oikos®

What if you have to add radiant floor heating to an existing structure or to a small area of a new house? The usual tactic is to spread the heat from the tubing to the floor with pre-formed aluminum plates. The plates wrap around the tubing and lay flat against the underside of the plywood subfloor between the joists. The tube is threaded through holes in the ends of the joists then pulled into each joist bay, one at a time. The layout appears below. A single length of tubing generally covers 200 to 400 square feet of floor space, depending on the tube spacing.

In an underfloor installation, the tubing is threaded through holes in the joists then pulled ino one joist bay at a time. Aluminum plates are then used to transfer heat to the floor.

An underfloor system may not be suitable for every home. For one thing, it usually costs more than a concrete slab. There may also be too many obstructions. The average house has lots of plumbing and wiring beneath the floor that can interfere with tubing placement. And if the underside of the subfloor is peppered with nail points from a hardwood floor, you won't be able to fasten the plates. On a new house, you can avoid these problems by keeping the radiant tubing in mind when routing pipes and wires, and by using a glue-down or "floating" hardwood floor instead of strip flooring.

Another problem with underfloor systems is heat transfer. Not only is plywood a better insulator than concrete, but the aluminum fins only touch about half the tubing surface. This makes heat transfer less efficient than with a slab. To compensate, the water temperature of an underfloor system needs to be 10 to 20 degrees F higher than the water temperature in a slab. The higher temperatures aren't a problem for the tubing, but may prevent you from using lower-temperature heat sources. Because of these restrictions, John Siegenthaler sees underfloor plates as most practical in small areas like tiled bathrooms.