Chris: No, closed-cell foam isn’t a bad idea, not from a building science perspective anyway. Because of the upfront carbon emissions (https://bit.ly/3IsAcHh), though, it’s not great for the atmosphere. For cold climates, closed-cell foam is the way to go if you’re going to use spray foam. Here in the South, open-cell foam works well, but, as you said, you’ve got to have a way to condition the attic to deal with the humidity.

Gene: I’ve seen roof leaks in regular attics that went undetected because the leakage was small enough that it didn’t wet the ceiling. I’ve also slipped on my bathroom floor because of a roof leak. It came through my open-cell spray foam and landed on the floor not far from where the leak was.

The truth is that if the leak is bad enough, you’ll find out about it one way or another. With closed-cell spray foam, the water may come out somewhere farther down the roof. But it’s got to come out somewhere. Regular inspections of your roof, including the gutters, fascia boards, and soffits, is essential no matter where the insulation is. And as I’ve learned with my house (https://bit.ly/3A8UMJ5), the flashing around roof penetrations may not last as long as the shingles.

I’d like to hear Allison’s point of view on your question as well Gene. I’ve had that same question asked of me more times than I can recall in the past. I’ve debated with contractors and homeowners regarding the roof leak dilemma.
To be even more specific with the question, closed cell or open cell on the underside of the roof deck? Is closed cell a bad idea on the roof deck as so many claim? That takes Gene’s question to another level. Which do you prefer to see on the underside of the roof deck, and why?
I believe that climate zone and outdoor humidity introduces another variable when deciding which one to use. Dehumidification of the attic is highly recommended in the humid southeast where I am. Especially with open cell. It’s not a vapor barrier. That’s another topic of discussion though. Let’s stick to the roof deck and roof leak debate. Let’s attempt to keep it brief. Not my specialty as you know Allison!
Thanks!

Arthur: Yes, there’s certainly a lot of room for improvement, and making building enclosures more efficient should be done along with electrification. Interesting point about the “marbling.” I’ve seen it a lot and just chalked it up to old, dirty, compressed insulation. (Just to clarify, though, that photo is from a different house, not the one with the frosty attic.)

Good point about attic ventilation in wildfire country. That’s a huge issue now.

Abercrombie’s photo with two yellow arrows also shows us the subtle evidence of this with the dust deposits, appearing to marble the fiberglass and powder the blown product. We often see this marbling with on the wrapping of our duct systems, especially returns.

The Habitat, raised heel truss baffling is brilliant. Our local fire marshal urged us to use a fire rated product for this exercise as wind driven embers can get trapped in the cardboard and defeat our other fire proofing steps.

Thanks again
Arthur Beeken