Our radiant heat system is what hydronics contractors call a "staple up". We have a crawlspace and the house originally had forced air heating. Unlike houses with radiant installed from the start, where the heating pipes are threaded through a poured concrete floor or slab prior to pouring the concrete, the pipes in our case are fastened to the subfloor. The contractor who installed our radiant system crawled around under the house for 2 months threading pipe between the joist bays beneath the floor.
For insulation, he stapled polyethylene/foil bubble wrap radiant barrier insulation immediately under the pipes, with a small gap to allow the radiant barrier to function properly. Below that, he installed 4" of fiberglass batt. The house previously had fiberglass batt insulation under the floor, but, as I've mentioned before, fiberglass batt is a poor choice for insulation, and even worse in an application such as this. It has a tendency to sag, and in this case, because there is nothing across the bottoms of the floor joists, the tendency is even worse. In a wall-based application, at least the drywall is holding it in place, but I think the insulation contractor used fishing line or something to hold it up.
So as part of our quest to tightly seal the thermal envelope in our building, we decided to have closed cell foam blown into the spaces between the floor joists, just like in the walls and ceiling. But the situation is more complicated there, we can't simply foam on top of the pipes, since there needs to be some clearance. The foil radiant barrier provides that clearance, but will it withstand the pressure of the foam expansion? At first, Forrest, our architect, thought not, so we discussed installing some metal plates in place of the foil radiant barrier. But Paul said that would result in a much larger expense and delay. I asked Paul whether Ponzini, our insulation contractor, could do a test item that would determine whether installing the foam on top of the radiant barrier would work. Here's a picture of the test item:
The test item consists of two 2x8's with a plywood top simulating the floor assembly, and some radiant barrier stapled partway down. Below, you can see the radiant barrier and the gap:
The foil radiant barrier seems to hold up well, the foam pressure isn't enough to collapse it. So we are going forward with foaming on top of the radiant barrier. I have, however, asked Paul to check whether there is enough clearance on all the pipes and to ensure that the barrier is firmly stapled in place and hasn't collapsed in the 4 years since we had the hydronic system installed.
One issue that does concern me is leaks. A large earthquake could separate the PEX tubing from the manifolds and zone pumps. Leaks within the PEX tubing are less likely, but still possible if a pipe fails due to long term deterioration. With closed cell foam insulation, the water would accumulate between the insulation and subfloor, ultimately leaking out upwards and possibly warping the subfloor. The only indication of a leak is that the hydronic system starts drawing a lot of water out of the domestic water supply and the system pressure gauge indicates a drop in pressure from the normal 7 psi. But the pressure gauge is in the hydronic closet and we mostly don't look at it (though of course if there is a large earthquake we probably will check it to make sure the system is OK).
I briefly searched the Web for some water sensors that we could put into the floor to detect a leak directly. They all require replacing AA batteries at periodic intervals, a nonstarter since the sensors would be sealed into the floor by the insulation. This is a general problem with current sensor technology. If you want to put a sensor in an inaccessible place, even if it could have a wireless data connection, you still need to supply it with power through a wire. I called Kevin Smith, our hydronic contractor (he now has his own company, IntuitiveClimateControl) to discuss options and he said there are a variety of sensors we could put on the water line to the hydronic system that would measure and report if the hydronic system began to draw water at an accelerated rate. So when we finish the job, I'll probably look into putting in a sensor, but not now. There are too many things to finish up, and we just want to get this job done so we have our living space back.
Neither Paul nor the insulation contractor seemed to know much about our situation, which leads me to believe that they haven't run up against a hydronic stapleup that needs closed cell foam. I wonder if anyone else has tried closed cell foam insulation together with a staple-up hydronic system?