I am watching something new (to me) happen at a jobsite down the road.  A “floating” solid hardwood floor (4-1/2″ wide t&g cherry, 13/16″ thick and sanded and finished after) is going down over a plywood deck with hydronic heating stapled to the bottom face of the deck.  Sika’s system of 3mm foam mat and one-part polyurethane adhesive is being used to bond the flooring to the plywood through the slots in the mat, which run perpendicular to the flooring planks.  Have any of you done this?  Works over concrete slabs, also.  What are the pros and cons?  It certainly must carry a higher cost than through-the-tongue nailing, but for solid wood over radiant, maybe it is best.  The mat, 3mm or 5mm, is supposed to do a great job deadening the sound transmission from floor traffic, and one gets that neat cushion feel when walking on the wood floor.