Small question: is there anything wrong in framing the basement wall on the footing before pouring the slab instead of waiting to pour the slab and then frame the wall on top of it? The slab will have a vapor barrier so there shouldn't be humidity concern that would allow rotting of the bottom sill wich will be encase in concrete...

Is this a walkout? It is NEVER a good idea to encase wood in concrete as it will eventually rot.

no, it's not a walkout.

There will be 2" of cc spray foam underneath the slab and 3.5" on the interior of basement wall. Framing the wall on the footer would allow the spray foam crew to come only one and spray everything... that's why I'm thinking of doing it that way...

so they could encase the bottom of the wall with ccsf? Much safer than concrete. If you do this, use some type of capillary break between the footing and bottom plate such as sill gasket. the other consideration is that there needs to be a separation (thermal and capillary break) between the wood studs and the concrete wall. We've done basement walls where we set the stud wall 2" away from the concrete, then they foam the space between and overlap onto the studs, then we fill the studs with batt insulation, giving us R13+13, or 13+20 if we use 2x6's.

Living on Hudson Bay.

Posted By Alexis on 29 Aug 2013 01:27 PM
no, it's not a walkout.

There will be 2" of cc spray foam underneath the slab and 3.5" on the interior of basement wall. Framing the wall on the footer would allow the spray foam crew to come only one and spray everything... that's why I'm thinking of doing it that way...

Spray foam under the slabs has a very spotty track record in my neighborhood, and I simply wouldn't do it.  Its cheaper  and more reliable to use EPS (the stuff of cheap coolers and coffee cups.)  EPS runs about 9-10 cents per R-foot compared to closed cell foam at 17-19 cents/R-foot, and it arrives at the job site completely cured, no funky PITA shrinkage or mis-curing issues that can happen with closed cell polyurethane sprayed onto dirt (preferably clean 1/2-3/4" screenings).  It's the right stuff.

Spraying closed cell foam between 2x4 studs is a waste of good foam, since the thermal bridging of the studs ruins the average performance.  The difference in "whole-wall-R" with the thermal bridging factored between R15 rock wool and 3.5" of R7/inch foam is R15.2 vs. R11.7, a difference of less than R4. And that's at a fairly low framing fraction of 15% (24" o.c. stud spacing, few or no windows & doors, single top plate). That difference could be made up with as little as 1" of EPS (or a 1" shot of closed cell on the concrete, separating it from the stud edge.  And in practical terms you're really not going to get 3.5" of foam between studs since it's not easily trim-able- figure on 3". 

And putting stud edges in contact with the sub-grade concrete is just asking for moisture & mold issues getting started even before the house is finished.  It needs both a capillary and thermal break to be robust. Rigid or spray closed cell foams can provide both, but it must be placed between the framing & concrete.

Massive quantities of closed cell polyurethane also have the issue that the HFC245fa blowing agent is a powerful greenhouse gas, about 1000x as potent as CO2 on a lifecycle basis.  An inch or two in select locations for managing vapor permeance and air sealing can be sorta-green, but there's no way that's true in under-slab or foundation wall applications at 2-3.5". In those locations at those thicknesses it will have a greater lifecycle greenhouse  effect in excess of the source energy use it's offsetting.  XPS (pink/blue/green board) has similar issues, but EPS and polyiso are blown with pentane, with only ~7x CO2 greenhouse potency.

If you were shooting for ~R20 on the foundation wall (3.5" of R6/inch foam is R21), a standard R20-R22 insulated concrete form would get you there, probably for less upcharge than the $3.50 per square foot you would have paid for the closed cell polyurethane.  But if the foundation has already been poured, that ship has sailed.  At a 15% framing fraction the studwall with R15 rock wool comes in at nearly R12, and putting 2" of EPS between the concrete & studwall brings it to R20.  Alternatively, 3.25" of polyisocyanurate held in place with 1x furring through-screwed to to the foundation with TapCons 24" o.c. gets you there (also at ~10 cents/R-foot)

Thanks a lot for the answer...

the 2x4 wall wouldn't be flush with the concree, I would be spaced 2" for the thermal break.
I'm interested in the spotty track record your speaking of for under slab insulation. A lot of manufacturer are beginning to push this and I must agree that I don't see why it wouldn't be better: perfect coverage, no seams, faster to install, vapor barrier...

thanks

I only have 3rd hand info- never used in on any project I've been involved with, though it almost came to pass on a deep energy retrofit I was involved with about a year ago, with a last minute change to XPS (which I objected to, but was overruled, not being in charge of that end of it) after some of the installation & performance problems began coming to light on one of the energy consulting company's other projects. (They did put 2" of cc foam on the foundation walls, which worked out fine.) What would seem like an obvious and good solution turns out not to be- too many things have to go right the first time for it to work.

Between the reported unanticipated shrinkage/cracking, goods that didn't end up with the projected compressive strength, non-uniform as-installed thicknesses creating thermal bridges it seems like a really expensive NON-solution compared to cheap EPS with cheap 6-10 mil poly as the ground vapor barrier, even it if DIDN'T have the high lifecycle global warming and high cost issues.

The vapor retardency of 2" of closed cell polyurethane is still over 0.5 perms, more permeable than kraft facers on batts, and more than an order of magnitude more vapor permeable than EPDM membranes or 10 mil-poly. Whether that's an issue or not depends on site factors. (Soil type, distance to water table, etc.) But a true vapor barrier it is not- if you go that route a sheet of poly between the foam & concrete is cheap insurance.

With 2" of space that means you'd only be wasting 1.5" of the 3.5" with thermal bridging. If you go this route save yourself the money (and the planet) and put only 2" of cc foam on the walls to where it just touches the stud edges, and install R15 rock wool in the 2x4 studwall, which would yield a true R20+ whole wall. The material cost of the rock wool is about 4-cents/R-foot, and swapping out the 1.5" of foam for 3.5" of rock wool is a net increase in whole-wall performance for a net reduction in cost.