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How to fix an attempted insulation retrofit on a ’70s cathedral ceiling?

Gregerathome | Posted in Energy Efficiency and Durability on

I discovered this website 6 months too late!

After losing heat through a 70’s era cathedral ceiling built on 2 x 8 rafters for more years than we liked to count, we decided to replace the roof, sheathing, and insulation in one operation. The area of the hip roof was bid at 2,230 sq. ft., the cathedral ceiling comprising 2,040 sq. ft., leaving a small attic space above bathroom and hallway areas under the peak.

The building contractor proposed a couple of options to gain R-value, but the real challenge was in providing an airtight ceiling – cedar T&G had been applied directly to the bottoms of the rafters when the house was constructed. The boards were butted to the wallboard on the perimeter of the interior walls and trimmed with molding, but no vapor barrier or other attempt at making a contiguous, sealed, ceiling was evident.

We chose to employ two layers of 3″ hard foam (foil faced) placed in the bays, giving us an R-38 ceiling. Not as high a value as I would have liked, but better than the R-19 (maybe) we had previously, plus, we couldn’t really afford to completely rebuild the entire ceiling and roof structure/facia/gutters to get more. A great amount of ‘pink’ foam was used to seal the T&G at the perimeter of the wall and ceiling joint, plus each foam block was sealed against the sides of each rafter bay and to each other. The contractor assured us that we would achieve an ‘air-tight ceiling’.

The roofing was nicely done on a 4:12 pitch, incorporating continuous ridge venting along the long hip ridges and a short ridge vent on the top ridge over the small attic space. There is also continuous venting around the eaves.There were also three skylights in the cathedral sections. These clearly blocked the airflow in their respective rafter bays, but cross bay vents were planned. The space between the insulation and the underside of the roof sheathing was minimal (and I suspect not consistent).

The house has been more comfortable this winter, but after a recent week of sub freezing weather (but no snow – unusual for Seattle), we had a sudden rise in outside temperature – hi 40’s during the day – resulting in multiple drips in the the living spaces, no doubt the result of the melting of frozen condensation on the underside of the sheathing. The contractor is being pretty slow at responding to my request to mitigate the problem and I am at a loss to envision an expedient way to solve this problem.

My question is this: Is there now any way to positively create an air-seal for my existing T&G ceiling? Do I need to remove the existing T&G or can I build it up with a new membrane and new paneling. If that were done, can we ignore the inconsistent air gap?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm sorry to hear about your problems.

    Your contractor chose the "cut-and-cobble" method of insulation, which is not recommended for cathedral ceilings due to the risk or air leaks. Unfortunately, you are now experiencing problems due to air leakage.

    For more information on the cut-and-cobble method, see Cut-and-Cobble Insulation.

    Your contractor made two other errors, unfortunately: he tried to created a vented roof assembly on a roof with a configuration that is not conducive to that choice (because your roof has hips and three skylights); and he failed to install an air barrier on the interior. Both of these issues are serious.

    For more information on the proper methods for insulating a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.

    Now you are stuck with a leaky ceiling with very limited R-value (because the air leaks are bypassing the cut-and-cobble foam).

    Here's the bad news: to fix this ceiling, you'll need to install at least two layers of rigid foam on the interior side of your tongue-and-groove ceiling. These layers of foam should be installed with staggered seams, and should be installed with careful attention to air sealing. You'll then need to install strapping and a new finish ceiling.

    Personally, I recommend that you install a drywall ceiling. But if you like tongue-and-groove boards, you can install a new T&G ceiling. But before you do, make sure that you have a bulletproof air barrier (verified by the use of a blower door).

    Since your current ceiling assembly has failed, an argument can be made that your contractor is at fault. It's up to you to decide whether to proceed by negotiating or by taking your contractor to small claims court. I would start by negotiating, if I were you.

  2. Gregerathome | | #2

    Thanks for your excellent assessment and recommendation. Application of a contiguous foam thermal break either inside the roof, as you describe, or on top of the sheathing as described in the How to insulate a cathedral ceiling article never came up in my discussions with the contractor. That is the next step, and undertaking an install from inside the house is the obvious choice as the new roof and solar PV panels were installed after we thought the roof had passed inspection (and a few rains).

    I do have one question that I can't fully understand in this debacle. Why didn't we experience the condensation with the previous fiberglass batts? The ceiling should have been even more of a leak factor before the installation of the ridged foam insulation – we'll see how much more after a second blower door and IR camera test next week. Thanks again for your comments.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Why didn't we experience the condensation with the previous fiberglass batts?"

    A. Clearly, one of the changes made by your contractor made the situation worse. Without a site visit, it's hard to guess what factor cause the degradation in the roof's performance, but I'll hazard a guess.

    I'm guessing that your contractor's decision to provide "continuous ridge venting along the long hip ridges and a short ridge vent on the top ridge over the small attic space" tipped the roof assembly into failure mode. As I have pointed out in the past, adding ridge vents often causes problems -- because the air that escapes from the ridge vents pulls warm, moist indoor air through ceiling leaks into the roof assembly, where the moisture can condense.

    Roof venting is a double-edged sword: it can sometimes help, but it can also cause lots of problems. For more information about this issue, see All About Attic Venting.

  4. Gregerathome | | #4

    A year has passed, without further incident of condensation, though I expect this was due to a relatively warm winter. There have been two IR camera sessions, one with a blower door, the second with cold temps out side (low 40's). Both sessions revealed air (and moisture??) areas behind the T&G in the ceiling, and in one location, a significant area of air leakage into the wall cavity below the area of the most predominant ceiling distress. You pointed out the detrimental effect of ridge venting on this project and it was further explained to me by a building envelop engineer that the ratio of eave venting to ridge venting was too low (too much at the ridge compared to amount of vent at the eaves. While debating which is going to be the preferred method of attacking the overall solution – more insulation above the roof sheathing or more insulation and a complete air seal applied to the interior space – we have now discovered that the insulation at the eaves ia actually cut off the flow from the eaves, The thought now is to take off the roof, remove the foam insulaition between the rafters (2 x 8)
    s and replace with compressed fiberglass batts, filling the cavity completely and eliminating the vents and at the eaves and the hips and ridge. Then add plywood sheathing, 3 inches of foam, followed by another layer of plywood, and weather barrier. finally reinstall a roof, possibly metal this time around.

    After reading the details of creating a "Thick Roof' on the GBA site, and the admonition to provide a semi permeable system of vapor control from the batting to the living space, the big question in my mind is weather we can leave the ceiling T&G in place as is and just concentrate on the perimeter seal with the walls?

    Thanks in advance for your advice.

  5. GBA Editor
    Martin Holladay | | #5

    I agree with your "big question." Your proposed solution will raise the temperature of the lowest level of roof sheathing. This approach should make condensation impossible -- if the work is performed perfectly and there are no air leaks.

    The problem is the lack of a ceiling air barrier. If any of the roof sheathing is cold (due to construction imperfections), the warm interior air still has contact with the roof sheathing -- which isn't ideal.

    One approach would be to go forward with the proposed solution and cross your fingers. If the roof assembly performs well, you're done. If there are condensation problems, you would have to retrofit an interior air barrier by installing drywall.

  6. user-659915 | | #6

    Ah, the 70's cathedral roof with t & g ceiling. Possibly not the absolute worst idea in residential building history. But close.

  7. user-1072251 | | #7

    Your proposed solution of adding exterior foam on top off the rafters is the best way to salvage the cedar ceiling, provide a continuous air barrier and increase the R value. I don't understand the purpose of removing the existing foam and installing fiberglass; why not simply leave the foam in place? To the extend that it is not vented it is acting as insulation. You might need to seal the edges from above - which should be pretty straightforward. If you feel you must remove it, insulate with cellulose or rock wool - both of which work better than fiberglass.

  8. Gregerathome | | #8

    I was unsure on how to treat the existing gap between the top of the hard foam and the underside of the plywood sheathing – hence the removal of the sheathing and the hard foam now in place. Doing so, we can ensure the joints at the bird-blocking, rafters, and top wall plate would be completely air tight. I read that there is now a spray on 'caulking' that should be perfect for this method. This would also provide the opportunity to make a good seal between the perimeter of the T&G ceiling and the wall junctions. All done from above and eliminating the need to tear out the ceiling – which is prevalent in the entire house – thus also eliminating the need to move out for the duration of the project. I should add that we plan to re-use the foam we remove from the rafter bays 2 layers of 3" foam and apply it to the top. but possibly only one 3" layer. Thanks for the tip on use of rock wool or cellulose alternatives.

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