Can you put WarmWire right on a plywood subfloor without backerboard?
Yes you can. We have the UL listing for direct application on plywood. But, consider using a cement based backerboard on top of the plywood, as recommended by the Tile Council of North America. It costs a little more money, but the TCNA likes it because the thinset bonds better to a cement surface than to a wooden surface. It also stiffens the floor and reduces flexing. Backerboard means less chance of cracked tile over the lifetime of your floor. WarmWire Does Not require backerboard.