I've been reviewing several ICF manufacturers installation manuals and some, but not all, show a connection between the basement slab and the concrete in the ICF wall, either via rebar or by cutting away the inside foam on the wall and pouring the slab up to the concrete in the wall.  Is there a technical reason or code requirement to do this?  If so, can some please describe it?

Background on my project:

Full walk-out basement, 34' x 62', and one story above, all with 8" then 6" core ICF walls extending to the roof.  The footing is poured.  Next will be pouring the slab over 1" foam and vapor barrier that will extend up over the top of the slab to the edge of the slab forms.  Then I will begin stacking the ICF forms directly on the footings around the basement slab.  This will result in the cold joint between the wall and the footing being ~5" below the top of the basement slab with gravel and drain tiles inside and outside the footing.  This will also result in the basement slab "floating" on foam except for a near center interior load bearing wall where the slab will rest on grade with extra steel reinforcement, and being completely surounded by Type IX EPS foam which has a compresive strength of ~25 psi.

The wall will be tied to the footing via 5/8" Grade 60 rebar dowels which are already installed 16" oc.

Is there a technical reason that I need to consider installing a hard connection, either rebar or cutout inside wall foam, between the concrete slab and the concrete core in the wall?  I'm building in the country in the south where no codes or inspections are required, but if there is a sound reason for doing it, I'm all for that.

If the slab is being used as a structural component of the builidng system than it must be tied in with the footing and walls to create a 'single' unit. I don't have your location, but if no codes or inspections are required I would wager it doesn't matter which route you choose.

Don't mix vapor barrier with the foam under the slab, one or the other is all you need, use both and you are double vapor barrier'ing and could create a condensation issue.

Chris said it wright ,if not structural don't tie it in. For the floor we usually lay 1.5'' foam w/vapor lapping over the footing to the wall and extending the vapor over the floor height so no condensation can penetrate. Ps. Never personally seen residential basement floor tied to walls.

Thanks for the input guys. It doesn't look like I need to tie the slab to the wall concrete.

But I'm not completely clear on the concerns with having insulating foam and a plastic vapor barrier under the slab. I didn't think you could credit the foam as being a vapor barrier?

I was planning on taking the plastic vapor barrier over the top of my slab forms (2x lumber) and stapling it in place during the pour to keep the 2x lumber clean and then folding it back over the slab when the forms are removed. After the ICF forms are in place on the footing and poured, I was then going to trim the plastic vapor barrier flush with the top of the slab. OK or not?

It isn't structural so it isn't required, but here is the real question- If the soil expanded under the slab and the slab rose up half an inch would you be upset? Soil expanding and contracting with the season is common, even in the south. If you are fine with the possibility of the walls and slab shifting in relation to each other, don't worry about it. I have seen brand new homes shift and actually pull the trim away from the wall, but its not like that is a catastrophe. I don't think you will ever have a structural problem, but it might cause you to have to readjust your trim work or patch a drywall crack.

Most of the foams on the market are used in place of a vapor barrier, contact the manufacturer and ask them.

2x vapor barrier can cause issues

Regarding the vapor barrier, what if I put the plastic vapor barrier over the top of the foam? I just don't feel comfortable trusting the foam as a good vapor barrier. Also note that under the slab will be 4" perforated pipe all around the inside and across the middle which will be draining to daylight. There will also be ~8" of coarse crushed and washed stone under the slab.

Also, what's the consensus on the best foam to put under the slab? At first I thought XPS was the way to go because of its strength, high insulation value/inch, and low water absorption, but after reading up a bit more on it, it turns out that it loses a lot of the insulation value over time. EPS Type IX foam looks like another option since it is almost as strong as XPS, has relatively low water absorption, and its insulation value doesn't degrade over time. Plus, it costs less and I should be able to get 1-1/2" of EPS (R=6.4) for what 1" of XPS (R=5.0 initially) costs.

Another question regarding the slab to wall connection. I have a couple of load bearing 2x6 walls in the basement. I intend to not put any gravel or foam (will put plastic vapor barrier) under these walls so that the thickened slab under these walls is resting on undisturbed ground and will make the thickened area ~12" wide. I also intend to lay two (2) #4 rebar in these wall support areas, holding them ~2" above the ground on chairs. The question is, would it be prudent to extend or tie these rebar out of the slab so that they can be tied into the wall concrete? Another idea I had was to drill into the side of the footing where these load bearing walls will intersect and epoxy grout the rebar right into the footing.

Chances are high that the EPS is a vapor barrier, if the seams are T&G or shiplap that's the best, if it is butt edge (square), you must tape the seams.

Yes, epoxy your interior load bearing wall to the existing footing, no do not put gravel or foam over the footing, the slab must have 100% contact with the footing to maintain it's load bearing capabilities.

Why don't you install the first course of block (without being filled with concrete) and use that as the form for the slab? It seems like you are doing extra work.

I am building a house that has 12 feet of fill on one side and the engineer is having me attach the slab to the wall to help with the shear from the amount of fill. I formed my footings so they are resting on grade.

Brain,

Your slab is a structural component, the engineer (smart guy by his design) is building a giant 'L' with the wall and slab as one. Arkie is not having his slab structural, just a floor.

What your engineer has done is saved you from building some ridiculously oversized footing and is working with the materials that you were going to use during the course of construction anyways, kudos to him...We call that value engineering!!!

I don't mind doing a little extra work if I get better results.  I intend to stain and seal the concrete floors in the basement, so I want the floor to be as level and smooth as possible.

It is much easier to finish the concrete slab, and get it flatter and smoother, if the top of the slab is flush with the top of the forms and there are no outside walls to deal with.  That way you've got a solid, level edge to run the screed over and you can run the power float all the way over the edge without worrying about damaging the foam.

Can foam ICF wall forms, if not filled with concrete, even be considered strong enough and stable enough to use as a form edge for the slab?  I would be concerned with damage to or movement of the foam, even if glued to the footing.

And it isn't that much trouble to build the form for the slab.  I already have the 2x lumber that was left over from forming my footings (I used Fast Foot for the footings).  All you have to do is drill a small hole in the 2x lumber and footing every ~4' and drive 2 concrete form nails into each hole (wedges tight in the concrete, but is easy to remove).  Then come back and screw the other 2x edgewise onto the 2x that was laid flat and anchored to the footing.

Regarding the empty icf panels as your form to hold the concrete is a no. Most forms are designed in order to withstand pressure coming from inside the wall. If opposite the ties are prone to bend and break. Bottom line the forms need to be stable and solid with nowhere to go. In my opinion it would be much easier to put up the walls pour them then the floor. By doing this you are eliminating a substancial amount of work. Chalking on the block to obtain your concrete levelness is quick and easy (the screeder won't damage the foam to compromise the system). This also eliminates any holes in the slab that would be required for bracing the walls.
Cheers!