How does that work logistically to provide a nail base for shingles? If you laid Rock Wool board on top of the roof deck, you would still need solid material for shingles. That’s likely more heroic than more $$ for spray foam option. Although I surely like the idea better.

@Brian, the issue with flash-and-batt in open rafter cavities is gravity. In order to perform properly, the batts must maintain good contact with the spray foam.

As it turns out, that’s pretty difficult to do without compressing the batts (keep in mind, the insulation will protrude well beyond the rafters). I would say impossible to do, but I recall seeing pictures of a beautifully done attic that involved a wire grid with stand-offs. The flash layer was done professionally but I think the R-30 unfaced batts+retaining grid was a DIY project. In a production environment, I think the labor would offset the savings over simply adding the equivalent R-value of open cell spray foam.

Knauf’s sprayed-on fiberglass product (Spider, requires no netting) used to be approved for this application but several years ago, they eliminated the glue from the recipe. Spider is now only suitable for wall cavities, according to the manufacturer.

I’d be curious to know if someone has come up with a cost-effective flash-and-batt system using mineral wool batts.

I think knowledgeable contractors must be doing this a lot, but there are not a lot of knowledgeable contractors that are easy to find for an average homeowner, at least not in southern CA. There doesn’t seem to be a way to know short of interviewing them, which is a problem if you want to get value out of any energy efficiency project.

Most recently for an air sealing project the responses ranged from “we don’t do that” to “that’s unnecessary… don’t believe everything you read online” to “what do you mean by air sealing”

I’ll let Allison quote the exact study, but essentially closed cell foam impacts heat retention in the shingles less than both the direction/pitch of the deck and also the color of the shingle. Does insulating them make the temp go higher? Yes, but not as much as coloring them black and aiming them south.

The CCSF photo in the article would be exceptionally poor construction in a cold/snow environment.

William, yes, the difference is that in a humid climate, you have to control humidity in attics encapsulated with open-cell spray foam. Here are some articles I wrote about that:

Humidity in a Spray Foam Attic

High Humidity in a Spray Foam Attic, Part 2

Can an Exhaust Fan Control Humidity in a Spray Foam Attic?

Could you please comment on what that difference in care is?

(Perhaps you have a past blog article on the subject of open cell vs. closed cell for attic insulation that you could provide as a link.)