Graig,

We are currently dealing with a radiant floor heat in a 1780's Cape in western Mass. We have used Climate Panels on other jobs, which are similiar to warmboards. It makes for a neat install, but I always feel there should be some insulation to help prevent heat lose. Plus nailing the floor itsself can become an issue, as you are bound to the spacing of the panels. I have also worked with the system Dave described, but would you need to increase your joist size to help support the added weight of the gypcrete?

As for the Cape, to limit an increase in the floor thickness of the original house we omitted the ridgid insulation and layed 3/4 inch tubing on the sub-floor. We then layed 5/8 inch plywood strips in between the loops for nailing. The heating contractor recommended this to ensure that the 1 inch wide pine floor would have full contact with the tubing, thus creating the optimal heat distribution. Of course that then leads to the expansion and contraction factor. The flooring was delivered at 5% moisture content. I will add that I personally would still include a layer of 1 inch High-R ridgid insulation between the ceiling boards/sub-floor and the tubing. Just to clearify this is the system I would recommend:

1 inch Shiplap or t&g sub-floor with a finshed side for 1st floor ceiling

1 inch ridgid insulation

3/4 tubing (confirm exact product with HVAC/ Plumbing Contractor)

5/8 plywood strips layed between tubing loops

1 inch softwood flooring (I think any hardwood would move too much with this system) Hemlock may be a good durable choice

I hope this helps. Good luck. Keep us posted on how it worked out.

Dan