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Degradation of closed-cell spray foam when applied in cold climates

Hey GBA,

I have a 3rd floor cantilevered overhang that has a conditioned space above. Underneath that cantilever I plan to do 9" of closed cell spray foam to maximize the insulation so I don't run into comfort issues with the floor space above it.

The floor assembly is:

3/4" plywood subfloor
9.5" cantilevered joists and beams with 9" spray foam in between.
3/8 soffiting

I ended up buying the home Depot caniaters of closed cell spray foam so I could do it on my own time rather than waiting for the spray foam crew to show up.

The instructions on it say the min ambient temperature to apply is +16C (about 61F). It doesn't look like that weather is coming anytime soon, woth next week looking steady at +4C (39 F) so may be forced to do it no near the min application temp.

Does any building science nerds on here have any info on degradation of R value of foam based on such parameters?



Asked by Jamie B
Posted Mar 7, 2018 6:51 PM ET
Edited Mar 8, 2018 5:24 AM ET


6 Answers

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At colder temps, it does not foam up properly.
The foam GUN is a good investment: you get a better bead and can pause without penalty (the cans must be used in 2+ hours).

How about renting a propane heater?

Answered by Bryce Nesbitt
Posted Mar 7, 2018 7:04 PM ET


Is it possible to return the foam kits?

The high framing fraction of a cantilevered overhang buys you almost nothing in overall heat transfer by going to R50+ at center cavity. The floor temperature of the striping of the joists will be pretty much the same as if it were R30 or R38 in the cavity, and the floor temperature over the filled cavity won't be more than a degree warmer. R30 between joists would meet IRC 2015 code minimum in US climate zone 6 for exposed floors, but it takes R38 for zones 7 & 8.

R30 batts in the cavity and 1.5" of foil faced polyiso or 1.5" of unfaced EPS applied to the underside of the cantilever would cut the heat transfer through the framing fraction in half, leading to less temperature striping on the floor, with less heat loss overall. If 1.5" polyiso isn't sufficient for dew point control use EPS, which can dry toward the exterior, held in place with MDF or asphalted fiberboard sheathing.

Spraying kit foam when the substrate is too cold out isn't so much an R-value issue as it is an adhesion & curing issue. It'll be less even, more likely to have shrinkage gaps & separation along the sides, etc. If it's possible to pre-heat the surfaces with a heat gun or something prior to spraying some of that can be avoided, but once it's in there and screwed up it's a complete PITA to chisel it all out to fix it.

Answered by Dana Dorsett
Posted Mar 7, 2018 7:14 PM ET


Before you decide to ignore the instructions came with your spray foam kit please read these articles.

If for any reason the foam does not cure correctly, low R value may be the least of your problems, what if foam smells and fumes sicken you and drive you from your home?


Answered by Walter Ahlgrim
Posted Mar 7, 2018 9:47 PM ET
Edited Mar 7, 2018 9:50 PM ET.


You've gotten good advice here. I vote for filling the joist bays with some type of dense batt, carefully installed, and then installing a continuous layer of rigid foam insulation on the underside of the joists -- installed with attention to air sealing.

If you want to use your spray foam kits, the standard solution by contractors in cold climates is to set up some pipe staging, tarp the entire area, and heat the area for several hours with a big propane heater before the work begins. Of course, you would need to keep your spray foam kits in a warm room for at least 12 hours before you begin work.

Answered by Martin Holladay
Posted Mar 8, 2018 5:29 AM ET


Thank you all. Excellent advice here!

I will take a look tomorrow, but I don't think rigid foam on the underside of the joists will be an option. So essentially my options are R31 rockwool batts, or tarp the area underneath, heat it and apply spray foam.

I do have a 200 BTU propane heater on site. As much as I hate using it in confined spaces, I might just go this route being careful for air quality. Also, I've read on here about prespraying the surface with water, I assume that was written for gun foam, is that still a good idea for 2 part kit foam?

That being said, since I'm putting slim led downlights in the soffits, I think I will switch my plan to do 6" of spray foam and 3.5" of Rockwool batting. This way I can have some play with moving around the batting when installing the downlights, and I can get direct contact on the led driver and have a bit of a fire break (the led manufacturer doesn't want spray foam closer than 2" to the driver)

That assembly doesn't help for striping as discussed, however the joists are I joists, so I believe the thermal bridging would be minimal compared to dimensional lumber. (Tell me if I'm wrong)

Assuming I can do a good application of spray foam, What do you think of this?


Answered by Jamie B
Posted Mar 8, 2018 3:21 PM ET


Spray foam insulation plus mineral wool will work. Conscientious installation is more important than material choices.

Answered by Martin Holladay
Posted Mar 9, 2018 6:32 AM ET

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