Copyright Goodheart-Willcox Co., Inc. 250 Hydronic Heating: Systems and Applications 12.4.2 Concrete or Gypsum Thin-Slab Floors There are several methods of installing a hydronic radiant heating system over an existing surface such as a conventional wood-framed floor. When the installa- tion of a slab-on-grade radiant system is not possible, one alternative is to install a thin-slab radiant flooring system. This system is a more practical application for existing homes and buildings where radiant heating is desired. Thin-slab sys- tems incorporate either a formulated concrete or poured gypsum underlayment material over the radiant tubing, Figure 12-23. The installation procedure for thin-slab floor heating is similar to a slab-on-grade radiant system, with a few exceptions and requirements. One issue that must be considered is that thin-slab installations typically add 1 1/4″ to 1 1/2″ to the floor height. This requires adjustments to the rough-in openings of doors and the riser heights of stairs. Another issue to consider is that the thin-slab system will add substantial weight to an existing floor. Typically, a poured gypsum floor will add 13 to 15 pounds per square foot to the dead load, or constant weight, of an existing floor structure. Standard weight concrete thin slabs add about 18 pounds per square foot per 1 1/2″ thickness. This is typically not an issue for basement floors or slab on grade, but should be considered for wood-framed floors on upper levels. Do not assume that the floor structure can support the additional weight of either type of thin-slab application. When in doubt, have a qualified structural engineer or designer verify that the proposed changes to the building can be supported by the existing framing. Otherwise, determine what changes are necessary to support the additional load. 12.4.2.1 Installing Thin-Slab Floor Systems The installation of a thin-slab system begins by stapling or securing the tubing to the existing subfloor. If the tubing is being installed over an existing concrete floor, install a vapor barrier and insulation as described for slab-on-grade sys- tems. Consult the tubing manufacturer for the acceptable method of fastening the tubing to the floor. Once all tubing circuits are in place, pressure test each Concrete or gypsum underlayment Subfloor Insulation Floor joist Tubing Staples attach tubing to subfloor Finish flooring Goodheart-Willcox Publisher Figure 12-23. Another method of radiant floor heating is the thin-slab system. It incorporates either a formulated concrete or poured gypsum underlayment material over the radiant tubing.