I am planning to put 2" XPS on the outside of my foundation wall from the top to the bottom (top of footing). Questions arise where the garage is. The garage is going to be heated so the plan was to put 2" XPS on the INSIDE foundation wall there. Can I have XPS on the inside AND outside of the foundation wall around the garage exterior perimeter. Since both inside and outside won't breathe, I am wondering if this is ok to sandwich the concrete between XPS like that?

My other consideration is to run exterior XPS and "overlap" the garage wall start/end where it meets the rest of the house with maybe 3 or 4 feet and then stop the outside XPS around the majority of the garage.

There is no problem at all having concrete remain damp. In normal ground conditions, unless you are in a desert climate with bone-dry soil, the soil and concrete always will have some moisture in it. The concrete never really "dries."

I have a similar situation, if I understand yours correctly, a house with attached garage. The garage rectangle overlaps the back corner of the house rectangle by 20 feet or so. There is a 2" XPS foam layer outside of the house foundation (another 2" inside as well). The garage is not heated, but is insulated. The partition wall (2x4) that separates the garage area from the shop in back of it also is insulated. I ran 2" XPS around the inside of the garage foundation walls, down to the footer. The 18 foot garage door is a supposed R-18 insulated one (Wayne-Dalton), although garage doors rarely perform to better than 1/4 to 1/3 of claimed R value. The slab is uninsulated. I reasoned that since I don't heat the garage I may as well just let ground heat temper the air to well above outside air temperature. This actually works. In the recent cold weather, ranging to below zero (F) at night into the single numbers and low teens daytime, the coldest I saw the garage inside temperature was +35 F. Most of the time, with more normal outside temperatures, it runs around 40.

At the two places where the garage foundation, poured much later than the house foundation, ties into the latter, a strip of the foam was removed from the foam layer on the house foundation, for a concrete-concrete connection. This resulted in two small thermal bridges, with those two garage foundation walls serving as "fins" for sucking heat out of the house foundation wall. At one of those two places I did what you suggest for yours, which has a few feet of XPS also outside of the garage foundation. One place is hidden by a porch hiding it, and the other was covered with the same surface bonding cement I applied over the rest of the exterior house foundation foam, and in a location not easily seen from most places on the property. In terms of thermal bridging, four feet of concrete gives a nominal R value of almost 4; with the R10 inside the house foundation I'm not really worried about a cold spot.

I covered the exposed foam inside the garage foundation, about a foot high, with Durock, fastened with adhesive. A 2x6 with a groove dadoed under the outer edge of the 2x6, captures the top of the Durock and butts up against the sill of the framed wall. The slab was poured up against the Durock. The Durock provides thermal and abrasion protection for the foam.

Well if the concrete never really dries ( makes sense ), I think I will just put 2" XPS around the inside and outside. The house footprint is like two rectangles. One is 30x60 running N/S and the other is 20x30 running E/W. The 20x30 is the garage and it's 20' wall butts up flush against the 30x60 60 foot wall with the edges flush to form a sort of L shape. I'll just run 2" XPS around the whole "L" outside, and also the inside of the garage. I guess I won't worry about it being sandwiched between two XPS layers...

This will be good because there also will be a 2" XPS wall (the inside of the garage perimeter) where the basement and garage share a common wall. This warms that wall with the inside garage XPS layer functioning as an exterior 2" wall of XPS on the basement.

Not to sound like a broken record, but 2" of EPS would be nicer to the environment, and in 50 years after losing most of it's environmentally damaging blowing agents the XPS would only be performing at EPS levels anyway. Same polymer, different processes, huge difference in lifecycle global warming potential.

It's common to have foam on both sides of a poured concrete foundation- its called "insulated concrete forms" ( ICF). From a "time is money" point of view it's usually cheaper to go with ICF than adding BOTH interior AND exterior foam after the fact.

Both XPS and EPS "breathe" sufficiently at 2" thickness that moisture saturation of the concrete high to levels high enough to rot the foundation sill is rare, and can be rendered impossible with an EPDM sill-gasket (not a cheap foamy). A kraft facer on a batt is more vapor tight than 2" of XPS, and more vapor tight than 6" of Type-II (1.5lbs density) EPS. A polyethylene capillary break between the footing and the foundation wall helps quite a bit too.

On the exterior, my apologies I have EPS on my lumber list. For the inside under-slab I have XPS there. I will have to look up the compression psi of EPS for garage floor and basement as I am unsure of that figure.