No. of Recommendations: 1
Then I thought, "That's a LOT of copper pipe" - expensive.

I didn't find it that expensive. I got a 60' roll of "soft copper pipe" at HD; I forget the price but it didn't sticker shock me. (I know I used 58 feet for the runs because I lopped off 2 feet to see if I could bend it to the 14" diameter I needed in some placed without kinking it, and I could.) When I was all done, I had 4 inches of pipe left over. How often does that happen? <grin>

I thought maybe one could drop the sub-floor a couple of inches below the top of the joists, embed the pipes in concrete in that cavity and tile directly over the concrete.

Maybe, but you have to get in between several joists, which means you have to jump through them at some point. And structurally, the worst thing you can do is cut in the top two inches or the bottom two inches of a support joist. (Not a perfect example, but if you take a piece of paper and cut a hole in it, you can still hold the two ends and snap-snap-snap it without tearing it. Start a small tear in one side, and your snap-snap-snap will probably rip it in half.)

Otherwise you'd have to raise the level enough to jump the pipe across the top of the joist, but then it would be right next to the surface, making a hot spot, or you'd have to make it so deep as not to do that, or you'd have to run down and below and back up in each joist cavity. Seems like a lot of jumping around. That's why they like to raise the floor level by a couple of inches instead of fooling around like that. Building it into the design as you built the house, you could have avoided that by using smaller joists or depressing them or whatever.

Which is why I liked the "air heat" method, particularly for a retrofit. My floor height is about 1/8 inch higher than it was. I lost a teensy bit (1/8") between the old subfloors and the new, but made it up (and them some) in the two layers of thinset and the thicker granite tile.

And, if it's just a matter of keeping the floor somewhat warmer, w/o trying to heat the room, it probably isn't worth the extra cost, time and work.

Agreed. But I was doing the floor tile anyway (which meant taking up at least one level of the subfloor), already had the AutoCirc pump pulling hot water down the line anyway and thought it would be a simple matter to add some piping. It turned out to be more than that, but it was fun, and it WORKS!
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