Staple-up under floor systems can be the best solution of getting radiant floor installed when the flooring is over a joist system. Since installing tubing on top of the floor will either require a thin-slab over pour or a plywood sleeper to build up the floor to allow for the tubing, you will have to allow for the increased floor height with doors, stair and cabinets. Putting the tubing under the floor eliminates the need to increase the floor height.

Since the subfloor will resist heat transfer, you will need to account for that in the system design and installation. Adding heat transfer plates and increasing water temperature can keep the same heat transfer rates as an above floor system. But since you don't have the cost of a sleeper system or the thin-slab you will usually find the under floor installation cheaper than an above floor one. Most under floor systems can be designed to work at temperatures well within the "condensing" boiler range so you will receive the high efficiencies you expect with a radiant floor install. To keep water temperatures low be sure to:

• Stay away from "suspended" systems where you install tubing without heat transfer plates.
• PEX tubing will expand with temperature so you can't staple it tight to the subfloor. When the tubing is heated it will push on the fastener and over time will either break the fastener or the tubing. So the alternative is to use a "loose" fastener that will allow the tubing to slide as it expands. This reduces heat transfer rates and usually requires higher fluid temperatures to get enough heat to the room above. This method of installation is called a "suspended" system.
• Use good quality heat transfer plates.
• Heat transfer plates increase the surface area of the heating system to give better heat transfer at lower water temperatures.
• The thicker the plate the better the heat transfer, are easier to install and do not make noise
• Carbon fiber plates can increase heat transfer rates over that of metal ones and are noise free
• Use a quality tubing like Watt's EPDM rubber Onix that produces high heat transfer without noise or metal plates.

Noise is always a concern with staple up under floor systems. Since the tubing is not fully encapsulated in concrete or held tight in a factory made sleeper panel it will move under the floor rubbing on fasteners, plates and joist members. Make sure you account for the tubing expansion in your installation by:

• Increasing joist penetration hole size.
• Using fasteners that allow the tubing to slide and not hold the tubing tight.
• Using a tubing with low expansion coefficients like PEX-AL-PEX or Onix.
• Using a tubing with a low friction layer like Watt's RadiantPEX+.