Recommendation for unvented cathedral ceiling retrofit from outside
I have a 1980’s cathedral ceiling section (2×12 raftered) of my home with soft sheathing problems. Hence, I need to remove the sheathing and re-roof. Most recommendation are to spray foam against the sheathing, but this requires an additional interior drywall removal and the interior mess and disturbance I’d prefer to avoid. There are also recommendations for large 6+ inch of rigid foam to the outside of the sheathing, this causes difficulty with the home appearance at the eave and problems with adjacent roof lines that aren’t cathedral. So is there a recommended acceptable practice for sheathing removal, close cell foam spray against the backside of interior drywall,(thickness TBD), then open cell foam or batt insulation (thickness TBD), and possibly ~3/4″ rigid foam before the 7/16″ sheathing & asphalt shingles?
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First things first--what's your climate zone and/or location?
Assuming it's somewhere that gets cold sometimes, I wouldn't have confidence in the plan you suggest. The thing that makes spray foam OK is having it air tight against the roof sheathing. You won't have that.
What you could do is add a vent space between the top of the insulation and the sheathing. If that was 1", you'd still have 10.5" of insulation. Depending on your climate, you might want more than that, but you'd said you'd like to avoid raising the roof significantly. If you could tolerate another 3", that might be worth it.
Given that this is being asked on Green Building Advisor, you probably care at least some about your global warming impact. If that's the case, you should avoid spray foam except as a last resort, because the blowing agent used to make the bubbles in it has a global warming impact drastically worse than that of CO2. See https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-global-warming-impact-insulation The good news is that with a vent space above the insulation you can avoid needing to use spray foam.
For more information, see https://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling
This is a great reference and case study on what you're looking to do.
http://www.nrel.gov/docs/fy13osti/56145.pdf
The sanest (and cheapest, and most DIYable) choice here is probably going to be adding ventilation channels under the new sheathing like Charlie suggests, filling what remains of the cavities with mineral wool batts, then applying a smart vapor retarder membrane (detailed as your air barrier), and then covering that with the ceiling drywall. You could also thermally break the rafters by putting a few inches of rigid mineral wool or foam between them and the smart vapor retarder if you're willing to lose a few inches of ceiling height.
David,
The short answer is: Yes, you can install spray foam from above, against the drywall ceiling. Your R-value goal will depend on your climate. If your goal is R-49, and you intend to install closed-cell spray foam, the goal can be reached with 7 1/2 inches of spray foam.
Some green builders will criticize this approach because closed-cell spray foam has a high global warming potential. Others will note that you will still have thermal bridging through your rafters. However, there is no denying that this approach works: Your new roof will not suffer sheathing rot in the future (unless you get a roof leak), and your roof assembly will be far, far better insulated than it used to be if the roof was formerly insulated with fiberglass batts.
Since everyone has to make compromises in a retrofit situation, and all solutions are imperfect, this isn't a bad approach.
It seems that don't have a clear consensus on whether installing the spray foam from above, against the drywall, is OK from the perspective of protecting the sheathing from moisture. Martin is more knowledgeable and experienced than I, so his saying it's OK carries more weight than my saying I wouldn't be confident in it.
But maybe we can reach more of a consensus by talking through the reasons why it might or might now work well.
That design has little air pockets between the foam and the sheathing. If there were a path for warm humid inside air to get into that space, you could have trouble with condensation on the sheathing without any way for it to dry. That's unlikley, because the foam should seal any possible leaks from the inside to the air in that region. As long as that really works, you'll be fine. One of the advantages of spray foam is that you tend to end up with good air sealing even without thinking about it. But that's not always true--I've seen several examples where there were substantial air leaks in a fully spray foamed envelope because the detailing where two facets met didn't quite work right. So you'd want to make sure that the junction between walls and roof and penetrations like chimneys didn't leave opportunities for air from inside the house or inside the wall to get above the spray foam. If you get those details right, then it should work fine, and you are left with just the cost and global warming disadvantages to consider.
Charlie,
In general, spray foam installations like this work better with an air space between the top of the insulation and the roof sheathing, for several reasons. Small amounts of moisture in the sheathing can evaporate to the air space and can be absorbed by the rafters, and that can allow for hygric redistribution, lessening the chance of moisture problems. And any future sheathing repairs are much, much easier.
Of course, paying attention to air sealing, and avoiding air leakage paths, is important no matter what type of insulation you are installing.