Built a SIPS HOUSE in Alaska (think Minneapolis) last fall, radiant slab on grade. Due to impending winter we did not finish the final grade, hence we have a 4-5" face of the slab exposed. Even with 2" of foam covered by a step flashing I know I'm losing a LOT of heat along that face. I have 4' x2" of insulation horizontally around the entire perimeter, this insulation is only covered by a couple inches of dirt. I think I'll be a lot better off just bringing the grade up to the top of the slab, but with a good bit of hand digging I can add 2" or even 4" to the face of the slab although this would be outside the step flashing. Will this be a problem? How deep does this perimeter insulation need to go? I'm thinking as deep as the slab or does it need to go deeper?. I'm also thinking of adding another layer to the 4' horizontal insulation, or possibly just adding a 2' wide layer to save money and to accommodate the grade.Do I need to remove the dirt covering the existing insulation or just put down another layer and cover it to final grade? Finally if this is a low/no traffic area is a couple inches of dirt over the insulation sufficient?

Is there insulation under the slab?

A couple of inches of dirt over polystyrene foam is never going to be sufficient.

A zip code would get us to a better handle on the local climate & subsoil conditions than a vague reference to Minneapolis, which is more than 2000 miles away, and on a glacially scoured pancake-flat plain.

Wing insulation is both for frost-heave protection and for controlling the heat loss out of the slab. But if you're in perma-frost country you may have some issues if you trap TOO much heat rather than letting the sub-slab soil cool to the outdoors. My guess would be that a 2' perimeter wing is either too much, or not enough. :-) But a better description of the construction and climate would help.

There is insulation under the slab. I'm south of Anchorage they say.our climate is very similar to Minneapolis although very locally we get a lot more snow. There are no permafrost issues . Thanks!! To be clear I already HAVE a 4' wing 2" thick but I'm thinking of adding another layer or maybe just another layer 2' wide. Since the first layer is 8" or so below final grade if I ad another layer there would not be much dirt over it. My other question concerns adding another layer of vertical insulation over the "face" of the slab but it would be outside the step flashing which is already on.

Anchorage is about a degree warmer than Minneapolis in mid-winter, but being a maritime climate, much snowier.

Anchorage also has MUCH longer shoulder seasons (and a near-zero cooling season), and an annual mean temp of 35F, which is ~12F colder than the annual mean temp in Minneapolis. I'm not sure there's any good climate to compare it to. But at least it doesn't have the permafrost melt issue to contend with that you run into in the AK interior.

If it's going over a 4' frost-protection wing the 2' addition isn't an issue. You might want to protect it with a non-structural 2' fiber-reinforced concrete slab rather than a couple of inches dirt though. Dirt can wash or blow away, concrete can't. Roughing up the surface of the vertical foam is usually enough to get concrete to stick, but it may eventually lose the bond due to frost pressures over time. (The drier it can stay, the longer it will last.)

I would add a block (perhaps made from laminated 2" pieces) of foam to the slab edge and then stucco over it. I see no reason to thicken the 2" foam wings out to 2'.

Dang! Talk about order-of-magnitude typos!

That was intended to be a 2" non-structural slab to protect the very shallowly barely-buried 2" foam, not a 2' slab or 2' foam!