thermometer in a place we could get at between the rock wool and ccsf, and temp was around 37 degrees. So , 2″ ccsf is not enough in general and certainly not enough to keep the foam face warm enough with the rock wool we have. At one time when this first started the rh showed around 60%, but we have been using a dehumidifier ( in the winter) to try to keep it under 35. ( we do have kitchen and bath vented outside). We now have a wood stove, which helps with a “dry heat”, but also have a mini split, which is what we were originally using when this started.
It’s a new metal roof so that isn’t coming off- so need to fix this from inside. The house is now finished, so while I know this will get messy, the less the better. And of course now we are really house poor. We will find a way, but, the more reasonable the better.
I want to keep it at about r38 which is code here. Could I take down the ceiling and add 6″ ocsf, assuming that since it is an air barrier at about 4″ that it will stop the majority of the vapor? Or do I need to add another inch or two of ccsf ( which I was hoping to avoid- mostly because I don’t want to take down and reinstall the rock wool, which is a pain). I can’t afford full depth ccsf. Do I need to add either an air barrier or drywall before putting the ceiling back up? I would prefer something other than drywall ( just because of what that would do to trim details, etc), but not sure of our choices. Any and all options that would work and not ruin my house later would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    I think you understand your errors. But if any GBA readers are confused, here are the errors:

    1. The unvented roof assembly has about R-12 or R-13 of closed-cell spray foam. According to most building codes, the minimum R-value for this layer is R-15 for Climate Zone 4, or R-20 for Climate Zone 5.

    2. [Edited] Every insulated cathedral ceiling that includes fluffy insulation needs an interior air barrier. Since the tongue-and-groove boards leak air like a sieve, the ceiling allows the humid indoor air to have unimpeded access to the cold spray foam.

    More information here: How to Build an Insulated Cathedral Ceiling.

    When I have more time, I'll come back and post suggestions for fixing this ceiling.

    -- Martin Holladay

  2. user-6758344 | | #2

    The thin foam is on the walls, but I did tape the joints. No air barrier on the ceiling at all.
    Thank you for your help!

  3. GBA Editor
    Martin Holladay | | #3

    One possible (inexpensive) solution is to install taped drywall on the interior side of the tongue-and-groove boards, and to paint the drywall with vapor-retarder paint. You'd lose the board ceiling, but a tight ceiling would probably be enough to pull your risky ceiling assembly across the line from "damp" to "safe" -- especially if you pay attention to your indoor humidity level.

    If you choose this solution, do it when the outdoor weather is warm and the indoor relative humidity is low -- after you are fairly confident that the wet assembly has had a chance to dry out.

    -- Martin Holladay

  4. user-6758344 | | #4

    I don't think my husband would go for that. Even if it was the least expensive. I'm not sure how to post a picture, but that ceiling is a feature. I figured I was going to have to take it down and reinstall it. Could it be nailed up under drywall if we only went into studs or would that be too "leaky"? Looking to other options I might have if we take it down. I kind of wish I didn't need to monitor the rh- don't like having to use the electricity to run a dehumidifier. But that may be a fact of life.

  5. user-6758344 | | #5

    I edited this, but see below.

  6. GBA Editor
    Martin Holladay | | #6

    If you are willing to remove the boards, there are lots of possible ways to proceed. (If you work carefully, most of the boards may be in good enough shape to reinstall.)

    The best approach would be to remove the board ceiling, temporarily remove the rock wool, and then to install another inch of closed-cell spray foam. At that point you could reinstall the mineral wool (slightly compressed), followed by a new layer of taped drywall. Then you could reinstall the boards. (The fasteners used to attach the boards will not significantly degrade the integrity of the air barrier created by the taped drywall layer.)

    A slightly cheaper option that would probably work: Temporarily remove the board ceiling; install a layer of MemBrain; install a layer of taped drywall -- note that you could install a layer of vapor retarder paint over the drywall if you wanted, in which case you could omit the MemBrain; and then reinstall the boards. This slightly cheaper option would probably work, especially if you keep an eye on the indoor RH. You might not need a dehumidifier -- you can lower the indoor RH in winter with ordinary bathroom exhaust fans.

    -- Martin Holladay

  7. user-6758344 | | #7

    Thank you so much!!!

  8. user-6758344 | | #8

    Is there a page or link that explains how rh really works ? If I know the inside phone temp and house temp, how can I tell what my rh goal should be? Or is there another way to tell, like if the windows aren't sweating should it be ok. I did notice them sweating just before we figured out what was happening. It hasn't happened since then with the steps we have taken. My husband seems to like it at 72 degrees, and I can't control the foam surface temp.

  9. GBA Editor
    Martin Holladay | | #9

    If you are worried about condensation on cold surfaces that are hidden in your roof assembly, I'd aim to keep the indoor RH under 40% during the winter.

    For more information on these issues, see:

    Relative Humidity Doesn’t Tell You How Humid the Air Is

    Do Humidifiers Create IAQ Problems?

    Relative Humidity and Makeup Air at a Tight Minnesota House

    Preventing Water Entry Into a Home

    All About Dehumidifiers

    Why Is It So Humid In Here?

    Measuring (and Understanding) Humidity

    -- Martin Holladay

  10. user-6758344 | | #10

    Awesome. Thank you
    ( this site rocks!!)

  11. Expert Member
    Dana Dorsett | | #11

    The R15 prescriptive for air-impermeable insulation on the exterior side of the fiber insulation in the IRC for class-III vapor retarders in zone 4A presumes an R49 total R, or 15/49= 30.6% of the total R. You have R12 out of R42 total, or 12/42= 28.6%, which is almost but not quite there, and would thus need something closer to 1-perm vapor retardency to keep moisture inside the cavities bounded in winter.

    Instead of painted gypsum air barrier/vapor retarder, it could be quicker & better to use 3/8" fan-fold XPS foam under the t & g, detailed as an air barrier using the appropriate tapes on the seams (housewrap tape is fine), with every seam supported by framing. That would add another R1.5-R2 to the stackup, and almost all non-perforated XPS siding underlayments have thin plastic facers that reduce the vapor retardency to 0.75-1.8 perms, which would be fine as long as it's air tight:

    Plastic faced 3/8" fan-fold XPS costs ~25 cents per square foot, sold in 4' x 50' fan-folded sheets.

    There are both perforated and foil faced version of this type of product, but you DON'T want to use them, and DO look up the vapor permeance specs- you'll definitely want it to be under 2 perms, but given how close you are to the IRC prescriptive ratio it doesn't necessarily need to be under 1 perm (the definition of Class-II vapor retarder.)

  12. user-6758344 | | #12

    Thank you, Dana.
    Will definitely consider that. Our builder put the rafters at 16" on center vs 24", so we would have a littl extra waste, ( since every other stud would fall at 36" vs 4 feet), but not a big deal. Will the house wrap tape really be fine? I thought I read somewhere there was a better tape to seal them but can't recall. Just want to make sure it sticks and is air tight, if we go this way. It would be less messy than drywall at this stage.

  13. Expert Member
    Dana Dorsett | | #13

    There are product-specific tapes for XPS, but there isn't much difference between them and housewrap tape, especially at the much milder range of temperatures & humidity that it would experience on the interior side as opposed to under exterior siding.

    You'll note that the graphic in step 4 in Pactiv's installation guide refers to "Greenguard contractor sheathing / housewrap tape" :

    My expectation would be that anybody's housewrap tape would be sufficient.

  14. user-6758344 | | #14

    A related question: in a couple of places we will put some plywood on the ceiling. If it is cailked on all edges ( with caulk that stretches) would that work as an air barrier or do I still need an air barrier under that?

  15. GBA Editor
    Martin Holladay | | #15

    I'm conservative when it comes to ceiling air barriers (especially for cathedral ceilings, which often have moisture problems). If I were you, I would install a European membrane like Intello between the plywood and the rafters -- and I'd caulk the seams, too.

    -- Martin Holladay

  16. Expert Member
    Dana Dorsett | | #16

    Plywood sealed at all seams works fine as an air barrier, and is a "smart" vapor retarder to boot. Taping the seams with appropriate tapes can be easier to achieve true air tightness than with caulk. To get there with caulk every seam would have to be supported by framing, sealing the sheets to the framing with the caulk.

  17. user-6758344 | | #17

    We Are going with the drywall installation with Vapor barrier paint and then reinstalling the tongue and groove boards. What is the perm rating needed for vapor barrier paint? Found one that says it is .5 perms. Would that be too much in this situation or is that ok?

  18. GBA Editor
    Martin Holladay | | #18

    It sounds like you've got the right stuff. A permeance of 0.5 perm is typical for vapor-retarder paint. That will work fine.

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