I forgot to add one other interesting detail about our solar hot water system. We were not terribly excited about the off-the-shelf insulation for the exterior piping for the solar hot water system. It seemed that it would leave exposed gaps. Like the perfectionists we are, we thought we could do it better.
Adam suggested cutting 4″ PVC in half, reassembling it around the pipe, and then blow in spray foam insulation. The process certainly took more time than using off the shelf materials, and we experienced some problems with the spray foam. It turned out there wasn’t enough moisture in the PVC after we sprayed in the foam and it never cured. Rather than take the whole thing apart, we drilled in new holes and sprayed in water and foam. This seemed to solidify it all, but I am sure that the resulting foam is not as insulative as it could be.
In the end, I am not sure we would use this method again–even if we did spray in some extra water to start. Probably the way to go is to try to double up Armaflex pipe insulation.