Radiant ceilings are ready for the next generation of houses.

Ask almost anyone in the heating trade about radiant panel heating and they’ll probably start describing tubing embedded in floors. True, radiant floor heating is by far the biggest part of the radiant panel market, but floors do not constitute the entire market.

Site-built radiant walls and ceilings, as well as some panel radiators, also meet the Radiant Panel Association’s definition of a radiant panel: “A radiant panel is defined as a floor, wall or ceiling surface designed to heat and/or cool a space where the panel has a controller surface temperature of less than 300 degrees F and the heat transferred by radiation is 50 percent or greater of the total heat transferred between the panel and space.”

Radiant floor heating is an excellent approach in many projects ranging from residential all the way up to heavy industrial applications. However, it’s not necessarily the ideal solution in the coming generation of low-energy-use houses.

When viewed only from the standpoint of heat-source performance, the low operating temperature of a bare concrete slab with closely spaced tubing (6-inch to 9-inch spacing) is very beneficial. A well-insulated house on a design day may only require supply water temperatures in the range of 85 to 90 degrees F to maintain the interior space at 70 degrees F. Condensing boilers, solar collectors and geothermal heat pumps all love to operate at these low temperatures and show their gratitude by operating near the upper end of their performance range.

That’s the good news. The downside is two-fold. First, the average floor surface temperature required of a heated floor in a well-insulated house is only a few degrees above the room temperature. You can estimate this average floor surface temperature using Formula 1.

Where:
TS(ave) = average floor surface temperature (degrees F)
TR = room air temperature (degrees F)
q = upward heat flux from floor (Btu/hr/ft2)

For example, imagine a house with 2,000 square feet of heated floor area and a modest design heat loss of 30,000 Btu/hr. The required upward heat flux under design load conditions is:

Assuming the room air temperature was to be maintained at 70 degrees F, the average floor surface temperature would be:

This temperature is at or slightly below normal bare skin temperature. A bare hand or foot would perceive this floor as slightly cool. During most of the heating season, the floor surface temperature would be even lower, perhaps around 74 degrees F when the outdoor temperature is 35 degrees F.

True, the floor is still warmer than it would be with convective-type heating. But it’s not delivering that “barefoot-friendly” effect so widely advertised as a benefit of radiant floor heating. The fact that the room is still maintained at 70 degrees F is unlikely to placate the unmet customer expectations of warm floors.

The other drawback is thermal response. Low-energy-use homes are especially susceptible to rapid temperature changes from internal gains. In many new homes, this is further exasperated by above-average passive solar heat gains.

These characteristics don’t bode well for high-mass heat emitters. Spaces will quickly overheat when the sun comes out, and much of the solar gains will be lost through the ventilation necessary to keep the house from turning into a sauna.

Over Your Head

Low-energy-use houses need heat emitter systems that are capable of rapidly changing their rate of heat delivery. One good candidate is a radiant ceiling panel. Heated ceilings deliver more than 90 percent of their heat output as thermal radiation. They “shine” thermal radiation down into the room much as a light fixture shines visible light downward. They offer the following benefits:

Low thermal mass. Low-mass radiant ceilings can quickly warm up following a cold start. They are ideal in rooms where quick recovery from setback conditions is desirable. Low mass also means they can quickly suspend heat output when necessary. This helps limit overheating when significant solar heat gains occur.

Higher heat output. Because occupants are not in contact with them, radiant ceilings can be operated at higher surface temperatures than radiant floors. This allows greater heat output per square foot of ceiling. For example, a ceiling operating at an average surface temperature of 102 degrees F releases approximately 55 Btu/hr/ft2 into a room maintained at 68 degrees F. This is almost 60 percent more heat output than a radiant floor with a mean surface temperature limit of 85 degrees F.

Not affected by changing floor coverings. It’s probably safe to say that the days of shag-carpeted ceilings are over. Ceilings are the least likely surface of a room to be covered. Thus, the output of a heated ceiling is unlikely to be compromised by future changes in floor covering or furniture.

Warms objects in the room. The radiant energy emitted from a heated ceiling is absorbed by the surfaces in the room below. This includes unobstructed floor area as well as the surfaces of objects in the room. The upward-facing surfaces tend to absorb the majority of the radiant energy; the top of beds, tables and furniture are slightly warmer than the room air temperature. The surface temperature of floors below a radiant ceiling is often 3 to 4 degrees above what it would be in a room heated by convection.

Easy to retrofit. Radiant ceilings are usually easier to retrofit into existing rooms than radiant floors. They add very little weight to the structure and require minimal loss of headroom.

Figure 1 shows one construction detail we’ve used to build a low thermal mass panel using standard construction materials, 1/2-inch PEX-AL-PEX tubing and aluminum heat transfer plates.

Figure 2 shows this panel during installation. The foil-faced foam strip and aluminum heat transfer plates are held in place by contact adhesive. The 1/2-inch PEX-AL-PEX tubing is held in place by the formed groove in the plates.

Notice the 2 1/2-inch drywall screws, driven under the watchful eye of my longtime buddy Harvey Youker, don’t go through the aluminum heat transfer plates. They only pierce the drywall and foil-faced polyisocyanurate insulation, and finally bite into the 7/16-inch oriented strand board substrate.

The heat output of this ceiling can be estimated using Formula 2:

Where:
Qroom = heat transfer rate to room side of ceiling (Btu/hr/ft2)
Tw(ave) = average water temperature in the wall circuit (degrees F)
Tr = room air temperature (degrees F)

Here’s an example: What’s the heat output of this radiant ceiling if operating at an inlet water temperature of 115 degrees F, a circuit temperature drop of 10 degrees and supplying heat to a room at 68 degrees F?

The average water temperature in the panel would be 115 - 10/2 = 110 degrees F. Now, just plug the numbers into Formula 2 and run them through your calculator.

This output is almost twice the 15 Btu/hr/ft2 required for the low-energy-use house in the first example. The implication is that one square foot of heated ceiling could supply the heat for almost two square feet of floor area. The implications: less material and lower installation cost compared to filling the floor up with tubing.

Glowing Performance

Figure 3 shows a finished radiant panel ceiling constructed using the details shown in Figure 1. It is indistinguishable from a standard drywall ceiling. However, the infrared thermal image of this same ceiling shown in Figure 4 gives a very different picture.

This infrared image was taken about five minutes after warm water began flowing through the single ceiling panel circuit. The red stripes on the left side of the image show excellent lateral heat diffusion by the aluminum plates. They are also evidence that these plates are making good contact with the back of the drywall. Can you tell which direction the flow is moving across this ceiling?

Adding Some Style

Figure 5 shows another installation of the same radiant ceiling panel design. In this case, the heated portion of the ceiling is only about four feet wide and used to supplement the output of a radiant floor near a wall with plenty of window area. The edge of the panel was detailed to give the overall appearance of a coffered ceiling.

If you’ve never tried installing radiant ceiling panels, I urge you to do so. I’ve used this panel construction on several projects, and have always been delighted with the installation ease, performance and lack of problems.

This type of radiant panel, as well as its equivalent installed on walls, could play a key role in keeping hydronics the preferred form of heating in the next generation of low-energy-use houses. You need to have it in your arsenal of techniques.

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