johnfrwhipple: He's already building it with more than 3/4" of foam on both the wall and the slab, and it needs to be more than 3/4" in his climate. With only 3/4" he'd be putting the studs at risk for mold growth from either wintertime condensation on the cold foam.
An interior vapor retarder would cut the wintertime load, but raise the average load from ground moisture, which he clearly has, so to be safe he has to skip the poly, and keep the average winter temp at the foam/fiber interface above the interior dew point of design interior temp of 21C/35% relative humidity, which is +5C. This would not meet code in Canada, which requires the foam/fiber interface to be above +5C at the outdoor design temp, not the winter average temp. But in any climate warmer than Saskatchewan it would lower rather than increase mold risk with this approach.
His average winter outdoor temp is about -4C, so with R13 batts in the studwall and R3.75 (3/4" XPS) the average temp at the foam/fiber interface on the above-grade section would be:
-4C + [(21C- -4C) x (3.75/(3.75+13)]= +1.6C
which is well shy of +5C.
Bumping that to 1" would be code-legal here, but only yields about +3C.
At 1.5" XPS (R7.5) or 2" EPS (R8) hie's in good shape though:
-4C + [(21C- -4C) x (7.5/(7.5+13)]= +5.1C
Any condensate that forms on the above grade portion in the overnight hours or during cold-snap re-evaporates before reaching the stud plate.
Vancouver's average winter temp is about +3C, but the wintertime interior moisture is also much higher, so it's safer to design for the dew point of 21C/40% RH air, which is +7C.
With 3/4" XPS you'd have:
3C + [(21C- 3C) x (3.75/(3.75+13)]= +7C, which is perfectly safe...
...but not code-legal without interior poly in Canada.
To meet code in Vancouver (outside design temp= -4.5C), if you bumped that to 1.5"/R7.5:
-4.5C + [(21C- -4.5C) x (7.5/(7.5+13)]= +4.8C
That's close enough to code legal without the poly that it should fly with inspectors, but you could use unfaced low density R11s rather than R13s, or unfaced 2"/R8 EPS rather than R7.5 XPS if they're they type that want to quibble.
In Vancouver I'd either do that, or use 2.5"/R10 EPS or 2" XPS held in place with furring through-screwed to the foundation on which to hang the gypsum for a thinner stackup.
In my own home (not too far from diyfun) I went with 3"/R19 fiber-faced iso + furring. The footings in my house are fully saturated with water 4-5 months of the year (high water table), yet the foundation dries adequately through the somewhat-permeable facers of the foam to keep from rotting out the foundation sills. If I went with foil-faced or poly I'd have to jack up the house and insert membrane capillary breaks under the foundation sills to protect them from ground moisture.