I'm in Minnesota, where we now have to put in a radon control system which involves putting in a vapor barrier under the slab. Now that got me wondering about using spray foam to get both my vapor barrier and insulation. As far as compression strength of 2 lb foam I believe it is stronger than your standard Dow blueboard. As a vapor barrier I was told by my spray foam contractor he can go as shallow as 1.5" to make a vapor barrier. At this thickness he has an R-value of about 10. And by the time I would install a 6-mil poly vapor barrier and insulation, the cost is very close to that of the spray foam applied at 1.5"

The only concern I've got is about the vapor barrier. I'm not sure of the brand he uses, but a vapor barrier at only 1.5" doesn't seem possible to me. If anyone out there knows, any information would be greatly appreciated.

interesting idea but seems like your thinking to hard..... .... .. .
6 mill barrier and then 2 inch dow board make a flat level surface to staple your pex pipe to. That spray foam if I am not mistaken will have a surface that will be un level and inconstant. Pipe attachment will be fast and easy on a flat surface.
Dan

Crete-Heat has the 10 mil built already into its 2 inch board and the way it goes together makes laying the tubing a snap! see it at www.GreenInsulationProducts.com if you are not familiar with it.

If the cost is close, do it. You will have a much better vapor barrier and insulation barrier, with no openings that you get with 4x8 sheets of foam and plastic sheeting. I talked to my spray foam guy about it and I wish I would have done it. They put a grid of strings down to guide the spray foamer and they can get it level within 1/4" according to him. They have been doing this kind of thing on roofs for years. You will probably actually save money on concrete, because if the spray foamer can get within 1/4", I guarantee that it will be much more accurate than the gravel layer beneath it. You will save more money there. 1.5 inches is certainly enough to achieve a vapor barrier with 2 pound foam.

Thanks, kjseuferer

The cost is almost a wash, so I'm almost certain I will try this approach on my next project. Problem I have always had with foam board is if there is any voids underneath it can all crack up when walked on. Also the foam will then be attached to the ground. Won't have to worry about it blowing away before concrete is placed.

This is interesting,
I saw one spray foam job where the shower pan leaked and absorbed moisture like a sponge, wasn't pretty after they tore out the marble, fixtures and Glass doors and replaced the rotten sub floor & TGI's. I assume that the foam you use will be closed and not absorb moisture? You can walk on the stuff? Staples will hold so you can staple directly to it, 1/4 error is nothing. no vapor barrier? How many square feet are you doing? I wonder where the diminishing return on cost is, certainly the absence of voids under foam and the fact that blue or pink board does blow away being eliminated is good.
Interesting.....
Dan

2lb foam (polyurethane OR iso) is always closed cell. (XPS is also nearly universally ~2lb/Ft^3, and closed cell.) The compressive strength of SPF or XPS varies with absolute density & manufacturing process, but both are typically ~25psi, +/- 10psi, in the direction we care about under a slab. (If you need more, strength, 3lb SPF runs ~40psi). In testing 2lb. SPF takes on less water than XPS.

Half pound foam and open-cell iso can soak up water, as in your shower pan situation, but the 2lb stuff just can't- it's as waterproof as anything.

Can't comment on 2lb SPF's staple-retention qualities other than to say my gut tells me it'll be similar to XPS, better than low-density bead-board EPS and way better than open cell iso or half-pound SPF.

I would consider adding more insulation under the slab, maybe another inch or more. I considered doing this on my last project but here in MA the cost numbers didn't work out as a wash. The 2lb foam costs $1 bd/ft, I used 3" of 2lb density EPS with a R-15 and costs $1 sq/ft. I wanted more R-value than the 10 you quoted and the cost to do this with the poyliso would have been twice the cost of materials. The closed cell poyliso would have eliminated the need for the vapor barrier and simplified the project, maybe next time. Let us know it works out. works great but smells like #$%^.

Tom Pittsley
ecobuilder@aol.com
www.eebt.org

Here in Arkansas, I can get DOW Agboard (typically used on chicken houses and farm buildings) which comes in 1" thick x 4' wide x 21.5' long sheets (the long sheets reduce the number of joints). It also has ship lapped edges. This is your typical 25 psi rated DOW XPS blue board. Cost here is $0.55/sq ft. This is what I plan to put under my basement slab, along with a 10 mil Class A rated poly vapor barrier such as Orange Guard. My cost for this is $195 for a 20' x 100' roll (~$0.20/sq ft).

You are doing yourself and your customers a disservice trying to sell Crete-Heat as a vapor barrier and radon barrier. There is not an unbroken membrane across the surface area of the slab as would be provided by poly sheet. The interlocks are not vapor or radon seals and selling it as such is just plain wrong.