I have a hot water heated floor in a sun room that is on a concrete slab in Michigan. The hot water pipes exit the basement at 40 in below grade and run horizontally inside a 3 in diameter schedule 40 PVC pipe 15 ft over to the slab. I was having huge heat losses in the winter.
My solution was to cut two 4 ft x 8 ft sheets of 2 in thick Owens Corning F250 rigid foam insulation into strips to precisely fit around the 3 in PVC pipe. See the attached drawing for exact dimensions for the 8 pieces that fit like a puzzle around the pipe. This pattern, with the 45 degree corner angles, utilizes 100 percent of the 4x8 sheet. With an R value of 5 per inch, the insulation provides over R15 all around the pipe.
The drawing shows scallops in pieces 2, 3, 4 and 7 that fit the curvature of the pipe for a snug fit. I did the scallops with a 7 in diameter carbide tipped saw blade in a 10 in capacity radial arm saw. The trick is to set the blade at the correct angle and depth to match the profile of the 3.5 in OD of the pipe. I first cut the two 4 ft x 8 ft sheets in half to get four 4 ft x 4 ft square pieces. I set up a rip fence, with wood strips on top before and after the blade, to keep the foam in place as I pulled the 4 ft sections through the saw.
I dug a 12 in wide trench to a depth of 4 in below the bottom of the pipe. I also made a template tool that allowed me to be sure that I had enough clearance between the pipe and the trench wall and floor. See attached photos. The 3 piece template has long handles and 4 hinges that allow it to be folded to drop around the pipe and then be pivoted to extend the template below the pipe. Just pull the template through the entire length of the pipe to insure proper clearance.
I laid a 4 mil thick plastic sheet under the pipe, placed the foam pieces around the pipe and then wrapped the plastic around the insulation. After backfilling the trench, the project was complete.