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Radiant Floor Heating - Dry System Hydronic

Radiant heat installed beneath a finished floor that isn’t embedded in concrete.

Hot water tubing is attached to the underside of the existing subfloor.

Dry system radiant flooring is radiant heat installed beneath a finished floor without material poured over the tubing. Several manufacturers offer dry radiant systems that position radiant floor tubing above floor, between two layers of plywood, or below floor under the subfloor. Hydronic radiant floor systems pump heated water through tubing positioned in loops beneath the finished floor. The heated water flowing through the tubes heats the surrounding air and flooring material. The floor emits energy as a result of its temperature.

Above floor systems are installed above the sub floor and below the finished floor. These systems use a grooved wood panel installed beneath the finished floor. The dimensions of the panels may vary, depending on manufacturer. Cross-linked polyethylene tubing (PEX) is inserted in the grooves of the panels and sets flush with the panel surface. Manufacturers claim their panels work under a variety of floor coverings: tile, marble, vinyl, wood and carpeting.

Below floor systems are installed under the subfloor. These systems include attaching the PEX tubing to the bottom of the subfloor or suspending the tubing from the subfloor. This system is popular for retrofits and is less costly to install than the above floor systems. Below floor systems require a higher source temperature to perform equivalently to an above floor system.

An above floor system uses 7x48-inch or 10x48-inch and ½-inch thick panel. These panels can be purchased individually or accordion style. The accordion style is six panels of desired width adhered together using fiber tape, which unfold to cover a larger surface area. This floor system is grooved for 5/16-inch PEX tubing and the panel bottoms are covered with aluminum. One hundred and eighty-degree return panels are available in 7 and 10-inch widths.

Another system is considered to have a dual function panel. This panel offers the structural requirements of a subfloor diaphragm in conventional construction, as well as a radiant floor heating system. The 4x8-foot, and 1-1/8-inch thick, engineered Comply panel is tongue-and-groove. It has a top surface with a modular groove pattern 12-inch on center. The panel is designed for ½-inch PEX tubing. A sheet of 0.025-inch thick alloy aluminum that contours with the groove pattern is permanently bonded to the surface. These panels come in three variations, panels with straight grooves, 90 or 180° turns.


Energy Efficiency

By heating the space directly above the floor, the space may feel warmer than a forced air system, where the heated air tends to rise, making the lower area seem cooler. In this way, the thermostat settings remain stable and not as much energy is used.


Not-so-easy

Wirsbo, distributor of grooved panels that attach to the subfloor, has regional warehouses and a delivery time of one to two weeks. Warmboard has stocking distributors around the country and maintain inventory at the plant for quick shipping.

It takes more time to feel the heat from a radiant floor heating system than a conventional forced air system due to the transfer of heat through the thermal mass of the floor. Retrofits using above floor systems can be costly and time consuming, since the existing finished floor could require removal. Below floor systems require drilling holes through the floor joists so that the PEX tubing can pass through. They also require a higher source temperature to perform equivalently to an above floor system.


Prices for panels attached to the subfloor are $10.80 and $12.70 for the 7x48-inch and 10x48-inch, respectively. The accordion style panels are $74.00 and $87.00 for the 7x48-inch and 10x48-inch, respectively. One hundred and eighty-degree return panels cost $16.00 for 7 and 10-inch widths. The dual function panel costs approximately $125.00 for straight grooved panels and $130.00 for curve grooved panels.


Claims have been made that a 20% to 40% reduction in heating bills can be realized.


The Radiant Panel Association has Standard Guidelines for the Design and Installation of Residential Radiant Panel Heating Systems available on their web site. Warmboard, dual function panels have been tested and stamped by the APA (Engineered Wood Association). They have been evaluated by the ICBO, report number ER-5525.


Schenectady Habitat for Humanity Home: Schenectady, New York

Thad Farnham Construction: Hailey, Idaho


Panels should be attached to the subfloor using approximately 10 screws per panel. A bead of caulk should be placed in the grooves before the tubing is installed. The tubing is laid in the grooves and pressed down flush with the surface of the panel. The dual function panel is suitable for joists spaced at 12, 16, 19.2, and 24-inch increments. Manufacturer recommendations include screwing and fastening adhesive when installing the panel. An elastomeric bonding material is placed in the grooves prior to tube installation. The tube is pressed into the groove flush with the surface of the panel. There are two common below floor installation procedures. The first installation is inserting the PEX tubing in a grooved aluminum sheet. The aluminum sheet with the tubing is attached to the bottom of the subfloor with staples. The second installation procedure requires hanging the PEX tubing several inches beneath the subfloor. Insulation is installed beneath the tube with a 2 to 4 inch air space between the top of the insulation and the bottom of the subfloor. These below floor systems require drilling holes in the floor joists for the tubing to pass through. Floor thermostats control many radiant floor systems. Some systems are designed to constantly circulate water through the pipes while the thermostat controls the burner. Other systems regulate the flow of hot water through each loop using zoning valves to control room temperatures.


Anywhere from 10 to 25 years can be found for the entire system, depending on manufacturer. The boards and fittings can differ; the boards are sometimes warranted for the life of the structure and the fittings form 18 months to 2 years.


Dry radiant floor systems allow even heating throughout the entire floor. The heat radiates from the floor and warms objects near the floor as opposed to forced hot air that tends to rise to the ceiling. Radiant floors eliminate dust, draft, and noise problems associated with forced air systems. They can be more aesthetically pleasing than other forms of heating because there are no heat registers or radiators to obstruct interior designs. Manufacturers claim radiant floor heating saves 20 to 40 percent on monthly heating bills.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.