I am in the process of building a 2100 square foot ranch home in central Illinois and I have a couple of spray foam guys telling me different things. One is telling me that 2” of 2lb foam is all I need and the other guy is telling me 3”. What is the truth? Thoughts? Thanks in advance.

Neither is telling you the whole story.

An IRC 2015 framed wall with no continuous insulating sheathing in US climate zone 5A (all of the green counties in this map: https://energycode.pnl.gov/EnergyCodeReqs/images/illinois.png ) OR zone 4A (all of the yellow counties) needs a minimum of R20 cavity insulation.

At 2" there is no foam that gets you there- the assertion that "it's all you need" is utter crap- it simply doesn't meet code, period.

https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2

As it happens 2" is the maximum thickness that can be safely applied in one pass for most HFC245fa blown 2lb foam. To go more requires a cooling/curing period, or there is a fire risk during curing. (HFC245fa is the industry standard blowing agent, but it's also an extremely powerful greenhouse gas, ~1000x CO2. Not so green.)

But at 3" some 2lb foam meets or exceeds R20. Most of those foams are blown with HFO1234ze, which is MUCH greener, at about 1x CO2, but they're also more expensive. But if you care about it, specify HFO blown foam.

But that's only a matter meeting the letter of the law, not the thermal performance. The framing fraction is typically 25% of the total wall area, after accounting for all the top/bottom plates, jack studs, window framing, etc) is only about R3.6. With 3" of foam the framing fraction has an R-value of only about R3.6, and would be conducting MOST of the heat through the R20 nominal wall, but a fully filled 5.5" deep 2x6 wall with R20 of open cell foam has an R6.6 framing fraction, which is a significant improvement in actual "whole wall " thermal performance even if the cavity fill is still R20.

This is not just a subtle point- it's a very real difference in performance, and a huge difference in cost/performance. The performance of 3"/R20 closed cell foam in a 25% framing fraction is less than R1 better in "whole wall R)" than a 2x4/R13 wall, despite the R7 improvement in center-cavity R. It's also more than R4 LESS in whole-wall performance (about 30% more heat conducted) than a 2x6/R20 full-cavity fill wall. For the math on that, see:

https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf

So a 2x4/R13 wall with a layer of 3/8" XPS siding underlayment would actually outperform 3" /R20 of closed cell foam in the stud cavities, at a lower cost. But even that doesn't meet code minimum on a U-factor using either R-value or U-factor. But a 2x6/R20 open cell foam wall would. The 5.5" of open cell foam is blown with water (fairly innocuous), air seals at least as well as 3" of closed cell foam, and even uses less polymer than 3" of closed cell foam.

If you are located in zone 4A you don't need an interior side vapor retarder to use open cell foam. In zone 5 if the siding is back ventilated such as vinyl, or any type of siding installed with at least 1/4" of "rainscreen" gap between the sheathing and siding, it meets the code definition of "Vented cladding over wood structural panels" exception, and doesn't need a vapor retarder either- regular standard interior latex paint (a class-III vapor retarder) is sufficient to prevent excessive wintertime build up of moisture in the sheathing, and gives the assembly MUCH better drying capacity than a close cell solution.

https://up.codes/viewer/wyoming/irc-2015/chapter/7/wall-covering#R702.7.1

So, if you're going to insulate the cavities with foam, a full cavity fill of half pound open cell foam is the cheapest, greenest, highest performance, and most resilient R20 option. Expect a a lot of whining and push-back with more BS from the foam guys on that though.

An even better option (lower cost, almost entirely recycled content, comparable overall wall thermal performance) would be damp sprayed cellulose with water activated adhesive completely filling the stud cavities.

It's not necessarily lower cost to go with damp sprayed cellulose. Lately quotes for damp sprayed cellulose seem to be coming in more expensive than half-pound foam in my area, not cheaper. YMMV.

Cellulose is definitely a greener option, but requires more air sealing detailing than a half pound foam (which air-seals the cavities at least). It's worth getting it quoted both ways when the time comes, along with the air-sealing detailing.

It must be a regional thing because in my area (in the south) you can get sprayed cellulose for about 1/2 the cost of half pound spray foam.

Dana1, dumb question. Which vendor are you getting a quote of R-5.5/inch for OPEN cell foam from?
Most are in the high 3.*, with some just saying "4".

Normally you see CLOSED cell foam in the 5-6 range.

Don't get me wrong. I think exterior sheathing is a better way to go to raise whole-wall values. Just I'm not seeing numbers line up and I'm curious.

Posted By Dilettante on 15 Jun 2018 04:20 AM
Dana1, dumb question. Which vendor are you getting a quote of R-5.5/inch for OPEN cell foam from?
Most are in the high 3.*, with some just saying "4".

Normally you see CLOSED cell foam in the 5-6 range.

Don't get me wrong. I think exterior sheathing is a better way to go to raise whole-wall values. Just I'm not seeing numbers line up and I'm curious.

It's not R5.5/inch, it's 5.5 inches (of R3.7/inch= R20 cavity fill in a milled 2 x 6 framing cavity.)

No he said the 5.5" wall cavity is R20, which adds up to 3.6 per inch.

Okay!  Knew I'd missed or mis-read something!

Thanks!

Wow and way over my head. We are building a house in the Florida Keys. Is closed cell better? Does it need to be put in in 2" depths as well? Thanks

Posted By MartyK on 01 Oct 2018 09:57 PM
Wow and way over my head. We are building a house in the Florida Keys. Is closed cell better? Does it need to be put in in 2" depths as well? Thanks

Closed cell foam is not particularly better if it's being thermally bridged by framing. The framing is so much more thermally conductive than the foam it essentially robs it of performance. Even though the cavity R-value might be nearly twice that of the same wall filled with open cell foam, after caluculating the heat transfer through the whole assembly including the studs it's less than a 15% improvement.

The only foam that is limited to 2" lifts per pass is closed cell foam blown with HFC blowing agents. Closed cell foam blown with HFO blowing agents can be safely installed in lifts of 4-8", depending on the particular product & manufacturer. Open cell foam can be blown in lifts of 5.5"-6". When installed in higher than recommended thicknesses per path it's both a quality issue and a fire hazard issue during the curing period. HFC blown closed cell foam can catch on fire if installed in lifts of 4", and will often have shrinkage separation from the framing and uneven thickness. The fire hazard is also true for open cell foam blown at 1.5-2x the manufacturer's recommended max depth per pass. Most HFO blown closed cell can be installed in R38-R49 per pass without issues- it just doesn't produces as much heat during the curing process, which is the cause of the shrinkage & fire hazard of HFC blown closed cell foam.