All ive read references either heated or un heated garage. I'm close to having my garage slab poured. I will heat the garage when i'm in it working but am unsure at this point if i'll foot the bill to heat it 24x7, and if i do it will only be up to 50deg or so. I do plan on insulating the garage well as when i do heat it i dont want to spend a fortune. I do plan on having the correct wing and vertical edge insulation. My question is if a well insulated garage is not heated every day or even possibly for over a month, does the slab require under slab insulation? My reasoning is that it will be insulated, just not under the slab, instead the garage over it will be insulated. I dont want to insulate the slab and increase the insulation on the wing if its not needed. This is for mid Maine, air freeze index of 2000-2500, Slab has a good compacted gravel base of 18"-24". It will be a 6" slab with 1'x 1' outer edge sloping back up to the 6" interior thickness. So a monolithic slab. Any advice would be appreciated. thanks.

Perhaps consider doing a heat loss analysis so you can determine how much heat (BTU/hour) will be lost by an un-insulated slab versus an insulated slab. Then you can estimate what it would cost you depending on how and when you will heat the garage.

http://www.borstengineeringconstruction.com/Heat_Loss_Analysis_Calculator.html

http://www.borstengineeringconstruction.com/Integrated_Heating_System_Performance_Calculator.html

if a well insulated garage is not heated every day or even possibly for over a month, does the slab require under slab insulation?

A non heated FPSF works by conserving ground heat. It would have to be a very well insulated/sealed garage to lose as little ground heat as a FP slab alone (the slab has much less surface area and no air infiltration). On the other hand, you could get close and then set the garage thermostat to just above freezing.

Why a 6" slab? How will you heat the garage?

You'll probably regret not insulating under the slab. Check out Craigslist for reclaimed insulation. There is a lot available from commercial roofing projects at great prices. I just bought some 4" eps for a project for about $.26 per sq ft in truck load quantities.