Insulation UNDER concrete slab?

[rsevilla]

I'm building a 40x28 garage in a few weeks. I've seen on here how folks recommend putting insulation (4'x8' sheets) under the concrete slab. I am building with footers and concrete block walls up out of the ground and then 2x6 stick built on top. The garage will be insulated but not heated. Don't know if I ever will heat it. Some people say it helps keep moisture out. Wouldn't just a heavy mil plastic do this as well? Thanks for your insights!

 

[ed_h]

The best use of insulation in this case might be to put it on the outside of the foundation walls all the way down to the footings.

What this does is thermally isolate the ground under the floor from the ground outside the building. The ground under the floor then effectively becomes a thermal mass that can help to damp temperature swings in the building.

And yes, a plastic film would make a better moisture (but not thermal) barrier.