Heres a easy one, what r value / thickness of rigid would you recommend for under a basement slab. No radiant heat, unfinished (for now) in Northwestern Ont (Cold as hell) I was thinking 3". Let me know what you think.

The floating slab will be very close to bedrock, min 5 inch crushed rock to a max of 4 ft

Also I am offering CAD services for residential drafting...... jk

Thanks for the help

Hi Ben,

This Canadian Mortgage Home Corp study might be of some help to you.
http://www.cmhc-schl.gc.ca/odpub/pdf/63728.pdf

Your description is unclear - Basement Slab (below grade)?, Floating Slab (on grade)? These are two different animals and approaches will vary based on the slab location. You may have both if you have a walk-out basement on one side and a full depth basement on the other.

Don't forget the slab edge insulation and be sure to provide a thermal break in the slab if it continues from the house out into the garage,

I would recommend as much insulation as you can affors up to 4" of high r-value rigid board foam or closed cell spray foam. As you mention it is unfinished and un heated "fo now" you will not have the opportunity to get under there after construction.

You also mentioned bedrock. If there is a good deal of ledge or bedrock do not skip installing Radon piping inder the slab. Follow the EPA guidance document for this installation. Where there is bedrock there is a higher likelyhood that you will have Radon. Radon is the number two link to the cause of lung cancer. It is cheap to install this now (several hundred dollars) and far mor difficult to do later and more costly (over a thousand dollars). Radon levels cannot be determined till the space is enclosed and tested and you cannot rely on testing of other houses in the area. I have found homes within eye sight of each other test with a difference of ten times the other.

Typical specs will require 2" - 4", engineered slabs often require thicker insulation around the perimeter edges of the slab. I have found that it is often more cost effective to increase the insulation thickness and eliminate some of the required fill materials.

I had a customer that wanted to increase his slab by 12" (old garage converted to main-floor residence) I sold him an 11" R45 sleeper floor (without concrete) for a lower price that what it would have cost him for sand/ gravel and 4" of insulation. Obviously this was a unique situation but I am sure that an extra inch of EPS or SM board is cheaper than yards of crushed rock and sand.

Remember the thicker the rigid insulation the better and the investment will pay off for the life of the structure.

FYI, "Passive Houses" (google PHIUS) in the US have used 16" plus under the slab, so don't think that 4" is the maximum you can use. Think of 4" as the minimum. Also, don't forget the foam between the footing & floor and wall and floor to thermally isolate the slab. One of the most common & most easily corrected mistakes most people make.
BobI

If he basement floor is below the frost line then I doubt that anything over 2" would be noticeable to comfort or utility bills.