We are in the design process on a new home and I have some questions on the installation of 1/2" x 2 1/4" tongue and groove oak hardwood floors over hydronic in floor heat.  There is a sub floor on joists over a full basement.  Hydronic heat tubing is embedded in 2" gypsum, with 2"x2" wood nailers 8" on center on top of the subfloor.  The gypsum and nailers are smooth and level. 

Do I need to install a second sub floor over the gypsum?  If yes, other than a nailing surface, what is the purpose of the sub floor?  To me it seems like it would only increase the insulation between the heating system and the room.  The water temperature in the tubing will be limited to 80 degrees Fahrenheit.

Will sealing the backs and edges of the oak flooring before installation decrease the expansion and contraction caused by humidity in the air?  I plan on doing the sealing and installation myself and have the final surfacing and finishing professionally done.

I am concerned about installing hardwood without a continuous nailer.  You do realize that without a continuous nailer, all butt-end joints should be over a nailer.  If so, then do you have the ability to tongue and groove each butt-end.  I may be wrong but I do not believe I would float a tongue and groove butt-end over gypsum.  Maybe someone who has a lot more experience than I can weigh in on this. 

John,

Yes, the subfloor provides a nailer for the loose ends. The nailers provide the strongest anchor provided the nails are long enough.  Local installers and retailers can tell you how long the nails should be for your application.

Gypsum is too soft to be left uncovered.  That is why it is always covered with something tougher such as tile. To me, installing a hardwood floor over gypsum is unusual. But I believe that by properly terminating the butt-ends over a nailer, the gypsum will be protected especially since your nailers are 8" on center.

If you have to use a subfloor under the hardwood, I am afraid that the delta T of the water in the tubes compared to the room temperature desired would be too small. For instance, 80º water in the tubes versus 72º room temperature is so close that the loss in exchanging the heat from the tubes to the floor might not be noticeable. In other words, the temperature in the room might not reach comfort level.

The best person to ask about installing hardwood over gypsum would be Norm Abrams from This Old House. The best place to ask about radiant heat and the necessary difference in temperature (delta T) would be the Radiant Heating forum within GreenBuilding Talk.

Do verify anything I tell you. Good luck.