Is there a basement under it or just the crawlspace? I have the same size and style house on a basement, the North half is always colder and I was looking into some underfloor radiant as well for the last few months. The Ultra-Fin, from what I understood, is typically sized for 1-on, 1-off in each joist space with 140 degree water. Like if the plates are 10" long, you'd space 10" without a plate and then add another plate, full length of each joist space. That's approximately $4k worth of Ultra-Fin to do your whole house, plus the PEX- are you only doing a certain section of the house or how are you getting away with 1200 ft of tubing?. I was using about double the propane you are, but again there's the basement if yours doesn't have one. Will you be adding a forced air coil under your furnace too, or relying solely on the radiant? I moved my furnace to the basement and put a hot water coil in, and have been heating that way for I think 7 years now, works very well aside from the fact that the North half is 5 degrees colder than the South half, this is in MN. Remember with the Ultra-Fin, it needs air space under the fin to the insulation, for convection to occur. I think I saw 2-4"?