Vaulted roof moisture problems
Hello GBA community,
I’m navigating a few ceiling-moisture issues with my house, and I’m hoping to get some feedback and perspective from this community. The house is a two-story, with a vaulted ceiling on the second floor. The house is in zone 5B, and has soffit + ridge vents. Folks we bought this house from (who flipped it to us), put recessed lights in some of the rooms upstairs (including a vaulted bathroom that was added upstairs). A few problems we’ve noticed:
- Brown liquid dripping through recessed light openings every winter, especially when it snows. I now understand this to be tannin-laden water, likely condensate from the roof cavity
- Water stains and brown stains on the ceiling sheetrock in the vaulted bathroom. We’ve taken down sheetrock and insulation in this bathroom twice now, and have noticed moldy OSB and rusted nail shanks.
- Some additional brown spots in the ceiling areas that do not have recessed lights, but have pendent light fixtures.
- Sagging of the roof in multiple areas (including bowing of a likely CDX panel, as we were told) on the side of the roof opposite to the recessed lighted rooms. As far as I can tell, there was at least a plastic vapor barrier installed behind the sheetrock.
We’re going to tear out the sagging roof and have it redone within the next year. In the meantime, I plan to close out all recessed light openings. One of the things I’ve noticed is the lack of air vent channels between the insulation and the OSB. If there was a channel (in the older section of the house), it was only by virtue of not having packed the insulation to the OSB. I’m planning to start redoing the ceiling in sections, starting with the bathroom. Attached here is my plan to create a dedicated air vent using furring strips and a board of polyiso/XPS, following by layers of batting and more foamboard: https://imgur.com/gallery/OXvsYaM
I plan to eventually replicate this to the rest of the house. The challenge is that the rafters in the bathroom are only 7.25” tall (they’re taller elsewhere in the house). Here are some of my questions:
- Does inadequate ventilation, exacerbated by openings in the ceiling seem like a likely cause of the problems we’re seeing, or am I missing something else?
- Does the planned installation of air channels (using foamboard, etc.) seem like a viable options? I’m shying away from pre-made plastic baffles because they’ll need to get stapled to the OSB, which I may end up needing to replace when we re-roof.
- Is the idea of putting foamboard under the rafters, sandwiched between the ends of the rafters and the sheetrock seem silly/untenable? I’m trying to get more R-value out of the limited rafter depth.
Thanks in advance for all your input!
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Replies
That's a classic example of what happens when you put recessed lights in a vaulted ceiling. Even surface-mounted fixtures, or "airtight" recessed fixtures, can cause the same problems, they just take a little longer. You should remove the recessed lights and replace them with well-sealed, surface-mounted fixtures.
Thanks for your comment. Definitely doing away with recessed lights!
Have you read these articles?
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
https://www.greenbuildingadvisor.com/cad/detail/unvented-roof-for-cold-climate-cathedral-ceiling-asphalt-roofing
Foam baffles work if they are thin and permeable or if they are thick enough for condensation control. Also you are adding rigid bellow which might make the whole assembly into a vapor barrier sandwitch.
I would skip the foam baffles and use either fiberboard/ cardboard or house wrap for the baffles. You can also skip the baffles if you go with high density batts(fiberglass or mineral wool both work). These don't suffer from wind washing so the baffles are not needed.
The rigid under the rafters is fine. Make sure to detail it as your main air barrier (ie taped seams and taped to the poly behind the wall drywall).
You can install drywall directly through the rigid with long drywall screws or strap it out. If you strap out the ceiling with 2x3 or 2x4 on flat, it will give you enough space for those slim LED pot lights and driver without having to put a hole through the foam.
Thanks for your reply. I hadn't thought about the vapor sandwich, but I totally see that now. Thanks for the input. I think I'll go with plastic baffles (from Lowe's), which aren't air-tight, pack fiberglass under that and have 1-2 layers of foam/rigid right under the sheet rock for the added R-value, and as my main vapor barrier.
Maybe a side question, but would you know what the "minimum" R-value is for zone 5B to prevent condensation? I'm left with 7.75" of usable insulation depth after carving out the air channel. I've seen numbers as low as R-20, and as high as R-49, and it's not clear which refers to a minimum for preventing condensation, and which is a recommended for energy efficiency.
User, 619,
Minimum R-values for condensation only apply to insulation needed to keep the sheathing warm enough when there is impermeable exterior insulation. Otherwise (perhaps counterintuitively) insulation actually makes wood framed walls and roofs more prone to moisture problems by making them colder. You could argue the safest house from a moisture perspective was one with good air-sealing and no insulation in the walls or roof. It would be wildly inefficient though.
Be careful with full length baffles. Most baffles are meant for vented roofs with loose fill on the attic floor, not for cathedral ceiling application.
https://www.greenbuildingadvisor.com/question/moisture-between-insulation-and-baffle
I would only use permeable baffles or skip baffles entirely.
This is helpful, again not something I had thought of. I have baffles that are ~48" long and 14" wide. My original plan was to lap around 1-2" as I "daisy chain" from the soffit to the ridge, but based on the article you linked, sounds like I might actually need to cut these down to 12" each, and space adjacent rafters by around 1" (instead of lapping them). Do you think this setup makes the baffles "permeable" enough to prevent condensation on the underside?
One little tip I will give you - if you buy a foil-faced, polyiso board - you can score one side near the edge, break it off in such a way that the foil on one face is intact & fold it over to form the depth of the channel you were going to nail up furring strips!
Hi again, everyone,
Based on everyone's feedback (here and elsewhere), I decided to take the plunge. I opted for the ADO Provent baffles, spaced 1-2in apart. Also opted for 2+2 in of polyiso to try and claw back some of the R-value from my ceiling. I realize this isn't the "best option" with the Provents, but I thought I'd post this as a sanity check for myself, in case I'm missing something obviously catastrophic.
Progress photos in a single bay and schema here: https://imgur.com/a/nkf2YLt