How should I air-seal and insulate a tongue-and-groove cathedral ceiling?
Six months ago, I bought a 1964 ranch house in Northeast Ohio (climate zone 5). The house has a cathedral ceiling throughout, with tongue-and-groove boards attached directly to the roof joists. There are currently old fiberglass batts up there for insulation, right on the tongue-and-groove boards, with some space above the fiberglass for air to pass through (we have large continuous soffit vents on the front and back eaves, and a continuous ridge vent).
In the summer months, we had problems with large amounts of condensation forming at the ridge of the ceiling, dripping down from the center ridge beam and dripping down the walls. Now, in the winter, there is some evidence of condensation forming (water marks on the ridge beam), but I haven’t seen actual condensation since the weather has been cooler. We’ve also had bad air quality issues, and we feel this is likely related, as all that moisture would be conducive to mold growth in the ceiling.
We feel pretty confident this is all due to the leaky tongue-and-groove ceiling (there are some visible gaps between some of the boards, and I’m sure there are plenty of smaller gaps as well). The poor amount of insulation probably isn’t helping.
We’ve had several recommendations for how best to air-seal the ceiling and insulate. Some contractors have recommended using a plastic vapor barrier, and putting rigid foam boards and/or closed-cell spray foam on top of that, but I understand that the vapor barrier could trap moisture above the ceiling boards, leading to mold and rot. Someone recommend putting rigid foam boards on top of the ceiling boards, then covering with closed-cell spray foam, leaving room for air to pass above the insulation, but I’m curious whether the rigid foam boards could have the same problem as a plastic vapor barrier, trapping moisture between them and the ceiling boards.
Several people have recommended getting rid of the ventilation altogether and dense-packing the ceiling. One recommendation was to seal up any gaps in the ceiling with a low-pressure spray foam, then cover with a layer of closed-cell spray foam, and fill in the rest of the space with open-cell spray foam. But I’ve head that open-cell spray foam can trap moisture and lead to roof rot.
It seems like the safest bet would be to fill the entire space with closed-cell spray foam, but is that even possible? I assume this would also be very costly.
Closed-cell spray foam directly on the ceiling would make it difficult/impossible to replace the ceiling boards, if we ever wanted to do that, but that’s a sacrifice we might be willing to make. In that case, maybe it would be best to put closed-cell spray foam directly on top of the ceiling boards, while leaving some space above the insulation for ventilation purposes?
We’re feeling overwhelmed with the number of different recommendations we’ve had, and would love some guidance. Mold and air quality issues are the biggest concern, so we’re looking to keep moisture to an absolute minimum. Any help would be very much appreciated! Thanks in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Albert,
Your current ceiling assembly is a disaster. You have no air barrier, so conditioned indoor air is rushing through your ceiling cracks and out your ridge vent. Your heating bills must be very high.
The fact is, there are many ways to address this problem.
If you want to address the problem from the interior, you'll need to install a thick layer of rigid foam on the underside of your existing T&G boards. Since the building code requires R-49 in your area, you'll need about 8 inches of polyisocyanurate or about 12 inches of EPS. That's a lot of rigid foam, and it would be somewhat daunting to attach that much foam to your ceiling.
It's probably easier to address the problem from the exterior. If you can open up the roof from the exterior in sunny weather, you can fill the rafter bays with closed-cell rigid foam. To make sure that the foam doesn't ooze through the ceiling board cracks, you would probably want to first install strips of cardboard in each rafter bay. If you go this route, you'll need about 7.5 inches of closed-cell spray foam.
Of course, this method requires you to temporarily remove the plywood or OSB roof sheathing, and to install new roofing when the work is done.
A third approach would be to install SIPs or nailbase (rigid foam with OSB on one side) on top of the existing roof, followed by new roofing.
None of these approaches is cheap, but your house is a disaster. You have no air barrier and virtually no ceiling insulation. Fixing these problems is a big job.
In New York State, the sellers could be on the hook for not disclosing your issues.
Solutions. Martin can point you to his blogs on topic.
Buiding Science Corporation much worth reading for you and your contractor. Get a contractor that has done this work before and get proof!! Go see his work and talk to the customer. Don't just hire the contractor that you like his mojo.
http://www.buildingscience.com/@@search?SearchableText=cathedral+ceiling
Lowest cost solution;
Do as much labor as you can.
Pull the T&G down carefully., Cut off any broken sections, toss and save the rest with nails pulled out of the way.
Check the insulation and the frame next for water and mold.
TSP for mold, some use dishwasher dry soap mixed in warm water. Scrub. Small amount of mold you can do. Lots of mold you hire a mold remediation company with proper credentials.
So now you are clean and dry. Next for me is standard vent channels from big box stores.
Then insulate as much as will fit in space.
Then add 2" rigid foam foil faced and tape the seams. seal it to the ceiling perimeter well.
Then you can put the T&G back or you can put a layer of taped drywall over the foam to improve air barrier.
if the ceiling is older pine natural, I use stain such as Golden Oak to match new T&G to old.
You may have moisture issues too.
Basement damp
Gutters
Exterior drainage from home
Running a humidifier too much
Lots of plants to water
Showers taken without exhaust fan
Etc.
The ceiling redo if you do it with minor skilled carpentry on the side could be accomplished for one to two thousand.
If I did it, and did my best work, figure twice that. based on 1500-2000sqft basic home.
Martin,
We will likely be addressing this from the exterior. The roof has some years on it and has had some leaks, so we expected to replace it sooner than later anyway. Your second approach sounds good -- just a couple questions:
- Could those cardboard strips result in any moisture/mold problems?
- This would still be a vented ceiling assembly, correct? 7.5 inches of closed-cell spray foam would leave a bit of room for air to pass through.
Thanks!
AJ -- thanks for the info.
The outside way is fine, much more cost, probably will do a great job if done right during a warmer season.
Albert,
The cardboard strips won't cause any problems.
I provided advice to a homeowner who contracted for the same type of work I described to you -- pulling the roof sheathing off, inserting cardboard strips, spraying closed cell-spray foam from above -- and I was a little nervous. But everything went very well. The performance of the roof assembly went from lousy to excellent. There were no odors.
Needless to say, you want to choose a good spray foam contractor with an excellent reputation.
Closed-cell spray foam doesn't fill the rafter cavities, so there will be an air gap on top. But closed-cell foam doesn't require a ventilation channel. Vented or unvented -- either way works.
"But closed-cell foam doesn't require a ventilation channel. Vented or unvented -- either way works."
Can you expand on that? I've heard it before but never got it. My understanding is the reason roofs need to be vented is to keep the shingles and sheathing from getting too hot. An asphalt shingle in the direct sun can get very hot, a layer of air beneath it allows some of that heat to dissipate. When the roof gets hot either the asphalt shingles themselves deteriorate, or the sheathing delaminates and the nails come out. Either way you lose your roof.
I don't see how closed cell foam on the sheathing keeps the roof cool.
The ventilation channel is to help the sheathing dry out if (when) interior moisture makes it to the sheathing. The closed-cell foam prevents the interior moisture from reaching the sheathing (or at least reduces the quantity of moisture enough that it doesn't cause a problem).
It's a "Does the rate of wetting exceed the rate of drying" type problem. Ventilation increases the rate of drying. Closed-cell spray-foam decreases the rate of wetting.
As far as concerns about the heat and the asphalt shingles, you could always double check that the manufacturer is still willing to warranty the shingles for the same amount of time when installed over an unvented assembly. My understanding is that asphalt shingles work fine over an unvented roof.
I've never heard roof ventilation described that way. Just for kicks I Googled "GAF Timberline ventilation warranty" (GAF being perhaps the most popular line of shingles) and this was the first hit:
"What is NOT covered by my GAF Warranty?
Any conditions resulting from anything other than an inherent manufacturing defect in the GAF Products (or their Misapplication if you have a Golden Pledge or Silver Pledge Limited Warranty) are NOT covered. This includes, but is not limited to, failure to install adequate ventilation. Please be sure to discuss ventilation with your roofing contractor prior to the installation of your new roof. "
The IRC certainly seems to approach roof ventilation from a moisture management standpoint: https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction Here are some snippets that talk about either reducing wetting via vapor retarders or increasing drying via vapor diffusion ports:
R806.5
...
4. In Climate Zones 5, 6, 7, and 8, and air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.
...
5.2 In Climate Zones 1, 2 and 3, air-permeable insulation installed in unvented attics shall meet the following requirements:
5.2.1 An approved vapor diffusion port shall ve installed not more than 12 inches from the higherst point on the roof...
---------------------
On the GAF shingles note, it looks like they hedge and say that they allow installation on unvented assemblies, but if the damage is caused by a lack of ventilation, they don't cover it: https://www.gaf.com/en-us/document-library/documents/documents/warrantiesamptechnicaldocumentsdocuments/steepslopetechnicaladvisorybulletinsdocuments/englishbulletinsdocuments/under_deck_sprayedinplace_foam_insulation__tabr2011135.pdf
For what it's worth, it appears that CertainTeed will warranty their shingles even when installed over an unvented sprayfoam assembly, but they reduce the length of the warranty: https://www.certainteed.com/resources/GeneralAsphaltShinglesWarrantyEnglish.pdf
So among shingle manufacturers, opinions seem somewhat divided?
Maybe someone with more years of field experience than me can chime in if they've observed shingles being damaged due to heat build up in an unvented assembly. This BSC article about unvented asphalt shingle roofs seems to focus entirely on moisture management and doesn't seem to mention the heat issue as being a problem in their mind (I only skimmed it, so I might have missed something): https://www.buildingscience.com/documents/building-america-reports/ba-1409-field-testing-unvented-roofs-asphalt-shingles-cold-and
Thanks Martin -- that is very helpful!
Martin, I'm considering a very similar operation for my stained pine T&G cathedral ceilings -- that is, remove the exterior roof sheathing and use closed cell foam against the backside of the tongue and groove.
My T&G is tight and my contractor says he has not had problems with foam oozing through, but I'm nervous. Why do you recommend cardboard against the T&G? Would poly plastic sheeting work just as well?
Thank you,
Eric
>"Why do you recommend cardboard against the T&G?"
I can't speak for Martin, but I'd want to retain the ability to replace/repair the finish ceiling, which becomes a lot more difficult to do when the whole assembly is glued together with spray polyurethane foam (which has a very similar chemistry to Gorilla Glue).
Going for a higher-R with closed cell foam between rafters is the opposite of "green", since the material itself (even with lower impact blowing agents) has a substantial carbon footprint, and the potential performance is being robbed by the thermally bridging framing. See:
https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
Using any more than necessary for air sealing or for dew point control isn't the best use of funds. A 1.5-2" shot of closed cell foam forms a Class-II vapor retarder (would qualify as a "vapour barrier" under Canadian code), which would be sufficient to control moisture movement with the rest of the cavity filled with open cell foam (at 1/4 the footprint per inch of depth of closed cell foam).
You can staple some housewrap to the top of the T and G boards and then spray foam against that. The housewrap provides a barrier so that the spray foam doesn’t adhere directly to the boards. If you ever need to take the ceiling apart, the staples will just pull through the house wrap leaving the housewrap stuck to the bottom of a block of spray foam. Make sure the joists are exposed and not wrapped with housewrap though since the spray foam needs something to adhere to for support in case you do remove the T and G ceiling at some point.
I have actually done this, but with a wall and not a ceiling.
Bill
If going to go from the top, you might consider how you'll handle the service channel needed for lights, especially if there's any chance you'll want to change their location in the future. If I understand Martin's suggestion correctly you will encapsulate any wires there in the foam and there won't be a way to run new wires in future. So you'd be limited to surface mount if any new locations were needed.
Working from the bottom would enable you to add a service cavity (ie criss-crossed furring strips) after the air-barrier and before your final surface (either new T&G or drywall).
Or maybe the cardboard to be inserted (that martin is referring to) would/could leave a wiring chase?
I assume the t & g is exposed as the interior ceiling. I would start with a liberal coat of urethane or paint to seal up interior cracks then go from there. A ceiling fan may also help.