As for the insulation: Fifteen years ago I attended 2 government workshops on hydronics before I put four 250 ft loops of half inch kitec
on 12 inch grid in my garage. At that time I was told that as long as my floor was well drained and I was going to use a wood boiler, it would take too many years to pay for the insulation. They recommended insulation around the slab and four feet around the perimiter. The computer layout for the grid takes the hottest water to the outer runs and the doorways. I have very good insulation and windows and doors. Ceiling is R50 plus and walls R20. Despite the fact that I live above the 54th Parralel and on a windy lakeshore I have had no problems. At startup in fall it takes two days with an infeed of 106 F to warm the slab, but after that it appears to run very little to maintain 60F in the workshop. I installed quite a few electric boilers in past years and they have all been insulated.
formula from the net = 1/R x delta T x area to heat = BTU per hour lost to the earth
1" styrofoam = R5 1/5 x 10 x 900 =1800 btu
2" styrofoam = R10 1/10 x 10 x 900 = 900 btu
4" styrofoam = R20 1/20 x 10 x 900 = 450 btu
now i am not sure what earth under your slab is for an R value but i would have to guess it is less than 1 inch of foam. lets say it is R1 (i think .1 or .2 but we will use 1)
1/1 x 10 x 900 = 9000 btu lost to the earth with no insulation..we can also make an average guess with a delta of 10 that you would require 10 to 15 btu per square foot to heat your room.. so you would require on the low end 9000 btu and on the high end 13,500 btu to heat your room, with no insulation under your slab you could be wasting as much into the ground as you are heating your room (on the low end)
i am not a heating or hvac guy but i think the formula from teh web is likely close as it was posted by an hvac guy in another forum