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Research Bolsters Hopes for Radiant Cooling

Demonstration project tests a 60-year-old theory on radiant cooling without the risks of condensation

Researchers built this experimental pavilion in Singapore to test a radiant cooling system that's especially well suited to hot, humid climates. Radiant cooling could have significant energy implications for residential as well as commercial applications. Photos courtesy Lea Ruefenacht.

Researchers have successfully demonstrated a relatively simple technique that could prove to be a boon for cheaper residential cooling.

Circulating hot water through tubing embedded in the floor has become a common type of space heating in the U.S. Radiant floor heating systems are quiet and all but invisible, and those advantages raise a good question: Why can’t the same system be used to cool the indoors in summer?

Radiant cooling systems are, in fact, available but they are uncommon. Particularly in high-humidity areas, radiant cooling has an inherent weakness. When moisture-laden air comes into contact with surfaces that are below the dew point, water vapor starts to condense into a liquid, and when the condensing surface is a wall or a ceiling that can lead to a variety of problems.

A dedicated dehumidification system would be required in all but the driest climates in order to limit condensation. That has helped make radiant cooling relatively rare. Now, a group of researchers is proposing another way of skirting the condensation issue with a surprisingly simple workaround that represents a potential breakthrough for high-humidity, high-temperature regions.

In a paper published September 1 in Proceedings of the National Academy of Sciences, 10 researchers representing several institutions outline their work on a demonstration project in Singapore that kept people feeling comfortable even when both humidity and temperature were high.

The idea is relatively inexpensive as well as simple. Surfaces in the experimental “cooling pavilion” were chilled while a thermally transparent membrane (nothing more exotic than 2-mil polyethylene) prevented condensation and unwanted air cooling. Both characteristics, they said, would be necessary for radiant cooling panels to be used without humidity control.

“The lack of widespread commercial adoption of radiant-cooling technologies is due to two widely held views,” the study says ” 1) The low temperature required for radiant cooling in humid environments will form condensation; and 2) cold surfaces will still cool adjacent air via convection, limiting overall radiant-cooling effectiveness.”

The work comes at a time of rising energy use for conventional air conditioning systems. Radiant cooling has the potential to lower cooling costs substantially, providing that consumers are willing to rethink conventional attitudes about what “comfort” actually means.

Building an experimental pavilion

Researchers built a modestly sized outdoor structure in Singapore in which a low-density polyethylene membrane prevented the condensation of moisture from the air on the panels, even when they were cooled below the dew point. The membrane did not block infrared energy, so people standing nearby could still feel like they were cooling down.

There was nothing fancy about the locally sourced polyethylene, lead author Eric Teitelbaum said in a telephone call. The plastic sheet was 50 microns thick, about 2 mils. “I’m pretty sure the supplier in Singapore usually sells it to people who are making shopping bags,” he said. “It’s basically a garbage bag, but a little more upscale.”

More than three-quarters of the test subjects who used the pavilion said they were “satisfactorily” cool even though air temperatures were about 85°F and the relative humidity was greater than 66%. Those are conditions that would have most people in the U.S. running for the AC thermostat.

Water circulating through the panels was chilled to about 62°F. Researchers said they didn’t see any condensation on the panels even though the dew point was more than 74°F. The temperature of the water could be lowered to about 55°F without any observed condensation.

“The cooling panel operates below dew-point temperatures, but is insulated from humid air by a membrane transparent to longwave radiation,” the researchers said. “It successfully makes people feel comfortable in conditions exceeding 30°C [86°F] and 65% relative humidity without modifying the air temperature or humidity circulating around human bodies.”

Idea was proposed but never studied

The report says this idea was first proposed nearly 60 years ago, but until now a full-scale test system had never been built or studied.

Conditions in which air temperatures are high and the mean radiant temperature of surfaces are low don’t occur naturally, researchers said, because chilled surfaces act as heat exchangers and cool surrounding air. In the Singapore experiment, commercially available low-density polyethylene prevented heat transfer via convection.

The inside of the cooling pavilion.

Ordinarily, people associate comfort with mechanical air conditioning, which rely on air temperature, relative humidity, and air speed.

“We had the goal of demonstrating that if radiant cooling is separated from comfort cooling it can be relied upon independently as a heat-transfer mechanism to provide comfort,” researchers wrote.

Although people often rely on fans and natural ventilation to cool down, but as air temperature and humidity go up, that becomes less effective. The aim of the experiment was to show that people could feel comfortably cool without air movement or the mechanical treatment of air.

Radiant cooling offers a way of separating, or “decoupling,” the process of lowering air temperatures (the sensible load) and lowering humidity (the latent load) that are normally combined in conventional AC equipment. In tropical areas especially, however, latent loads can be much higher than the demand for cooling.

“This study demonstrates potentials for radiant cooling to create comfortable conditions while doing nothing to the air,” the study says.

The potential for energy savings is substantial—although not spelled out directly in the group’s paper—because supplying panels with chilled water should be much cheaper than cooling and dehumidifying air with conventional AC equipment.

“In humid environments is where you’d expect to see the biggest energy reductions,” Teitelbaum, who wrapped up his PhD studies at Princeton earlier this year, said by phone. “And that is very simply because for air conditioning up to 60% of the compressor’s energy budget is devoted just to dehumidification. Dehumidification is very energy intensive. If you don’t have to do that, and you don’t have to do that for thermal comfort, that could be 60% off your energy bill.”

One of the more important implications of the group’s work, he said, is that it’s possible to introduce lots of fresh air into the space that’s being cooled without incurring a big energy penalty. With conventional AC, increasing the amount of outdoor air quickly drives up energy use because it ramps up both latent and sensible cooling loads. That’s not the case with radiant cooling.

More work lies ahead

What researchers called the “Cold Tube” in Singapore was built with 10 panels, each 4 by 8 feet, that were cooled with capillary mats. The maps were isolated from ambient air by 2 inches of polyisocyanurate insulation on one side and polyethylene on the other.

While “thin and flimsy,” the membrane had enough tensile strength and puncture resistance to last four a couple of months. Longer lasting installations would need stronger materials, but increasing the thickness of the membrane would decrease its infrared transmittance and make the panels less effective for cooling. “Future work could be spend designing better stabilized polymers,” the researchers noted.

Teitelbaum said it should be fairly easy to come up with another material for the membrane, although alternatives might not be quite as transparent to radiant energy as polyethylene.

A more difficult problem could be retraining consumers to accept that comfort doesn’t necessarily mean lower temperatures or lower humidity.

“Air conditioning is so conflated with cooling,” he said. “If you think of cooling, your go-to is not to go to the hardware store and pick up a radiant cooling panel. No, you’re going to get a window air conditioning unit and install it. Your average consumer probably has never thought about radiant cooling.”

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.


  1. pjpfeiff | | #1

    Very nice! I like the idea of being cool in the summer while still able to keep the windows open. I have a hard time imagining how this would be implemented into finished surfaces inside a home, but hopefully someone can work that out. Also it would be interesting to know what is the minimum view factor necessary for this to work. I see that they chilled the ceiling and all four walls. I suspect that the ceiling doesn't contribute much unless the person is lying down. And with that thought, perhaps the best way to start with this would be panels that install on the ceiling over your bed. Although it might be hard to retrofit something like that.

  2. Jon_R | | #2

    Interesting, the same principles as are used when radiant heating is used in an open garage. Heating the air is mostly ineffective, but raising the MRT (mean radiant temperature) works. I could see insulated radiant cooling possibly being practical in ceilings.

    > A dedicated dehumidification system would be required in all but the driest climates .... That has helped make radiant cooling relatively rare.

    Most places need the dedicated dehumidification system anyway. So I'd look to other reasons why radiant cooling is relatively rare. For example, the expense of hydronics compared to the efficiency gain.

  3. jameshowison | | #3

    Super interesting. My understanding is that the polyethelene surface does not condense because it stays above dewpoint. And it stays above dewpoint because it is being warmed by the air outside, but not losing that heat via radiation towards the chilled mats? Is there air between the polyethelene and the mats? If so, isn't there some convection cooling of the polyethelene? I guess not sufficient?

    So the infrared heat from the people is radiated to the chilled mats, and does not cool the polyethelene as it passes through it.

    Hmmm, I'm still having a little trouble grokking this. The article talks about using the membrane as a "convection shield". The illustration has a 6" air gap filled with desiccant packs (so very low humidity air on the cold side. So the air in there must be very cold. Why is that not convectively cooling the poly to below dewpoint?

    The early conference paper is here: it says "sensible heat in the air prevents condensation on the membrane". So I read that as the heat absorbtion from the outside air over powers the cooling from convection inside the apparatus?

    I wonder how this relates to the dehumidification via membranes discussed here:

    This seems a much better solution for accessing outdoor space than evap cooling via fans and water spraying into the air) something seen all over the south of the US.

  4. charlie_sullivan | | #4

    This is a cute trick, and could be of value for things like improving comfort in outdoor areas, similar to how radiant heating is used now for some outdoor restaurant seating, and could be particularly relevant to the current pandemic situation during the cooling season.

    However, its relevance to high performance buildings is pretty questionable. If you insulate a building well, the cooling load becomes mostly latent (dehumidification), unless you are in a climate with low summer humidity, in which case good insulation and night flushing often make the cooling load negligible anyway. Deploying this in a humid climate without dehumidification could provide some degree comfort without consdensation on the panel, but there are two problems with that. One is that maintaining a building at high humidity leads to problems with dust mites, mold and rot, and the other is that the comfort is limited. With mild exertion in low humidity, a human body can cool itself with a bit of sweat, but that doesn't work when the humidity is high. So you end up with conditions that are only comfortable if your exertion level and metabolism rate are just right.

    For most climates that require cooling, what we need for high performance buildings is better high efficiency dehumidification systems.

    1. jameshowison | | #5

      Yup. Although much worry about radiant cooling comes from worrying about condensation. Even in houses with good dehumidification. What if it fails? What if a door is left open? What if I ever want to rent the place out and they mess with the systems? So a system where condensation won’t happen, regardless of spikes in humidity, is appealing.

      Also: screened porch and radiant cooling for nighttime patios in the south. Almost like living somewhere comfortable (in summer!). And at just the cost of more carbon load.

      We are about to see the cost of conditioning the outdoors in NYC in winter with restaurants. Not saying it’s not the right thing to do, perhaps precautionary principle could now include: well, we should reduce carbon in the atmosphere in case we have a pandemic and all restaurants need to move outside! I guess that falls under the heading of flexibility as a benefit of ameliorating climate change!

    2. user-1121196 | | #9

      Those were some of my first thoughts. You have to control humidity indoors regardless. You could allow temporary spikes, but it must be kept down to prevent mold, rot, etc.

      If a durable solution were made that could do double service as radiant heating, that might potentially make it economical, but I would still have my doubts, since you would still need that humidity control.

  5. bill328 | | #6

    I am going to be using a Chiltrix air to water heat pump for in-floor hydronic heat. This system can also run cool water as well...about 60 degrees. I live in a low humidity area - generally 15%-30% RH. I am trying to figure out if I will have condensation issues. Can someone much smarter than me help me make that determination?

    1. Jon_R | | #7

      At 77F, if the room humidity is < 57%, you won't get condensation on 60F supply pipes or the even warmer radiating surface. And with many loads and radiators , you don't need water cooler than 60F. It's all quite doable, even in humid climates (where you need a dehumidifier).

    2. user-1121196 | | #8

      I'm so interested in Chiltrix! Seems like one of the best products on the market right now. Combine with their heat pump water heater.

    3. charlie_sullivan | | #11

      With more details on your climate we could help more, but it might work fine. Another option is to include some of the nice slim quiet fan coil units that chilltrix makes, and use those with 45 F water in addition or instead of the floors with 60 F water.

  6. david_king | | #10

    Couldn't there be a simple way to deal directly with condensation via a gravity drainage system? If less humidity is beneficial then perhaps there's a way to work with it. Many buildings in warmer parts of the world are concrete or cinderblock construction and wouldn't be degraded by transient moisture.
    I also wonder if the water could be cooled using a second capillary mat on the roof equipped with a 'Radiative Sky Cooling’ panel as described here:

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