Bricks, Masonry, Asphalt and Concrete - Insulating slab on grade garage floor...

I am building a 24'x24' detached garage/workshop with the intent of using it year around up in Calgary, Alberta. Winter here can be long and cold, so I would like to minimize heat loss thru the concrete slab as much as possible.

The garage floor will be slab on grade, and I would like to add insulation and a vapor barrier during construction ie. under the concrete. I have read some material regarding the use of rigid foam insulation under a concrete floor, but would like someone to more clearly explain the how&what is involved.

I have spoken to a contractor who told me that the vapour barrier (poly) should be under the insulation, but this seems contrary to other info I have read....

Any input would be much appreciated as I am total newbie to this homeowner/addition-builder stuff!

Brian up in the great white north

The slab of my shop is insulated from the ground and footers in order to use radiant hydronic heat in the slab. I used rigid foam insulation without any other vapor barrier, following the construction plans from the manufacturer of the radiant heat product. The floor is finished with epoxy. In three years, I have had no problems from moisture intrusion with this method. From my experience, I see no reason to use more of a vapor barrier than is provided by the rigid foam insulation.

If you wanted to use a polyethylene barrier, it would seem to me that it would be more beneficial to use it beneath the rigid foam as it would have no seams.