Does Radiant Cooling in a Humid Climate Make Sense?
Imagine a sunny spring day. You’re feeling warm from the sun as you walk down the street. Then you go into a big concrete parking deck. Suddenly, you feel a chill. That’s radiant cooling. (And it was radiant heating from the sun before that.) The massive concrete is still cool from winter. You walk through and your body radiates heat into the cooler concrete. We can use that physics to do radiant cooling in homes, too. But is radiant cooling in a humid climate practical? Humid air and cool surfaces can do things we don’t want.
It’s certainly being done, and the photo above proves it. Those are radiant heating and cooling panels hanging over a hallway in ASHRAE’s new headquarters here in the Atlanta area. I took the photo when I went to the ribbon cutting ceremony in 2021. So, let’s take a look at the issues and find out if there really is anything to be afraid of.
The benefits of radiant heating and cooling
Radiant heating and cooling can be one of the best ways to make people comfortable at home. The basic science of the comfort is in an article I wrote a long time ago called Naked People Need Building Science. And yes, it applies to clothed people as well.
The science I wrote about there was mean radiant temperature, which is the average temperature of all the surfaces in the room. Those temperatures are important because our bodies exchange more heat with our surroundings through radiation than through conduction or convection. It’s far more important than the temperature of the air. In fact, about 60 percent of the heat exchange between our bodies and the surrounding environment occurs through radiation.
Another benefit of radiant cooling is that there’s no ductwork for forced-air cooling. Hydronic tubes take up less space than air ducts. I believe they also have fewer performance problems. Check out some of the many horrific design and installation problems with air ducts I’ve written about in this blog (e.g., here, here, & here).
Then there’s the potential IAQ benefit. Air conditioning coils and drain plans can provide a nice habitat for a wide variety of microbes. You know, biofilms, slime, and gunk. And when it does, pollutants can get into your lungs because the air being cooled blows right over that stuff. I said it’s a potential benefit, though, because high-efficiency filtration and airtight ducts will keep the coil and pan clean for the lifetime of the AC.
Avoiding condensation
Now let’s get to the potential for trouble with radiant cooling in a humid climate. The knee-jerk reaction that often comes up when someone mentions radiant cooling in a humid climate is, “You’ll get condensation.” I’ve certainly been guilty of doing that myself. So, let’s discuss this.
First, radiant cooling works great in a hot, dry climate. You can chill the water without having to worry about dropping the temperature of the chilled surfaces below the dew point temperature of the air.
But in a humid climate, the dew point temperature of the indoor air is of paramount importance, especially with radiant cooling. Here in Atlanta, for example, percent relative humidity excursions into the mid-sixties do occur, even with air conditioning. Ideally, it doesn’t last very long, but the weather this summer has been tough for humidity control.
If you’re using radiant cooling, whether in a concrete floor slab or lightweight panels on the walls or ceilings, you’ve got to have tight control over the humidity. If the indoor temperature is 75 °F and the relative humidity is 65 percent, the dew point is 62 °F.
To avoid the potential for condensation, then, radiant cooling professionals usually recommend the radiant cooling surfaces operate at 65 °F or higher in a humid climate. So you get a temperature difference (ΔT) of about 10 °F. Is that enough to get sufficient cooling capacity? Keep reading.
You still have to deal with the humidity
Let’s contrast this with dry climate radiant cooling again. There, you install your radiant cooling system, and you’re done. You don’t have to worry about condensation or keeping the indoor humidity low. The dry climate does that for you. But with radiant cooling in a humid climate, you’re not done.
If all you do is install a radiant cooling system instead of air conditioning, you’re in for trouble. When I mentioned percent relative humidity in the mid-sixties above, I was talking about what happens in a lot of air conditioned homes.
Without air conditioning or other types of dehumidification, a house in a humid climate will quickly turn into a mold-farm. And if you’re cooling the house with radiant slabs, you’ll be slipping and sliding on the sweating floors, too.
That means you must install a dehumidifier. This isn’t optional for radiant cooling in a humid climate.
Radiant cooling capacity
In my last article, I covered the physics of radiant cooling. (OK, heating, too, but that’s not part of the discussion here.) I started with the Stefan-Boltzmann equation and its temperature to the fourth power and went all the way through to how much cooling capacity you can get for typical conditions. If you like math, you really ought to read through the derivation. I had a lot of fun putting it together.
In the end, I wrote that you can get 15 to 25 thousand BTU per hour of cooling capacity for each thousand square feet of panel area. I worked through an example where the result was ~24 thousand BTU/hr per thousand square feet.
Is that enough? In new homes, absolutely. Easy peasy. With radiant panels and a dehumidifier, you’ve separated the two types of cooling: sensible (lowering temperature) and latent (lowering humidity). The radiant cooling panels handle only the sensible load.
If you build a house to the current energy code—heck, even if you use the 2006 energy code—being able to cool a house with 1.5 to 2 tons of sensible cooling for each 1,000 square feet isn’t hard. That’s old-school rule-of-thumb levels of cooling capacity.
Radiant cooling cost
What about the cost of radiant cooling panels? I don’t have much hard data on this, but one manufacturer (Messana) of this equipment put some numbers on their website. “Typical cost for just the Messana components is around $15-20 per square foot and $40-45 per square foot installed.” The square footage they’re talking about is the conditioned floor area of the house. Therma-HEXX systems are also in that ballpark and include the heat pumps and everything you need.
But there are other systems out there with lower costs. I’ve seen some claiming you can do it for as low as $10 per square foot, although I’m not sure that includes everything (e.g., the cooling equipment).
Let’s say you’re building a 2,500 square foot house. At $40 per square foot, your total installed cost is $100,000. Whoa! And then there’s still the dehumidification. Fortunately, that cost will be a tiny fraction of the radiant cooling cost.
If you can get it installed in that same house for $10 per square foot, you’re now down to $25,000 for a 2,500 square foot house. That’s still significantly higher than you’d pay for a lot of heat pumps or air conditioners for that size house. But it’s comparable to high-performance equipment like the inverter-driven heat pumps in most of our HVAC designs.
Yes, radiant cooling in a humid climate can make sense
Radiant cooling in a humid climate has some nice advantages over air conditioning. It doesn’t eliminate moving air in a home, though. You still have to dehumidify. You also need to move ventilation air and get some filtration happening to catch the particulate matter.
The condensation issue is the one that gets mentioned first, but it’s exaggerated. And hey, it’s not like homes with air conditioning don’t have condensation problems. You’ve probably seen water droplets on a cold air conditioning supply vent, right?
We have the knowledge and the technology to design and install radiant cooling systems that will stay dry and provide enough (sensible) cooling capacity. The main issues with these systems are cost and finding someone who knows hydronics in hot or warm humid climates like the southeastern US.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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Interesting article. As an HVAC professional, the condensate issue has always been enough of a deterrent to radiant cooling systems in my area of the Mid-Atlantic. As long as condensate can be properly dealt with, I see no reason for not utilizing radiant cooling. We’re just a bit cautious, maybe overly so, about condensate issues causing mold issues, to the point we avoid radiant cooling altogether. Having more information from the building sciences could very well change my mind on this.
Robin: As an HVAC pro, you no doubt have dealt with many condensate line problems for ACs. Probably a good number of homes with sweating supply registers and ducts, too. So, in a humid climate, forced air doesn’t solve that problem. No matter the type of cooling we use, we have to design, install, commission, and maintain it to avoid condensation and other moisture problems.
Getting ready to break ground – so we can install PEX tubing in the slab. As mentioned before, Sophie designed the air-system for the cooling-load and a Santa Fe unit. (She also reduced the build costs). I designed the slab with Loop CAD. I am treating radiant cooling as a secondary stage with a surface temperature of 75F. Basically, the CERV2/Geoboost provides the first stage up to 88F along with hydronic fan coils.
I would suggest people look at the Chiltrix web-site – they have some useful calculators and provide useful information. In terms of cost- the only major cost that I am seeing is a mixing valve and dewpoint controller, roughly $1 per square foot to add cooling to the slab.
I was going with the Uponor radiant controls – so there was no additional cost for cooling. The system was designed with an Air-To-Water Heat Pump so other systems may require a buffer tank and some retrofit.
Bill: Glad to hear you’re close to seeing the house come out of the ground. Here’s the link to the Chilltrix page with the info you mentioned:
https://www.chiltrix.com/products/radiant-heating-and-cooling/
Like you said no reason it can’t work but there are so many reasons to not even think of a radiant system. First of all the numbers don’t work. Second even if they did work where are the panels, Ones on the ceiling will have issues with light placement and ones on the walls will have issues with furniture etc placement. Then we get into the niche product issues of proprietary supplier, installer and repair people and parts and when that company goes out of business which they will as we all know this stuff comes and goes like hair styles you have a pricey system that you can’t get parts for and no one will touch and you are swapping out to a new system.
It’s a neat thought experiment but not a realistic solution for residential. Maybe in certain commercial uses but those costs are too high for that.
My question is where are the chill water systems for residential. Europe has all kinds of them for heating but no one has any for cooling. It’s extremely popular in commercial.
A chill water system built in the stye of a multihead minisplit like those heating systems would be awesome. You would have a factory sealed refrigerant system outside that is far more reliable as you have no copper line sets to run. And then you are running just water lines to each head unit which are way easier to do on many levels than copper linesets.
And how about in houses with wells. What about running that cold water through a heat exchanger and cooling an area like a garage or I plan to do that in my shop. My well water temp is only 70* but that’s essentially free cooling while the yard is being watered.
Robert: Chilled water systems are here, and the number and range of models is growing. They’re called air-to-water heat pumps.
Does this use less energy than the inverter-driven heat pumps?
It’s very hard to beat the efficiency of minisplits. The only reasonable way is by going to Ground Source Heat Pumps which may be more energy efficient but the install cost far outweighs any energy savings unless you are using a lake or pond as your coil placement.
The chill water system idea I mentioned would not be as efficient as true minisplits but the install cost and complexity would be far less.
The issue with multihead minisplits is the long copper runs. On the low end you are looking at around $150 for a 100′ lineset which includes a power wire and drain hose. So figure up those vs the cost of insulated water pipes. Oh and you also need to add refrigerant to the system for those long line sets and you do loose effeciency on long line sets because the metering device is in the unit outside not inside.
Emily: The radiant cooling I wrote about here refers just to the distribution side. Rather than ducts, you have hydronic tubing running through panels or beneath the finished flooring. The hot or cold water has to come from a piece of equipment that can heat or cool water. Traditionally, radiant distribution systems were just for heating, so people used boilers for the hot water. The best way to make both hot and cold water is with an air-to-water heat pump, which can have an inverter-driven compressor.
Kinda looks like dewpoint control and system cost were the main items in the article. Are costs of radiant cooling that far out of whack with forced air considering cooling AND heating loads? I presume a radiant cooling system heats, too for the money.
In principle, the radiant cooling system could probably be a radiant heating system, but generally one would expect compromises just as when you use your air to air heat pump to heat and cool. We generally want cooling to start near the ceiling and heating near the floor. Trying to do both in one would force you to choose. A current result is that my heating-optimized setup can allow me a 70 degree floor with an 88 degree ceiling on a decent summer day, and the thermostat is okay since it’s 76 degrees at its height on the wall. [I do have R15 ceiling insulation, but there’s a dark asphalt shingle roof with only a 2/12 pitch, and the only attic to speak of is the 2×6 frame with this insulation.]. With better insulation, much of the radiant temperature issue would disappear.