We’re building our first house. It’s in the PNW near puget sound (zone 5). We have our footings and stem walls poured and are about to do the slabs shortly. The house is on a hill and will be partially in-ground. 

Unfortunately, we didn’t think a lot about foundation insulation during the engineering phase and now can’t quite decide what to do. 

The way the foundation and slab was designed, the slab is acting as part of the structure (we’re on a hill) and therefore needs to tie into the stem walls directly. This means I’ve lost the opportunity to do a floating slab with a proper thermal break via insulation. 

We’re going to insulate the exterior of the stem-walls with 2″ EPS from footer to top of concrete. 

What I can’t decide is if it’s worth the cost/time to insulate under the slab. From what I understand, the thermal break is key and if I can’t do that, it seems any additional under-slab insulation would be of minimal benefit. 

As we’re not in a terribly cold climate, I’m less worried about slab temp in terms of comfort, but am worried about condensation. The slab will be in livable areas so want to avoid condensation issue. I am going to go with an upgraded vapor barrier underneath. Is there any benefit to insulate under slab from a moisture standpoint? would the insulation assist the vapor barrier in helping prevent/slow water absorption? 

So the question: Is there much to be gained with adding insulation under the slab if the slab will be tied into the stem walls around the perimeter anyways (in terms of both heat and moisture mitigation)? Or is our time/money better spent elsewhere (better stem wall waterproofing, epoxy floor finishing…etc)?

Bonus secondary question: What is the pros/cons of adding a capillary break between the stem wall and slab? They’ll be tied together with rebar, but is there a need for there to be direct concrete-to-concrete contact between the stem wall and slab?