I’ve put pex tubes between joists under the subfloor. that is how my
radiant guy designed it. The whole theory that as long as you contain
the heat it will get to the room eventually etc, etc. I also couldn’t
put them above the floor due to a low ceiling.

Since then I have realized the virtue of using things to pull the heat
out of the tubes. Concrete is good and cheap. We all like heated
concrete slabs, right 🙂 Then you have aluminum fins. Good, but

So this week I’m going to experiment 🙂 I’m going to pile a mixture of
sand and small amounts of cement/water onto a strip of plywood and
then press and screw the plywood up against the pex tubes and
subfloor. The concrete will squeeze around the tubes and create a mini
concrete slab! The floor is strong enough to hold it.

The idea is that the concrete will pull out the heat. It is a dense
mass so it will take longer to heat up (and cool down). I don’t need
the instant heat gratification that aluminum fins would give because
the house is hyper insulated and will change temperature very slowly.
If outside temperature changes it will take at least a day for the
inside temp to adjust and that is plenty of time for the concrete slab
to catch up.

We’ll see if it works!