Spray Foam Quote Help
Short and sweet- I’m trying to enclose my attic HVAC into my thermal boundary & air seal all in one project. And I am stumped on how much foam thickness to have applied. Contractor’s initial “we typically do” was 6″ of open cell foam w/ a class II vapor barrier and I was too uncomfortable with that.
I’ve seen on here (in comments by experts) that 5-6″ closed cell is generally recommended for attic sprays (~R35-42). Almost a universal statement, but I may just read the same experts in different places. I’ve then seen articles saying that spray foam should have its own R charts in energy codes, but don’t yet. Read one study that said 3″ of CC foam (~R20) in a test environment retained 90% of heat, and 6″ retained 96%. Final result being that 3″ was the optimal amount to apply, and the additional 3″ was really not cost-or-comfort-effective So I’m still struggling to understand where I should be.
I’m Zone 4A I think (Kanwaha Co, WV, on the border to the next cooler zone). I just got a quote to bring my upper attic on my cape-style
$6,700 for 11.5″ R40 open cell with a spray-applied Class II vapor barrier to finish on the interior side. They’d do that 6″ for nearly half that price, which would be SUPER but is R21 Open Cell really ok…? They ALL seem to push this!
$8,500 or so for 4″ of closed cell (R 28)
$10,500 or so for 6″ of closed cell. (R42)
All quotes involve extending vent baffles to ridge vent and removing existing FG.
Help is appreciated! I’d really like to select a path and feel confident in it, but I’m spinning.
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Replies
I'm in 4a too. I had no insulation in attic. I went with two inches HFO closed cell foam, then furred out rafters with polyiso foam strips and put rockwool batts in between rafters (mostly R-23 thickness).
Also 2 inches on eave walls and drywalled those with slightly thicker drywall for fire safety.
It was a lot of work though!
Flash and batt, right? What's your total R value in the end? I could have them do just 2" of closed cell and save quite a bit if I batted it myself. Wouldn't be that hard considering the OC would leave me rafters to work with. Mineral wool isn't nasty like FG. I hate... HATE working in fiberglass.
I forgot to ask about fire safety. If they spray it all, do I have to cover it all with something fire retardant? If so that kind of shoots down that much OC right away- they'd be well over the thickest rafter depth...
If the attic will be used for storage, spraying the roof is the better option. In this case unless specifically rated for exposure, the exposed foam needs an ignition barrier coating. Once you add in the additional cost of the foam plus coating, the flash and batts with mineral wool makes much more sense. In zone 4 you don't need much closed cell spray foam for condensation control so it should not be an expensive install. With encapsulated attics, the important part is to air seal it (soffit area and any gable walls) and condition the space.
If you go for closed cell foam, there is no need for roof venting unless you are in heavy snow country, zone 4 doesn't sound like it. With any open cell, you need venting with open cell.
If you don't need to use it for storage, the better use of spray foam is to encapsulate your ducting and boots and air seal it that way:
https://www.energy.gov/eere/buildings/articles/buried-and-encapsulated-ducts-building-america-top-innovation#:~:text=In%20humid%20and%20mixed%20climates,sealed%20before%20encapsulating%20in%20foam.
Not an expert. From what I've read I would basically never use open cell. Closed cell does indeed hit diminishing returns around three inches. Last Nov my market was $1.40-2.00 per in per sqft, currently it is $1.26-2.00 per inch per sqft. Not sure fi that helps any
I did a similar thing up here in Zone 6A:
1. Vacuumed insulation out of attic.
2. Constructed baffles between the rafters.
3. Spray 2" of ccSPF
4. 16-18" of fiberglass insulation of floor.
This cost around $8K back in 2016.
In hindsight, we should have added a vapor diffusion port vent to get rid of humidity. I just added a dehumidifier to the attic to reduce the humidity to around 41%. (See attachment).
As shown in the graph of the attic sensor - the humidity was over 60% - which may cause mold/mildew.