Radiant Floors Demand
Careful Planning

John Siegenthaler remembers a time he was asked to price a hydronic floor-heating system in a home that had just been framed. The floor-heating system of choice would require that the hydronic tubing be fastened to the subfloor then covered with a 1-1/2-inch thick gypsum or concrete slab. But the framing couldn't handle the weight, and the doors and windows hadn't been sized to accommodate the slab's thickness. Adjusting the original design would have been easy enough, but making the needed changes after construction started was too expensive. In the end, he opted to install panel radiators.


Figure 1
It's okay to wheel the concrete over the tubing, but put a plywood scrap beneath the nose of the wheelbarrow as you empty it. The plywood protects the tubing from the wheelbarrow's steel noise piece.

The story makes an important point. Floor heating requires more planning by the builder than other heating systems. Siegenthaler has 18 years experience as a heating engineer, but he is limited by the building design. Installation details are often dictated by the floor structure.

Despite these complications, hydronic radiant floor heat is growing in popularity. Not only is it inherently comfortable and efficient, but the fact that it requires water temperatures as low as 90 degrees makes it a good match with lower-temperature heat sources like heat pumps and solar. The following guidelines for designing and installing hydronic heat were adapted from Siegenthaler's book, Modern Hydronic Heating.


Slab-on-grade homes make the easiest installations. The tubing is typically arranged in 12-inch on-center serpentine loops that are fastened to the slab's reinforcing mesh with wire or nylon ties. To reduce heat loss to the ground, the tubing should be placed over a minimum 6-mil polyethylene vapor barrier and at least 1 inch of extruded polystyrene insulation. (Tongue-and-groove is best.) Insulation thickness should be increased to 2 inches within 4 feet of the edge of the slab. Two inches should also be used as a thermal break at the edge of the slab.

Figure 2
As the concrete advances, a worker should pull the welded wire mesh up to half the slab depth.

Before the tubing is installed, make sure that all underslab plumbing or electrical conduit is in place, and that all utility trenches have been filled and compacted. When excavating, be sure to account for the combined thickness of the slab and the under-slab insulation, especially near the edges where thicker insulation is used. The welded wire reinforcing should be lapped 6 inches on all joints, and the joints tied together with wire ties.

Pressurize the tubing to at least 40 psi before the concrete is placed in order to test it and to protect it from damage during the pour. Then place the concrete so that it flows parallel with the tubes. This makes it less likely that the concrete will push the tubes out of position. Assign one person to pull the welded wire mesh up to half the depth of the slab while the concrete advances. A 3-foot long blunt steel hook with a loop-type handle on the end works well for this (Figure 2). The slab can be screeded and troweled in the normal manner, but keep the blades of the power trowel away from the tubes where they stick up out of the slab.

Thin-slab systems

When hydronic tubes are installed over wood framing, they're usually covered with a 1-1/2-inch thick concrete or gypsum-based underlayment to distribute the heat evenly across the floor. As with the slab-on-grade system, put plenty of insulation beneath the floor. Siegenthaler recommends at least R-11 if the space below the floor is heated, at least R-19 over an unheated basement, and a minimum of R-30 over a crawlspace.

Most thin slabs add 12 to 18 pounds per square foot to the dead load of the floor. This may require larger floor joists or narrower joist spacing than a conventional floor. In addition, stair carriages, door and window openings, cabinets and toilet flanges must all be raised 1-1/2 inches to accommodate the slab thickness (Figure 3). Most builders install wooden sleepers beneath the front and side edges of the cabinets, and block up toilet flanges with plywood shims.

Gypsum or concrete?

Underlayments can be gypsum or portland-cement based. Gypsum-based underlayments are self-leveling, but must be pumped in place by manufacturer-certified contractors using special equipment. If you can't find an applicator, you can substitute a portland cement-based concrete that's mixed with a superplasticizer. Depending on

Figure 3
Stair carriages, door and wndow openings, cabinets and toilet flanges must all be raised 1 1/2 inches to accommodate the slab thickness. Toilet flanges are usually raised up with plywood.

where you live you could pay only half as much for concrete as you would for gypsum, including labor. You'll need to give your redi-mix plant a recipe, which you can get from your tubing manufacturer. The concrete can be pumped through a hose or placed from wheelbarrows (Figure 1). Siegenthaler says to make sure your crew can handle at least 4 cubic yards in no more than an hour. You'll also need to put some type of bond-breaker over the plywood deck and wherever the edges of the slab meet a wall or other fixed object. You can use a 6-mil polyethylene slip sheet or a form-release agent for this. If you choose the slip sheet, it must be installed before the tubing. If you prefer the form-release agent, make sure it's chemically compatible with the tubing material.

The concrete slab will also need control joints. Control joints divide the slab into several smaller slabs, which can better absorb building movement without cracking. (Minor cracks won't hurt the tubing but might crack tiles or other floor coverings.) If the tubing doesn't pass beneath the joints, you can saw them after the slab hardens. Or you can fasten thin plastic angle strips or lightly oiled wooden strips to the subfloor before the pour. Make these strips 1/2 inch shorter than the slab thickness. Doorways are good locations for control joints. With proper planning a radiant floor can fit into just about any house plan.

The information in this article was adapted from Modern Hydronic Heating, by John Siegenthaler. Delmar Publishers, Albany NY, copyright 1995.

ęCopyright 1996 Iris Communications, Inc.

Source: Oikos

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