But the floors do work fine.

The maple has pretty much become unglued from the low voc water based glue we used. Water based glue just doesn’t have any real strength. So now it is a floating floor. It feels a little softer than a floor that is attached solidly to the sub-floor but it does not feel bad.

The floating floor allows it to expand and contract without any issues.

The sand/concrete/structolite mix does help spread the heat out more evenly.

Last winter the water was set at 120 F, which is high but not terrible. This winter we will have installed our new energy efficient doors and fixed our windows so I suspect we will be able to get the water temperature down to 100 F.

So altogether the floor is fine. I think next time I would not do the concrete mix. I would build my own warmboard out of plywood and aluminium like we experimented in the upper floor. That one works great.

One comment I’ve read is that the nails should be 6 to 8″ apart. Given your 16″ spaced stringers, you may always have problems with wood movement.

Also, at the Radiant Panel Association, they discuss the ability of media to transmit heat. Sand (by itself) is not a good media to transmit heat through for flooring. I did a search on structolite to see what it was and the USG description says that it is a good insulator. I take that to mean that you will have to have very high water temperatures to get sufficient heat out of the floor.

Good luck.