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Re: Slab thickness for radiant-hydronic floor heating

From: info@builderswebsource.com
Category: HVAC
Remote Name: 63.198.181.43
Date: 26 Jun 2005
Time: 07:36 PM

Comments

Radiant heat can be used in slabs as thin as 1.5" (such as in cases where it's placed over existing floors). However, for new construction, the slab should be typically at least 4" thick and is dependent upon the soils conditions in your area. The amount of insulation depends on your climate conditions. However, at minimum the tubing should be placed over a vapor barrier on typically at least 1" of high-density R-5 or greater tongue and groove extruded polystyrene (EPS) insulation. The T&G edge helps to prevent thermal loss between adjacent insulating panels. Insulate to at least 2" within 4 feet of the edge of the slab as well as at the outer edge of the slab to provide additional thermal resistance. Typically, the tubing is tied to the top of the reinforcing mesh or rebar. If the tubing is too low in the slab, you'll lose heat to the ground. An article by John Siegenthaler, author of Modern Hydronic Heating and Radiant Precision, suggests that the optimum tubing depth is approximately 25% of the way down from the top surface (about 3/4" - 1" in a typical 4" slab). Lower positioning increases thermal lag and reduces efficiency of the system. Since the positioning of reinforcement is often compromised during the pouring process, pay particular attention to proper supporting of the slab reinforcement. This is easier to do with rebar and dobies than with wire mesh, which often gets matted to the bottom of the slab where its usefulness in crack control is compromised.

Always pressurize the system before pouring the slab in order to check for leaks, as well as to prevent the tube from getting crushed during cement pouring. Try to pour in the the direction of the tubing to avoid moving the tubing out of position and to fill in tightly around the tubing. This will ensure the most efficient thermal transfer from the tubing to the slab. There are other types of insulating materials, such as so called "Low-E" radiant floor insulation. This consists of a thin layer of heat reflective aluminum foil sandwiched between two thin layers of foam, such as bubble polyethylene. Low-E insulation comes in rolls and is easy to apply in both slab and underfloor hydronic heating installations.

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