Opinions on insulation for under slab

[tankd0g]

So I've decided to spend the extra money on Celfort C-300, but since I have a basic slab with no footer it raises a question. If I go the full 24x24 with foam, the entire slab is going to be sitting on foam and the edges are going to exposed because the gravel base is 12" above grade in the front and it would require an unreasonable amount of back fill to cover that.

If I only do a 23x23' square of 2" foam, then the 6" curb wall will be sitting on just vapor barrier and rock, which will protect my foam but I am concerned there may be cracking between the curb and the insulated slab. I'm aware that either way I am going to lose heat out the 10" exposed outside perimeter of the slab, I plan to put my pex at least 12" inside the walls all around to try and mitigate this somewhat. Anyone have an opinion either way?

 

[blkhonda1991]

see this thread http://www.garagejournal.com/forum/showthread.php?t=91322

 

[tankd0g]

see this thread http://www.garagejournal.com/forum/showthread.php?t=91322

Thanks, I didn't see that one in my search. It looks like one guy there went 4" bigger on his forms and just made a 2" XPS box to fill with concrete.

 

[wssix99]

Why not just wrap the outside of the whole slab with more foam and pus some sheathing on it. You'd essentially have your whole slab sitting in a bathtub of insulation.

 

[tankd0g]

Why not just wrap the outside of the whole slab with more foam and pus some sheathing on it. You'd essentially have your whole slab sitting in a bathtub of insulation.

Do you mean make my forms 2" smaller all around and have the walls over hang so I can come to the ground with the sheathing?

 

[kbs2244]

Dirt back fill will be the cheapest thing to do.
It will keep that exposed gravel from getting washed away, and provide frost shielding.

 

[wssix99]

Do you mean make my forms 2" smaller all around and have the walls over hang so I can come to the ground with the sheathing?

That would leave part of your walls unsupported. I'd either add the insulation just around the slab and then put a sloped cap on top, or just run the insulation all the way up the wall and wrap the the rest of the building with it, also.