Method 1
A picture is worth a thousand words. Here is my attempt to put that concept into practice, while trying to put some “see and feel” component into radiant heating.

As a radiant contractor I have, over the years, been bombarded with all sorts of numbers, graphs, calculators, slide rules, software programs and more. All in an effort to explain radiant floor output facts and reasoning.

I heartily agree that any radiant design should always start with a room-by-room calc and design. Get some help if you are unsure how to do this step.

Numbers on paper, however, can sometimes cause potential radiant customers to glaze over. My hope would be for contractors to include these pictures as “show and tell” sales methods, also.

So here is my attempt to put some color and visuals to a normally numbers-intensive game. I felt it would be nice to somehow show in Technicolor what is actually going on within those warm floor surfaces. I hoped to present a picture over a six-hour period of time from a cold room start-up.

Method 2
Conveniently enough, a local utility had just the “vehicle” for me to put this idea into reality. A Flir infrared video camera was made available for me to film this process.

The utility rents this camera with an operator for various heat-seeking projects. Generally, it is used for filming transformers and overhead power lines and connections. Being able to pick out “hot spots” helps the utility with preventive maintenance and minimized down times.

Many industrial customers also use the cameras to check motors, bearings or any component for areas of high heat, indicating excessive friction and unnecessary wear.

Seemed to me like the perfect device to watch, in color, the operation of some radiant floor panels. I chose to film four of the more common installation methods. I followed the manufacturers' installation recommendations, while assembling the test panels.

Method 3
Method one used a rubber composite product stapled directly against the subfloor from below the space, with the tube installed 8 inches on center, two tubes per 16- inch joist bay.

Method two was a suspended tube application with the PEX fastened just below the subfloor, but not in direct contact with the subfloor. The tube was installed 8 inches on center, two tubes per joist bay.

Method three used heavy gauge aluminum transfer plates screwed to the bottom of the subfloor with PEX tubing snapped into them. Again, an 8-inch on center installation.

Method 4
Method four was a premanufactured subfloor product with channels to insert the PEX provided in the aluminum covering. This panel provides for 12-inch on center spacing only.

All were built over a typical floor joist framing assembly and insulated as per manufacturers' recommendations. All had R-11 fiberglass batts. An aluminum reflective surface was applied to the batts for the direct staple-up installation. The rest had kraft-faced batts installed to the manufacturers' spec regarding air space allowances. The test room was conditioned to approximately 65 degrees.

The panels were built with 3⁄4-inch Advantech subfloor sheets, measuring 4 feet by 8 feet, a standard sheet size. Half of the sheet was left bare and uncovered. The other half had a berber carpet, without any pad below. All were fed the exact same supply temperatures and same flow rate.

These still photos taken from the video tell the story. The video clip better shows the output over the six-hour elapsed run time. The biggest difference was the amount of time (acceleration factor) that the various installation methods took to get to the 85 degree F surface temperatures, and the temperature spread between the tubes. The output of course can be read in the colors.

Quick response can be a big plus in shoulder seasons or in areas that see wide outdoor temperature swings. All of these methods may have a place in radiant applications. The contractor's job is to match the best application to the building and the owner's expectation.

Over the years, various testing has been done to put some “real” numbers on the output expectations of the various methods shown here. Engineer and hydronics specialist John Siegenthaler has provided numerous FEA test results via his Plumbing & Mechanical columns, as well as in PM Engineer.

Most recently, a study funded by ASHRAE and the Radiant Panel Association put some nonbiased third-party numbers to paper. Kirby Chapman, Ph.D., led a team at Kansas State University National Gas Machinery Laboratory through the study (ASHRAE 1036) defined as “Develop Simplified Methodology to Determine Heat Transfer Design Impacts Associated with Common Installation Alternatives for Radiant Conduit.” There, I said it again!

Radiant Realism

The following gives a more realistic view of a few methods of installing hydronic radiant heating systems. The first listed below approximates Hot Rod's method one and two; the second outlines method three; and the last, method four.

Hanging Or Attached Below Subfloor

Radiant tubing is hung or attached to the underside of the joists in an air space with insulation below. This requires higher water temperatures and has more limited heat output than other systems. It is often used for retrofitting when access from below is possible. Hanging systems have more even joist cavity temperatures than when pipe is attached in contact with subfloor joists.
    Estimated Assembly R-value*:
    R-1.7 - R-2.2 (pipe + 3/4-inch plywood only)

With Plates Below Subfloor

Radiant tubing is attached to the underside of the joists with metal plates to diffuse the heat. Insulation is recommended below the plates. This has higher water temperatures and more limited heat output than above subfloor systems, but plates make it more effective than hanging pipe from under joists. It is often used for retrofitting when access to joist space is available.
    Estimated Assembly R-value*:
    R-1.3 - R-1.8 (pipe + 3/4-inch plywood only)

Structural Radiant Subfloor With Aluminum And Grooves

Premanufactured 1.125-inch thick panels have grooves for tubing and an aluminum sheet bonded to the board. In this case, the premanufactured panels serve both as the structural subfloor and as the channel into which the tubing is installed. The aluminum sheet makes the system accelerate rapidly and spreads out the heat. Tubing is installed 12 inches on center in the grooves.
    Estimated Assembly R-value*:
*Assembly R-values are illustrative estimates only, and do not include the R-value of the floor coverings, which must be added to determine total system R-value. System design and installation should only be done by qualified professionals. Note: Insulation is usually required under heating systems.

Reprinted from the 2004 edition of the Radiant Flooring Guide. Call 970/613-0100 to purchase a complete copy.

Convective Radiant
by Steve Smith

There are more ways to install radiant tubing than just the four methods outlined in Hot Rod's project. It's safe to say, however, that none look quite like the radiant system marketed by Ultra-Fin.

Like other radiant systems, Ultra-Fin circulates hot water through half-inch tubing installed in the underfloor joist spaces at specified intervals according to heat-loss calculations.

That's where the similarity ends. The tubing does not make direct contact with the flooring above.

Instead of a staple-up or transfer plates, aluminum Ultra-Fins attach to the tubing, heating the air itself within the joist space, creating a warm convection current to heat the floor above.

“I think that what separates us from other radiant systems is that most rely on surface contact, which can lead to 'stripping,' ” says Matt MacDuff, MacDuffco Manufacturing, the Canadian maker of Ultra-Fin. “With our system, we're actually warming the entire floor surface, which provides a more uniform source of heat.”

“It's actually a modern implementation of an old way of installing radiant before the use of tubing,” says PM columnist and radiant engineer John Siegenthaler. While not as popular as sealing copper pipes in cement slab, Siegenthaler says that finned tube, just like that found inside baseboard, used to be installed parallel to the floor joist.

With Ultra-Fin, holes are drilled into the floor joists and the tubing snaked through. This eliminates almost all the need for fasteners, except at the ends of whatever room installers are working on. The system works with any 1/2-inch tubing, however, the company recommends PEX/AL/PEX since the tubing stays straight on the straight runs, bends easily and stays bent, and, finally, provides a 100 percent oxygen barrier.

The tubing needs to be placed at least 3 inches below the floor and with another 3 inches left underneath the tubing, followed by fiberglass insulation. After the tubing is in position, installers attach the Ultra-Fins with six pop rivets.

What you end up with is a higher temperature, lower mass system than other radiant underfloor systems. The company markets the products particularly for new construction of wood frame homes. Due to the high temperature of the water, the system could also work in a retrofit with a home that already has baseboard heat.

PEX and Hydronic Radiant Heating Systems


Embassy Industries Inc.

FlorHeat Co.

Heatlink USA Inc.

Infloor Heating Systems


Performance Engineering Systems Inc.


Roth Industries


Schluter Systems

Slant/Fin Corp.


Uponor Wirsbo

Vanguard Industries


Warmboard Inc.

Watts Radiant


Zurn Plumbing Products