I just bought my first home, which is a 50-year-old time capsule.  It is single-story, wood-frame, on a crawlspace, with a full brick veneer.  Other than 0.5-inch foam panels behind the brick, there isn't a lick of insulation anywhere.  I can't think of any better option than spray foam, which will go under the subfloor, under the roof deck, and inside the exterior walls (the gyp board is coming out for many reasons).  My initial plan is to use closed-cell all around, for 3 main reasons:
1. Everything is 2x4 framing, so application depth is limited.
2. Closed cell will give me some extra points in the green certification scoring to make up for some difficult-to-achieve subcategories.
3. Closed-cell on the roof will likely be the only way I can achieve the Fortified Home certification and will possibly be crucial to meeting the hurricane mitigation criteria that will save me $100/yr in windstorm insurance premiums.

Excluding the opinions of those whose subject-matter expertise I question (SPF is very uncommon in my area), about 1/3 of the pros jump up and down screaming that closed-cell foam should never be used on a roof deck because it rots the decking if water gets between it and the shingles, 1/3 who say that this is a "possibility" and they require a warranty waiver to be signed, and 1/3 say that it's not a problem other than overkill for my climate.  The total survey population consists of 5 foam application contractors, 2 residential builder/GCs, and 2 energy auditors/engineers.

What's the deal?  This is the first time I am hearing about closed-cell being problematic (aside from increased cost and ventilation requirements).  I am willling to revise my specs, but I'm also willing to deal with a much more finicky building than the typical Joe.  Does anyone think some of this is due to latent paternalism (i.e., "We don't talk about the gold-standard solutions with residential clients because they are all too ignorant and cheap to handle such complex ideas as mass-balance equations and preventive maintenance requirements."  I have been dealing with that attitude with about 80% of the vendors I contact.

Also, if I am fundamentally mistaken about SPF being the best insulation option, please share alternatives.

I used closed cell foam in my new home and also used it to encapsulated the attic. Its great to go in the attic in August and it's only about 3 degrees warmer or in the winter and the opposite is true.

Closed cell foam is the opposite of "green" on many levels, so how does it manage to score any green certification points?



In a 2x4 cavity the performance difference between 3.25" of closed cell vs. 3" of open cell foam adds up to about R1 in "whole wall-R" performance, at best a 10% improvement, using nearly 4x as much polymer, and typically blown with high global-warming potential HFC245fa (about 1000x CO2) compared to water, the blowing agent for open cell foam.

The structural benefits of having 3" of closed cell foam at the roof deck may be "worth it", despite the only modest thermal performance improvement. If going that route there are a few low global warming factor HFO1234ze blown closed cell foams out there. (eg. LaPolla: https://www.lapolla.com/spray-foam-insulation/foam-lok-2000-4g/ ) as well as some 2lb water blown foams from Icynene (at slightly lower R/inch), such as MD-R-200 & MD-R-210, each at about R5/inch.

The hysteria about roof deck rot due to the inability to dry toward the interior is not well founded, but if you used a more vapor permeable 2 lb foam such as MD-R-200 that point would be moot, since it's still only a Class-III vapor retarder (1.3 perms @ 3" of thickness), where most HFC blown 2lb foam is deep in to Class-II vapor retardency at that thickness.

To reach code-min (R38) thermal performance for the roof you would need more than just 3-5" of 2lb foam, but the better approach would be to use rigid foam on the exterior when it's time to re-roof. With ~3" ( R15-R18) of foam between 2x4 rafters you can hit code performance on a U-factor basis with as little as 3" of roofing polyiso under a non-structural nailer deck, or 3.5" polyiso nailbase panels through-screwed to the structural roof deck.

Unless you're on a flood plane it's usually cheaper/better to insulate the crawlspace at the foundation walls than at the subfloor. In zone 2 you can take advantage of the favorable deep subsoil temperatures by not insulating between the floor and subsoil, just a modest amount of insulation at the crawlspace walls will "earth couple" the house to the thermal mass of the subsoil. IRC code doesn't demand ANY insulation on crawlspace walls in zone 2, but an inch (or two) of closed cell foam on the walls combined with a heavy ground vapor barrier air seals sufficiently, and would be a lot less foam than doing the entire subfloor. Insulating at the subfloor code would demand R13 which is more foam yet provides somewhat lower net performance.

Posted By Dana1 on 27 Feb 2017 04:54 PM
Closed cell foam is the opposite of "green" on many levels, so how does it manage to score any green certification points?



In a 2x4 cavity the performance difference between 3.25" of closed cell vs. 3" of open cell foam adds up to about R1 in "whole wall-R" performance, at best a 10% improvement, using nearly 4x as much polymer, and typically blown with high global-warming potential HFC245fa (about 1000x CO2) compared to water, the blowing agent for open cell foam.

The structural benefits of having 3" of closed cell foam at the roof deck may be "worth it", despite the only modest thermal performance improvement. If going that route there are a few low global warming factor HFO1234ze blown closed cell foams out there. (eg. LaPolla: https://www.lapolla.com/spray-foam-insulation/foam-lok-2000-4g/ ) as well as some 2lb water blown foams from Icynene (at slightly lower R/inch), such as MD-R-200 & MD-R-210, each at about R5/inch.

The hysteria about roof deck rot due to the inability to dry toward the interior is not well founded, but if you used a more vapor permeable 2 lb foam such as MD-R-200 that point would be moot, since it's still only a Class-III vapor retarder (1.3 perms @ 3" of thickness), where most HFC blown 2lb foam is deep in to Class-II vapor retardency at that thickness.

To reach code-min (R38) thermal performance for the roof you would need more than just 3-5" of 2lb foam, but the better approach would be to use rigid foam on the exterior when it's time to re-roof. With ~3" ( R15-R18) of foam between 2x4 rafters you can hit code performance on a U-factor basis with as little as 3" of roofing polyiso under a non-structural nailer deck, or 3.5" polyiso nailbase panels through-screwed to the structural roof deck.

Unless you're on a flood plane it's usually cheaper/better to insulate the crawlspace at the foundation walls than at the subfloor. In zone 2 you can take advantage of the favorable deep subsoil temperatures by not insulating between the floor and subsoil, just a modest amount of insulation at the crawlspace walls will "earth couple" the house to the thermal mass of the subsoil. IRC code doesn't demand ANY insulation on crawlspace walls in zone 2, but an inch (or two) of closed cell foam on the walls combined with a heavy ground vapor barrier air seals sufficiently, and would be a lot less foam than doing the entire subfloor. Insulating at the subfloor code would demand R13 which is more foam yet provides somewhat lower net performance.

Thanks for the many useful points!  I have only recently (after researching several older threads in this forum) begun to consider the differences in foam propellants and their contribution to the total impact, but I will certainly take this factor into account.

This is the first I've heard of a foam roof exterior.  I'm hoping not to need a new roof for >10 years.

Some followup questions:

1. Can you point me to a good primer on the alternative crawlspace insulation method you describe?  Would it require a certain type of wall construction in order to be feasible? I'm skeptical of the humidity factor if I don't do CC foam under the floor.  Also, the interior flooring is vintage oak hardwood laid perpendicularly above a 2x8 decking. No plywood, and lots of air gaps.  Some of the decking is spaced as much as 5-10mm apart!  The subfloor beams are also 2x8, so the foam could go on pretty thickly under there.

2. The type of foam notwithstanding, is there any other insulation/air barrier system that even comes close to the performance of foam?  Or can I safely focus my research into the granular issues of specific foam products?

3. Could I increase my foam depth by adding furring strips to the 2x4s under the roof?  If so, what could I reasonably extend to?  I think the code limits furring to 2" and/or 50% of original, but correct me if I'm wrong.  Is there some other clever trick to get more foam in an undersized cavity?  FYI, I will be applying ITW hurricane adhesive on the joints and reinforcement clips before the foam goes on.