I have seen a hundred different drawings and read articles discribing this need for shallow foundation insulation but I am not sure how to apply it in my situation.

I am pouring my basement floor on Saturday and need to decide how to detail the edges of my frost footer where the walk out stick framed wall of my basement is going to be.

I have THREE areas that will be poured with the floor that are frost footers.  The area is about 16 inches deep and drops away from the level floor about a foot away from the outside edge of where the wall will go.  It really just looks like an edge of a slab on grade foundation.

My question is this.  I am putting 3" of XPS foam over a 6 mil vapor barrier under the floor with radiant heat.  Do I stop the foam on the level portion of the floor and stop the vapor barrier as well?  That way when they pour the floor and the concrete starts to roll off into that thickened frost footer area it'll just fill up?
I plan to stick a 3" piece of foam on the outside edge of the thickened footer when the formas are removed and the patio that will be poured outside the stick built wall will also have foam under it so the frost heave should be controlled.

Don't want to miss a vapor bvarrier step or a foam placement so please advise.

It's really too bad that you're using 3" of XPS (R15) rather than 4" of EPS (R16) since the former has a huge global warming impact roughly 200x that of the latter. The greener solution is even cheaper to boot.

If you plan to put a finish floor on it it's better to put the vapor barrier on top of the foam, not under it. If you put it under the foam it will trap puddles of excess water from the concrete pour between the foam & poly, which will take many months (years?) to dry, whereas with the vapor barrier atop the foam all of the water stays in the curing concrete, and any excess beyond what gets used for the curing will wick & dry within weeks. The puddle under the foam can cause painted finishes to peel and flake, and cause mold issues under wood flooring, etc.

You know I am the first skeptic and the last critic, but I have not found a reliable and competitive source of EPS nor any evidence that excessive moisture is present, let alone a causal factor in any of the common negative outcomes you attribute to the very common practice of placing 6mil retarder under insulation. If I follow the argument correctly, I would, and have skipped the barrier altogether with flawless results, both inside and outside in my snow melting systems.

Well since I got the foam for free, and it's once used foam off a roof I felt it was the logical, gree. and economical choice to make. Don't think anyone can argue with me there.
Secondly like badger says I am not nearly as concerned with what is on top of the. Spot barrier as much as I am what is below it getting I to the house
Now, back to my actual question. Is the support under the foam a huge deal or will the concrete hold it down and level it pretty well. Do I need to worry about the fronts footer area???

Hopefully your pouring on a crushed stone base. I wouldn't worry too much about the vapor barrier. If done properly your drainage system should keep your floor dry. With this kind of slab you need to be dry under it. Just putting insulation under the slab won't stop the slab from heaving. I saw on tv this morning a building in Michigan that had the wall heave leaving cracks that you could see threw. Then they did a shot from outside and you could see that the sidewalk butted up against the exterior wall. The problem was obvious. Must keep your slab well drained or it will heave insulation and all. Water in the soil causes heaving dry soil does not.

As mentioned, having a crushed compacted stone base is very important. That will stop capillary action, along with drain tile and sump pump if needed.

If you have a high water table or poorly drained soil, then also adding stone under the footings is better than placing the footing in the mucky or wet soil (engineering approval probably needed here for correct base and compaction).

If you think you need to continue the vapor barrier to the outside of the footing, I personally would do that *under* the footing, assuming the wall is going to be poured (not stick). This my opinion anyway. I've seen pictures where they wrap the vapor barrier over the top the footing (after it's poured), but that seems to me to be a bad idea. You want a tight connection between the cold joint between footing and a wall and having plastic there seems to be a bad idea. Another option is to use an elastomeric material in the cold joint which I think would be better than plastic over the top, but to me not as good as just putting the vapor barrier underneath the footing.

If you don't run the vapor barrier to the outside edge of the footing, then at least seal the vapor barrier at the inside of the footing.

Posted By easyrider470 on 16 Apr 2014 08:36 PM
Well since I got the foam for free, and it's once used foam off a roof I felt it was the logical, gree. and economical choice to make. Don't think anyone can argue with me there.
Secondly like badger says I am not nearly as concerned with what is on top of the. Spot barrier as much as I am what is below it getting I to the house
Now, back to my actual question. Is the support under the foam a huge deal or will the concrete hold it down and level it pretty well. Do I need to worry about the fronts footer area???

When using reclaimed XPS you are extending the lifecycle of material that is already manufactured, which has far less environmental hit than using any virgin-stock.

I've never seen the issues involved with finish floor damage first hand, but have more than one second-party reference to the issue. Joe Lstiburek of the Building Science Corp thinks it's enough of a risk that it's worth swapping the layering in the stack, placing the poly above the foam rather than below.

Type-II EPS (~1.5lb nominal density) is easy to source from multiple vendors, and is almost universally cheaper than XPS.  I suspect Morgan's source problems are for Type-IX foam (2lbs nominal density), preferred by those radiant slab builders who staple the tubing directly to the foam for it's superior staple-retention.  Pricing on Type-IX EPS has always been lower than XPS in my (albeit limited) experience, but I could believe there would be markets/situations where you'd pay a premium for Type-IX EPS over 1.5lb XPS.

Raking the gravel layer reasonably smooth (even compacting it) prior to laying down the foam is good practice.  Don't count on the weight of 3-4" of concrete to flex 3" XPS to any sharp or deep anomalies.

easyrider470
When you pour the patio, make sure you add at least 2 feet but preferable four feet of horizontal insulation next to those frost footers. If you have placed foam under them, then there is not enough heat loss to prevent frost heave at those locations. As Smartwall mentioned it will heave insulation and all.

Also use glycol in your radiant heating.