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
[email protected]
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.

I'm considering 1.5" of spray foam as the thermal/vapor barrier break on my earthship in order to prevent the depressurization from drawing the heat out of my passive solar heated slab floor. Does anyone know if the depressurization has this wicking affect? If so, I'll insulate and up my adobe slab floors from 3.5" to 6" thick, but worry that the insulation would stop the solar penetration deeper into the thermal mass of the grade. Any thoughts?

depressurization? Why are you depressurized, for one? I know this can draw soil vapor through the slab (which may not be a big deal depending on where you are building) but I'm not aware of any effect pressure has on heat loss.

I don't know what your passive solar design is counting on for mass so I have no idea whether the thermal mass of the grade is of benefit or not... or if it's a greater benefit than the heat loss would be. That probably depends on where you are building too, as well as what kind of passive solar design you're doing. If this is 'passive annual heat storage' you need truly massive amounts of mass.

I saw this again on a google, and I have to note that papabear only posted one time. I guess he didn't have much to say on any other topic around here. I wonder what papa bear thinks happens to those "continuous" vapor barriers on most projects.

the vast majority of projects I've seen poke so many holes in the vapor barrier you wonder why it's there, until you realize that it's about percentage of coverage, not perfect coverage. No vapor barrier is perfect in the field.

To the extent that you don't have perfect air sealing, depressurization for radon control will suck heat out of the house. As will any other pressurization or depressurization of the interior.

Posted By NRT.Rob on 29 Mar 2011 02:43 PM
I saw this again on a google, and I have to note that papabear only posted one time. I guess he didn't have much to say on any other topic around here. I wonder what papa bear thinks happens to those "continuous" vapor barriers on most projects.

the vast majority of projects I've seen poke so many holes in the vapor barrier you wonder why it's there, until you realize that it's about percentage of coverage, not perfect coverage. No vapor barrier is perfect in the field.

Nor does it HAVE to be perfect to dramatically limit either radon or water vapor permeation.  It's just as you say, a percentage of coverage issue.

This comes up in discussions about vapor barriers vs. air-barriers in walls.  A ripped up slashed vapor barrier/insulation-facer is a piss-poor air-barrier, but can still be a good vapor barrier, which can sometimes be the worst of worlds- air-transported moisture gets in and condenses from air pressure differences, but can be slow to leave via vapor diffusion. 

I wouldn't sweat it too much in a slab though- concrete is a pretty good air barrier on it's own except at the cracks, which still would have a preponderance of coverage from any vapor-barrier even if holey, slashed or gashed.  But  concrete is a LOUSY vapor barrier- highly permeable to molecules as small as water vapor or radon, even though it's resistant to the larger diatomic gas molecules such as O₂ or N₂ that comprises bulk of air.  (Sharing their only electrons with their bound oxygen, the hydrogen atoms on the water molecules don't make H₂O much bigger in cross section than a naked mono-atomic oxygen atom.)

  A ripped up slashed vapor barrier/insulation-facer is a piss-poor air-barrier

I'll add that these holes can be as subtle as staple or nail holes that get enlarged due to pressure differences between inside and outside.