I have a 1-story addition on my house that is roughly 15'x30'. It was added to this house either in the 1930s or the 1950s. It has no foundation. (The bottom beams are either sitting on the ground, or sitting on fieldstones, so that the addition is *mostly* level.)  After at least 50+ years, I figure the building has settled as much as it is going to settle, so I'm not going through the cost/expense/hassle of trying to put a foundation under it.

My house+addition is build on top of an esker (small sandy/rocky hill). I don't have a water problem in my basement, and the ground under the addition doesn't have a water problem.

For all intents and purposes the addition has been gutted, all that is left is the frame and the exterior walls and the roof.

I'm located in Ontario, Canada. In the winter it sometimes gets down to -30 degrees Celcius plus windchill.

I got a few of my ideas for radiant heat flooring from http://www.legalett.ca

Here is what I'm thinking of doing:
 - putting down a layer of plastic sheeting (2 overlapping pieces of 8' wide "general purpose polyethylene film" (it doesn't say how thick it is)
 - putting down 6 layers of  1"-thick pink foam board insulation (R5/inch, the stuff you buy at Home Depot), with the joints staggered so they don't overlap, and the joints taped with red "tuck tape"
 - putting at least 2" of this insulation between the sides of where the concrete slab will go, and my existing building
 - putting down 8'x4' sheets of wire mesh over the entire surface (using "wire mesh chairs" if I can find a place that will sell them to me)
 - putting down 4" diameter piping (for air)
 - putting down more wire mesh and rebar and wiring everything together so it won't move when the concrete is getting poured
 - possibly putting in tubing to carry electrical wire and/or phone cable
 - pouring a 8" thick concrete slab
 - when the concrete has cured enough, framing a new interior wall, with the new wall attached to the concrete slab, with the studs in the new wall offset from the existing studs in the outer wall (to provide a thermal break between the outside of the addition and the inside of the addition)
 - at least a couple of inches of spray-foam insulation (I'm thinking Tiger Foam) in the outer walls, with the rest of the cavity filled with cheaper insulation (or I may just splurge and fill the entire cavity with spray foam insulation)

I realize that I'm super insulating my addition. My wife is tired of freezing in the winter. The addition is going to be turned into two rooms: a master bedroom, and a kitchen. Next year the rest of the house (built around 1880) will get renovated.

Any one have any comments or suggestions?

you want to be concerned about radon. look at using the Barrier , using Insul-Seal as a conduit for the pipes so they wont freeze. for walls. use MicroFoil radiant barrier reflectiver foil in adddition to solid convention/mass insulaiton. want to protect against radiant heat loss/gain also www.greeninsulationproducts.com

"using Insul-Seal as a conduit for the pipes so they wont freeze"

If it gets to the point that I have to worry about *air* freezing, I don't think Insul-Seal will help...

Don't count on it, especially if there is wood in contact or close proximity to the ground. It'll continue to rot and settle. The soil moisture level can change and possibly allow more settling. What about frost and frost heaves? That could cause the old wall to move with respect to the new inner wall. You need a foundation.

Two times 8' doesn't give much overlap. Plus you should consider bringing the plastic up the outer walls a couple of feet if you do the inner/outer wall thing.

Don't know that 6" vs. 4" inches will buy you much. You do need to come up with a scheme to get insulation down into the ground around the perimeter close to frost depth. That's where you'll lose the greatest amount of heat. Interior of the space the ground won't get much colder than maybe 13 to 16C.

Also, since the perimeter of the slab will be at, or near, ground level with essentially no frost protection you will need to pour a footing down to frost depth to provide stability.

If by "tuck tape" you're referring to duct tape, forget it. Duct tape adhesive drys out and crumbles in a few years. The one thing duct tape ain't good for is sealing ducts! Consider a foam adhesive.

Seriously rethink the air idea. You'll be hard put to find anyone who thinks it's a viable way to heat a concrete floor. You'd be far better off to use hydronic. Much, much, more efficient.
Four inches is more than adequate. Use hydronic and you won't need the thickness. Less concrete will also give you a much better response time when you turn on the heat, or raise the temperature. Use fiber reinforced concrete and you won't need to mess with the wire mesh. It'll take hours, if not days, to bring 8" of concrete up to temperature using air.

Of your entire plan, this part makes the most sense to me. However, if you put in a foundation you may be able to create a well insulated wall without adding the new interior wall.

I say you should just bite the bullet and put in a foundation under the existing exterior wall. Rebuild the bottom portion of the wall as needed to sit solidly on the the new foundation. Then you can never be accused of "cobbling it together!"

If you haven't done it, get a real heating/cooling load calculation done. Explore various scenarios and find the one works best for you. Sorry to be blunt, but when you talk about using 6" under the slab and 2" around the perimeter I'm questioning just how well you understand how heat flows out from a building.

Since you don't know what "tuck tape" is (and it is not duct tape), I'm tempted to disregard the rest of your advice, for obvious reasons.

I've decided against air radiant because of the added cost. Roughly $4500 CDN for my 15'x28' addition, just for the heater+distribution box+piping+connectors+piping supports/etc+engineering+plans is more than I want to spend, especially since that money can go towards solar heating panels at some point in the future.

My pad is sooooo thick (8") because I want to use solar to at least partially heat it at some point in the future, and thermal mass is needed to hold the heat  from the day, for release at night. Another factor affecting slab thickness is that it isn't much more expensive to add another inch of concrete to my slab, than it is to pay a guy to wheelbarrow in another in of gravel. (I want the floor level to be raised a bit inside my addition.)

I find, after doing an Internet search, red Tuck Tape is primarily a Canadian product. I've never seen it in the US. According to some forum discusssions I find I'm not the first State sider who didn't know what red Tuck Tape is! I was familiar with the Tuck brand name 40 years ago. That tape was pretty much what duct tape is now.

Now, can you find it in your heart to cut me a little slack, please? Thanks.

Your plan appears to insulate the ground from your addition very well. This will cause the actual frost depth at your addition perimeter to be deeper since there will be less heat from the addition escaping to the soil. This might cause more frost heaving than before - depending on how rocky/sandy the soil is and how much moisture is present. You might want to check with a soil engineer to see how susceptible to frost heaving your soil is.

Half of the addition used to be a woodshed, and has never been heated. The other half of the addition was used as unheated storage for the last 9 years.

Because of the amount of insulation that I'm putting under the pad, I don't anticipate the frost depth changing much after my renovations are done.