Insulating below slab-on-grade floors

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. 

That is a common question, and a hot topic that often brings dissagreement, but we like that though!  So much so that we have a page highlighting the pros and cons, check it out - 

Slab on Grade or Foundation and Basement; Which is Best?

But to address your questions specifically, I've included your questions in the response to make it easier for both of us - 

1. Cost - I know a slab is cheaper but having a basement that we can finish allows for a smaller footprint with less walls and roof material.  Is there a big difference in cost? 

We have typically found above-grade walls to be a more affordable building option, but no, it is not a huge difference. A basement will result in a much higher excavation cost to begin with, but also, concrete is a more expensive structural material compared to wood, so an above grade wall assembly can usually achieve a higher performance level for a lower cost since you can use wood as the structure. Yes, you will have a greater amount of above grade wall to build, but as mentioned, that can cost less than the same amount of below grade wall.

On the other hand, there are ICF advocates (insulated concrete forms) that would argue in favour of basements, including bottom to top ICF walls; those builders would argue there isn’t a great difference in cost.

Personally, I wouldn’t base the decision as much on cost as a few other issues, namely: durability, quality of life, and ecological impact. Cement manufacturing emits a significant amount of greenhouse gases, and sand that is suitable for concrete production is becoming an increasingly scarce global resource, where wood is  renewable. that's why we as an organization like to promote the more sustainable options, and limiting the use of concrete is a large part of that, ergo we like slabs.

As for durability – basements walls cannot dry to the exterior, so more caution must be taken in designing and building walls below grade. Also, basements flood, that’s just a part of life. We can take steps to mitigate that with drainage and sump pumps with battery backup systems in case of power failure during storms, but it is impossible to predict levels of precipitation long into the future except to say that it doesn’t look good. If your house is above grade and not in a flood plain, you will never have to worry about it flooding. 

2. Access for mechanical - I can't get my head around having to embed drains, plumbing, electrical, etc in concrete as opposed to putting it under floor.  Not to mention running ducting for ERV.   Is this a valid concern?   What if there is a leak in the drains, how would you ever know?

More than being a valid concern is the part about wrapping your head around it, so that's all relative. I have no problem with embedding the infrastructure in the concrete  but I would have a hard time wrapping my head around the idea of digging a hole in the ground to live in it when I could be living above grade. It's more a matter of what you're used to, if you go to California for example, you'd have a hard time finding a basement because they build all their houses on slabs and don't give it a second thought. And... if a drain DID have a leak and a the occasional drop made it to the ground below, no you probably would not know, nor would you probably care.  

Where concern is valid is - you cannot change plumbing in the future so you need to be sure you are happy with the house layout to begin with. In order for a drain to leak you’d have to screw it up in the first place. So hire a licensed plumber and don't worry. As for the water feeds and electrical lines, they should be put into conduits (a sleeve), so if there ever was an issue you can pull them out and replace them. HRV ducting should not be run in the slab, that should be run through walls and ceilings only. 

3. What about a hybrid - we don't need a full basement, what are the implications of having half of the footprint as a slab, and half basement?

A hybrid can be done no problem, but it would add cost and building complication, so if it were me I wouldn't do it unless there was a very good reason due to the building terrain a house was to sit on. A hybrid would  still leave you with embedding stuff in concrete as well as the durability concerns of a basement, so that sounds to me like it will ensure you are concerned out about it whichever one you choose :) 

We are currently building a new demo house, on a slab, here is a video of the install if you want to have a peek. It is entirely above grade, and as I write, the house on top is being built, which will be insulated with cellulose (recycled news print).
Here are some other pages on slabs for your interest. Don’t hesitate to write back with more concerns, we love slabs and are happy to keep the discussion going. 

• Best practices for insulating basements
• Choosing between a slab and a basement