Insulating below slab-on-grade floors

When building a slab on grade, EPS rigid foam insulation is the most common choice of insulation, for performance and cost. I wouldn’t go as far as saying you 'couldn't' use spray foam below a slab, but I’m not sure why you’d want to. Spray foam for sure has uses where it is the best choice by a long shot, but under a slab is not the place. On this page you can see what we like and don’t like about spray foam and why – 

4th generation blowing agents for spray foam

If you need it as a vapor barrier, air barrier, or to insulate difficult areas like basement rim joists or to create a radon barrier in a crawl space it’s a great choice, but in most other applications, and certainly with a slab on grade there are better ways. When an installer sprays the polyurethane insulation they really go by experience to get the appropriate depth, and of course a skilled installer could get close, but it would still be an uneven surface to an extent. That would mean your mesh and mesh chairs don’t sit at an even level, nor does your rebar, and then the thickness of concrete may in turn vary as well, so you may need a little more, or it may be too thin in some areas. 

The only reason I could even think of to use spray foam would be if you needed to maximize the R value in order to meet height restrictions for your build, but if your allowance is down to a couple of inches that’s cutting it pretty close! 

All that said, there may be something I’m not thinking of and I’m curious, why were you thinking spray foam? 

We often get asked 'whats the best way to insulate a basement' particularly when people are doing basement renovations and energy efficient upgrades. The honest question, is there simply is no single 'best way' to do it, so if someone tells you that,  they're wrong. It depends on your climate zone, the water table on your property, and your own personal preferences for material selection, and the list goes on...

So the best advice we can give you is to learn what NOT to do in a basement, then when a general contractor suggests doing it the way 'they've always done it', you can you can tell them no thanks, if I want mushrooms I'll go get them at the grocery store, I don't want to grow them in my walls. So start here with the building science basics, we've made it simple (and even fun we think) to learn - 

How to prevent mold when you insulate your basement

That's the science 'for dummies' behind it, and more directed for renovations. there is a page for building basements properly in new homes - 

Best practices for building basements in new homes

Now that we got that stuff out of the way.... here are some more direct answers to your questions. There are a lot of questions in there so I've copied them here below so neither of us get lost : 

Question:

Concrete Foundation: Durofoam EPS (1.5" or 2"?) +wood stud with Rockwool R14 +Drywall +moisture/vapour barrier latex paint. Which EPS thickness should I go with?

Answer:

You’d be better with the 2” EPS on the walls if you can spare the space. That would give you a bit more thermal protection, and at 2 inches EPS is about 1 US perm, which is the permeability rate at which a material is classified as a suitable vapour barrier for residential construction. That would better prevent moisture from the ground and concrete walls from migrating inwards.

Question:

Concrete Slab: DMX aiflow dimple mats +(3/4" EPS, needed?) +OSB +flooring of choice

Answer:

First on the floor EPS – yes it is a very good idea to insulate the floor, if it was built in 1983 it is unlikely there is currently any insulation under the floor. But ¾” is only R3, so how much head room do you have and how much can you spare? If you have the space I’d add more than that myself, partly for energy efficiency, but also for comfort as you seem to be planning this as finished living space. R3 would leave your feet pretty cold.

And from a construction point of view, you might find that the floor is a little spongy if it rests on foam only. Alternatively, I would suggest maybe you lay down some sleepers (2x4 or2x3) and insulate between them. This would allow for thicker insulation and give you a solid nailing surface for your subfloor and final flooring.

As for the dimple matt, that’s a great idea, but keep in mind there are limitations to how much water it can handle below it. Has there been flooding before or do you have a high water table? And will you be putting in a sump pump? That’ always a really good idea and a wise investment for avoiding future damage and expensive repairs.

Question:

Header/Rim Joist: Not sure... Since this is above grade, but in the basement, how do I layer this and tie it into the wall insulation method described above? Do I add Acoustic sealant around gaps in the rim joist and sill plate Or Expanding foam? Next would I fill rim joist cavity with Rockwool, then EPS + Drywall? or Just sandwich+seal EPS panels to build it up to the foundation wall insulation.

Answer:

This is one of those places where spray foam really shines, and I don’t mean the stuff you get in a can at a box store but the kind that is installed for you. It fills all the holes, it is an air and vapour barrier and has a high per-inch R value. Besides spray foam, yes, you could put Rockwool in each cavity, then  EPS in that same cavity (sealed to the wood at the sides) and install drywall over it. 
Accoustic sealant does a great job, but it is a nightmare to clean up, so keep that in mind.  

Other questions:
How do I adequately insulate around water meters, electrical panel or rough-in pipes that are very close/adjacent to the exterior foundation walls? 

Answer: 
Spray foam and tape. Which works best would probably change on a case by case basis. 

Question:

Will there be an issue with thermal bridging or vapour with having one insulation system (old) adjacent to another (new) and only separated by wood stud?

Answer:

I don’t fully understand the question. I’d need to know which portion you are referring to and what types of materials. Since that is a question to itself, maybe start that on a separate question thread with details about the current assembly you plan to keep and details about the one you plan to add and we can see if it makes sense.