Spray foam cathedral ceiling
We have a 1-1/2 story Victorian with sloping ceilings on the second floor that lead to a flat ceiling with small attic above. We live in climate zone 4C. I mentioned to an architect friend that I was planning to spray, or have sprayed 1″ – 2″ of closed cell foam to the underside of the sloped ceiling portions of the roof deck and use some sort of “fluffy” insulation between the foam and sloped ceiling. I will conventionally insulate the flat floored attic and will run the spray foam up the underside of the roof deck to the intended level of the top of the attic insulation. He told me that in the UK, lenders are denying mortgages to people wanting to buy homes with roofs insulated with spray foam of any type on the underside of roof decks. Why? And am I making a bad choice? I do not want to create an air channel between the deck and insulation under, nor do I want to tear off a new $30K roof that I had recently installed to retrofit continuous insulation above the roof deck.
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Replies
Spray foam in the UK was typically open cell, their roof designs are also a bit more moisture permeable than our roof designs - this combination resulted in moldy disaster. Open cell roof decks only work in very warm climates, preferably dry warm.
You should ask for a minimum of 2" of closed cell everywhere just to be sure. 1" would make me nervous in zone 4.
Other than that, your roof will be fine. Make sure they use a synthetic underlayment if theyre doing the roof at the same time, and maybe consider rigid foam between your roof deck (plywood) and shingles - that will cut thermal bridging and reduce any risk associated with spray foam being a bit too thin.
arioda,
I’m not a spray foam enthusiast, but wouldn’t take anything from the experience in Britain for a couple of reasons.
- Their typical house construction is very different than that in North America, and frankly betrays a lack of understanding of moisture issues I find surprising.
- In the stories I have read about the issue, they put down the failures to poor installation, the wrong foam, and a fundamental misunderstanding of what they were trying to achieve.
Make sure you meet the ratio of foam to permeable insulation for your climate zone, and include a good interior air barrier / vapour retarder. See assembly #5 in this link:
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Many of the problems in the UK, as suggested above, were associated with open cell foam. That's happened here- as have problems from poor workmanship. Installers aren't created equal, you can stack the cards in your favor by insuring the techs are paid by the hour, not the piece.
That said, your proposed assembly could be tricky and problematic. Extend the CCSF fully up the rafter bays and envelope the ridge. 1 inch isn't enough. Not sure what the appropriate ratio of non-permeable insulation is for your climate zone, but you're way off. Regardless, CCSF will outperform the permeable stuff, you should shoot for 50% CCSF and finish with 50% OCSF (batting is difficult in a roof assembly when paired with CCSF, OCSF is much easier).
If you don't carry the CCSF through, you'll have to vent the upper portion of the roof (nearly impossible at this point) AND contend with moisture at the ceiling level by incorporating a vapor retarder into the CCSF.
Edit: You can even strap the inside edge of the rafters with 2x4 at 24" centers. Have the OCSF carried through to this plane- you'd greatly reduce thermal bridging and increase cavity depth/ r-value of the insulation.