Cathedral ceiling woes
Thanks for taking the time to read my question. My problem is as follows:
I purchased a small home in zone 5B this last December. I uncovered a significant amount of mold in our only bedroom, after smelling an intense mold/mildew scent upon our first thawing cycle. I pulled the ceiling (cathedral) and walls down to the studs and found that everything was insulated with R-19 fiberglass faced batt insulation. After remediating the mold, I am now in a position where I can start putting everything back together. My problem is the previous owner built the room as an addition, and built the room with a 2×8 ridge beam with 24″ OC 2×6 rafters. This does not allow me adequate space for appropriate insulation without spending an excessive (for my budget) amount of money on closed cell spray foam. In addition, rebuilding the roof and adding rigid insulation on the exterior sheathing is out of my budget, as well.
Current plan:
– My current plan is to keep this an unvented cathedral ceiling, with R-23 Rockwool insulation. Then an Intello Plus smart vapor retarder, 2×2 paralel furring strips, and finally tongue and groove for the ceiling.
– Walls will be virtually the same, with drywall instead of T&G.
– I happen to have an outlet near the ceiling of the closet and I am planning on putting a dehumidifier near the ceiling, and have the fan on for air movement.
My concerns:
– The roof is likely under built and I am not sure how much furring/strapping/scabbing onto the rafters I can safely do to make adequate depth for increased R-value batt insulation.
– Closed cell spray foam seems to be my only (that I know of) option to achieve adequate R value in the ceiling with current rafter depth. For me, this is cost prohibitive and I have concerns of longer term off-gassing.
– If I add a ridge vent and soffit vent system to this with inadequate insulation (R-23 Rockwool w/Intello Plus smart vapor retarder) am I going to improve the situation or just going to have continuous mold/other issues with the setup?
In closing, my question to the people here is: Is there any other budget friendly solution that you may have that you can share? What effect is my current plan going to have? Will having a dehumidifier going in the room, near the ceiling have a desirable impact?
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Replies
While it’s true that you won’t have condensation if you are able to get your air sealing perfect I would be very reluctant to rely on that completely especially in a renovation where it can be very difficult to get your details right. I would lean towards adding some sort of ridge and soffit vents and venting your roof by installing the cheap cardboard baffles 1 to 1-1/2” below sheeting and furring out your rafters to provide appropriate thickness for your r-23.
Cannot comment directly on the structural situation with your rafters without seeing it, but the added weight from furring it out a couple inches should not be very significant - if you’re in an area with snow load that would be much more of a concern. Ideally you would get a professional to look at it before covering it back up; if there is any visible sagging I would definitely recommend you do so. I would just say that usually a vaulted ceiling requires a structural ridge, and 2x6 is almost certainly not appropriate for that.
JoeGro,
I appreciate your reply. For some reason, I missed your reply initially. I did edit my question to reflect that it is a 2x8 ridge beam, NOT a 2x6 like originally posted. I am assuming at this point, with the direction of multiple people that venting of my roofing is necessary. I have someone coming to take a look at the roof and construction Thursday and will likely be asking them a lot of the technical questions, in terms of what is appropriate for our area.
Thanks again!
Palmer,
Without taxing the budget limits you have, I'd suggest:
- Fasten 2"x3" furring on edge to the bottom of the rafters. With a 1" vent space separated from the bays by house-wrap baffles, that leaves you enough space for R-32 fg batts. If you run the 2"x3" parallel to the rafters it will increase not diminish the structural strength of the roof.
- Skip the Intello and instead sheath the underside of the rafters with 1" of foam before dry-walling. Build your baffles out of house-wrap for good drying potential into the vent space and you should have no mold problems.
Malcom,
Thank you for your prompt answer. For clarification (as I am new to this world), The house wrap baffling should extend from the wall plate all the way to the ridge vent? I assume the wrap I use should be vapor permeable?
As for the foam sheathing, should I be looking for a faced type foam board and tape it? and does it matter whether it is EPS, XPS, or polyiso?
Again, thank you for your time answering my questions!
Palmer,
First off - I sort of skipped over your initial question. Keeping the assembly unvented with only permeable insulation is very risky. With no way to remove moisture, mold and rot are almost inevitable.
- Yes the house-wrap should create a clear path for ventilation air from the eaves to the peak. Common house-wraps like Tyvek are very vapour-open (around 77 perms).
- You can use any foam. Pick it for the R-value per inch, or by cost.
I'd wait a bit and see what others suggest before committing to this assembly. Lots of good ideas come from posters here.
Thank you, Malcom, for the follow-up. This is likely the way that I will be re-installing my ceiling.
Thanks again for the help!
Palmer,
Good luck with your project!
I would prefer foil faced polyiso under the rafters here for two reasons:
1- Highest R per inch, which is more important when you're in a tight space and want to achieve the max possible R value since you're already light on R value here.
2- Easy to tape foil facer, which is going to give you both an air AND vapor barrier, both of which are helpful here to keep moisture from getting up into your ceiling/roof assembly.
I would add the vent space along with soffit and ridge vents. Adding ventillation will help to prevent moisture accumulation and thus will help to prevent mold growth. If you want to build this as an unvented assembly, then closed cell spray foam is really your only safe option. Vented roof assemblies are much more forgiving, and also more robust in terms of moisture resistance.
Bill