For radiant concrete overpour over rigid foam - fastening, seam-sealing, and vapor barrier

rossn

First off, want to say thanks to the great group of folks here... I have posted a couple questions related to my project, and have received some very professional and insightful guidance. What a great, positive forum!

Uninsulated slab, 4' below grade, which is getting a vapor barrier + 1-1/4" rigid foam insulation (25 or 40 psi EPS) + 1/2" pex + 1-1/4+" concrete pour.

Two questions:
1) Is it generally recommended to use a sheet vapor barrier, or a roll-on vapor barrier? Specific product recommendations are great... do want to be mindful of indoor air quality/product toxicity. Old slab, but can be high (up to 98%) humidity at times, but never any bulk water issues.

2) Do you loose-lay or fasten the foam in-place, and do you seam seal it? If so, what is your approach? Note: I do have some concerns about areas that might have a ridge, sloping area of slab, or uneven slab cut - where the foam may compress after the pour (esp @ 40 psi foam).

hot_rod

Still time to consider spray foam? Cost works out about the same and you get an excellent one piece vapor seal. It's becoming fairly common for slab insulation around here.

The wind doesn't take it away either

rossn

Well... tell me more

I wasn't aware there were spray foam applications on top of slab.... do they shave it flat, and what about what PSI is it?

I would want to also look closely at the environmental impact, as spray foams historically have had some pretty harsh blowing agents, though some are improving.

hot_rod

This insulator seem knowledgable, not a sales pitch video.
There are a number of other You Tube videos about spray foam under slab. Check with local providers to see if they have experience with putting it down over subgrades.

I would guess the actual chemical make up of spray or board foam is similar? If it doesn't outgas in walls, probably no issue under slab. seems like a much better radon barrier with spray.

https://video.search.yahoo.com/yhs/search?fr=yhs-ima-st_mig&ei=UTF-8&hsimp=yhs-st_mig&hspart=ima&p=spray+foam+for+under+slab&type=q3000_A1BWP_set_bcrq#id=1&vid=621929fd5a08f0c4691ca906b21ea6e7&action=click

rossn

Ahh, very interesting. Sounds like a pretty cool option.

I think in my case, it won't be very practical, mainly because I'm trying to go for absolute thinnest assembly... as it is, I will have maybe 7'-9" ceiling clearance downstairs... IF I'm lucky. I guessing they'd have trouble keeping the foam thin enough to keep my full assembly between 2-1/2 to 2-3/4", but it would be great for Radon management, for sure.

As to the chemical makeup of spray vs board foam... foaming agents represent a tremendous source of warming agents in the atmosphere... I think I read recently somewhere between 5-10% of all greenhouse gasses, which is really major. The traditional blowing agents included HFC-134A, which if I understand the science correctly, traps 1430 times as much heat as carbon dioxide from your car. Basically, 1lb = 1430lbs of carbon dioxide from a car. A year or two back some of the mfg's came out with next generation blown XPS rigid foams, and at least one mfg brought theirs down into the (if I recall correctly) maybe 70-90x range. Now consider EPS foam insulation (ordinary styrofoam). It is rated at 7x (and fully recyclable). So, one really can make a difference in their choices and the downstream impact, just speaking from a scientific perspective is all.

This Chemical & Engineering News article goes into the transition away from HFC-134A. I haven't checked into the spray-on foams lately, but at least as of a few years ago, I believe they were using the same blowing agents, so I've tried to use blown foam insulation only in the most important areas.

hot_rod

I believe they use much safer blowing agents now, check and see

I had them spray my shop walls at 1/2” thickness, then use fiberglass batts. I have  seen a paper thin application applied also, for a air seal barrier. its all about adjusting the gun and the skill of the applicator.

rossn

That's good to hear, as I'm also going to be doing flash and fill upstairs. I'll check in on the spray foam.

PC7060

You’ll be able to achieve close to R10 with 1-1/4” close cell spray. Very good option. 

rossn

Thanks, I'll check with some foam guys. I see it more like I might be able to drop it 1/4" to add a little more ceiling height. For some reason, I was thinking that spray foam wasn't rated for slab contact, but my information is a few years old, so that may no longer be the case.

hot_rod

About R 5 per inch for most foams. So 1/4" will not get much R value.
It will seal Radon, smells, air leaks, etc however.
There are a number of different foam formulas, I think it needs to be closed cell for ground or concrete contact. Maybe different psi ratings are available also. A good foam guy should be able to steer you.

PC7060

hot_rod said:

About R 5 per inch for most foams. So 1/4" will not get much R value. It will seal Radon, smells, air leaks, etc however. There are a number of different foam formulas, I think it needs to be closed cell for ground or concrete contact. Maybe different psi ratings are available also. A good foam guy should be able to steer you.

I believe @rossn was going to reduce the foam thickness by 1/4” from 1-1/4” to 1”; about r10 overall.