staple-up
JK Mechanical Staple-Up Radiant Heating HVAC SystemsTubing is attached to the underside of the existing subfloor. Aluminum plates can be used to spread the heat evenly under the subfloor. Insulation is placed in the joist space beneath the tubing. A 2” air space is usually left between the insulation and the bottom of the subfloor. If aluminum plates are used, the insulation may be pushed up tight against the plates.
http://www.jkmechanical.com/hvac/radiant-heating/

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With the staple-up system, aluminum heat emission fins are used to hold the radiant heating tubing up against the bottom of the sub-floor.
http://www.radiantec.com/retrofit/

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Note added at 3 hrs (2015-08-09 21:22:28 GMT)
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Subfloor plates: Aluminum plates supported by wood or plastic spacers contain channels to accept radiant heat tubing. Hardwood floors can be nailed directly to the subfloor plates, or carpet and pad can be placed over the plates. A second subfloor also can be nailed over the plates to produce a smoother finish for vinyl areas.
Staple-up or hanging: Tubing is attached to the underside of the existing subfloor, and insulation is stuffed into the joist space beneath the tubing. A 2 -to 4-inch air space usually is left between the insulation and the bottom of the subfloor, allowing the warmth to circulate in the air between joists.
http://www.angieslist.com/articles/3-ways-install-radiant-he...

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heat transfer plates that are stapled up with radiant heat tubing under your subfloor.
https://about.me/ledifiedcompetitionstem