Steve Mackay is already committed to radiant floor heat in the house he is building. Why not, he wonders, use the same system for cooling?
“I’ve read a lot of the pros and cons about using radiant floor heating in a well designed home and I understand that we probably should go with forced air,” Mackay writes in Q&A post. “There are a number of reasons why I am still looking at radiant floor heating, most of which are not technical justifications but more because we really really want it … Having made a decision on radiant floor heating, does it make sense to go with radiant floor cooling?”
In areas with high summer humidity, water vapor can condense on cold water lines. Similarly, water can accumulate on flooring cooled by a radiant floor cooling system. But Mackay writes that he is building in Climate Zone 6A where humidity ranges between 20% and 35%. The house will be well insulated and well sealed — walls insulated to R-30, the roof insulated to R-60, and airtightness at 1.5 ach50. In other words, Mackay is planning a much better than code-minimum house.
Mackay refers to an article that mentions several successful radiant cooling systems. But they were in airports and schools, not private homes.
“Is this even a thing in a residential home?” Mackay asks. “I understand that we will have to be careful about set points to ensure we don’t have condensation buildup on the floors. We are planning polished concrete floors on the basement and on the first floor. (I understand that extra structural support is required for the first floor.) There will be no wood or installed carpet; we will likely have a few rugs here and there. It seems like radiant floor heating combined with a cooling solution might make the decision to go radiant a little more justified.”
Good idea or not? That’s the topic for this Q&A Spotlight.
Radiant cooling works in dry climates
Dana Dorsett suggests such a system works well, provided that humidity is low.
“Radiant cooling works well in dry climates with mostly negative latent loads,” he says, “but gets more complicated to control when humidity is higher and condensation on the plumbing manifolds, etc. needs to be managed. Some amount of latent cooling with air coils may need to be included if the design water temperatures are otherwise too low.”
Radiant cooling panels in the ceiling or walls are more effective per square foot than radiant floor systems, he adds, due to the tendency of cold air to pool near the floor rather than mix with room air.
“Of course it all starts with the load calculations, from which the design water temperatres for doing it solely with the floor can be derived,” Dorsett says.
Jon R says the trick will be making sure that the temperature of the floor remains above the dew point. If that doesn’t occur naturally, Mackay may need a Dedicated Outdoor Air System (DOAS) or dehumidification. A heat pump and fan coils for dehumidification made by Chiltrix is one possible answer.
Yupster adds that an air-to-water heat pump similar to what’s made by Ecologix can produce both chilled and heated water. But finding an experienced contractor, he says, will be essential. He recommends that Mackay contact Robert Bean at Healthy Heating.
Not the easiest sell
Justin Gibbs details the system he is installing in his new high-performance home in Boulder, Colorado. It will use an air-to-water heat pump for radiant floor cooling as well as heating.
“Getting the system designed and installed has been challenging,” Gibbs writes. “As others have mentioned, radiant cooling in homes is rare. In our area, I couldn’t find anyone outside of commercial outfits that had installed an [air-to-water] ATW system — and the commercial guys had no interest in a project our size.”
“Harvey’s design package was detailed enough for my GC’s plumber to build and commission the system,” Gibbs said. “But getting my GC and plumber comfortable enough with the design to commit to doing it took a lot of work on my part. I had to completely understand the components of the system, walk them through how the system would function, and be available regularly to answer questions and perform inspections.”
The system will use a Nordic heat pump and a hydronic buffer tank, with distribution loops set in Warmboard-R radiant panels. Water temperatures will range between 55°F and 95°F.
Like Mackay, Gibbs recognizes there are potential disadvantages with a radiant floor system, but the benefits outweigh them.
“This is not the most cost-effective way to heat or cool a tight, thermally efficient home,” he says. “Minisplits or central forced air could have worked for a fraction of the installed cost of our system. We decided to do it because we’ve never been thrilled with any forced-air system in any home we’ve lived in, and using the floor avoided ducting, radiators, fan coils, etc. that would have impacted our floor plan.”
What about the extra costs?
Gibbs also addresses Mackay’s concerns about how much a combined radiant floor system would add to construction costs. Gibbs had already decided to go with an all-electric house, and that meant using a heat pump as the heart of the system. Simpler and less costly options are available.
“The heat source had to be a heat pump, and heat pumps are most efficient at moderate temperatures (our system will be calling for 85°F or lower water for the majority of the heating season),” Gibbs says. “Low water temperatures work best in low-thermal-mass radiant floors/panels, or you can use fan coils optimized for lower temperatures. This brings us to the first big jump in cost for our system. We used Warmboard-R radiant panels to achieve low thermal mass and fast response.”
Material and labor for those panels added up to $9.30 a square foot.
The rest of the specialized materials for adding cooling shouldn’t have amounted to that much, but putting the plan into action was more complicated and expensive than Gibbs would have imagined. There was the outside designer he needed to hire, increasing design costs by four or five times over a simple system, plus the added 20% in plumbing labor.
Gibbs adds that floors must be insulated when the house has a radiant heating (and cooling) system, and in his case forgetting to put that into the budget proved to be a “sizable omission.”
Is the insulation really necessary?
On the insulation question, Jon R says a “staple-up” radiant system (one in which distribution tubing is stapled to the bottom surface of the subflooring rather than installed in special panels like Warmboard) could work, particularly if left uninsulated. In that case, the upper floor might need some additional cooling.
“If the resistance below the radiant tubing … is less than the resistance above, very little heat will travel to the space meant to be heated,” says Richard McGrath. “It’s all about resistance. Heat energy, like water or air, is lazy and stupid and will travel the path of least resistance. Insulation is required.”
When the floor system includes Warmboard with tile above and a wood subfloor below, Jon R replies, the Warmboard/tile surface will have less resistance. “And it’s not all about R ratios,” he adds. “Air heated from above tends to sit right there, providing less heat to the room below than the ratio suggests. The opposite happens with cooling. If you want a radiant floor to also serve as a radiant ceiling for the space below, insulation gets in the way.”
Gibbs says he had similar concerns before he spoke with the system designer. The designer told him one concern was that an air space in the floor would increase thermal mass and therefore increase the system’s response time.
“I don’t know if this was explicitly modeled between the first and second floor, but the design calculations do include a 2.25 Btu/h/sq.ft. loss to the unconditioned crawl from the first floor,” Gibbs says. “(This assumed R-19, but we went with R-24 because that was the closest to R-19 we could get with Rockwool’s 24-inch ‘steel stud’ bats that fit our TJI spacing).”
Our expert’s opinion
Here’s what GBA Technical Director Peter Yost had to say:
I contacted a number of engineers for guidance on this topic, and to a one they referred me to my friend and colleague Robert Bean. About to head off on a business trip when I contacted him, Robert took the time for this detailed and thoughtful response:
“Let’s start with this: 100% of all condensation problems in buildings conditioned exclusively with air did not have a radiant cooling system to blame. It is time people stopped focusing on the radiant cooling panel as the problem and focus on the real problem, which is moisture.
“It is climate, system, and enclosure dependent…[and so here are] some examples:
“For cold, dry climates and good-to-great buildings with sensible cooling flux under 13 Btu/hr/ft2, fabricate the floor cooling system with masonry type surface (tile, stained concrete, terrazzo etc.), and no further sensible cooling is necessary. Ventilate for IAQ and interior moisture control.
“For cold, dry climates that can have a short wet season (ex. Calgary), space cooling is not necessary during the brief rains. Ventilate for IAQ and interior moisture control.
“For cold, dry climates and high-performance buildings with sensible cooling flux under 5 Btu/hr/ft2, mechanical cooling not necessary. Use night time flushing and elevated air speeds (ceiling fans). Ventilate for IAQ and interior moisture control.
“For marine climates (Pacific Northwest) and high-performance buildings with sensible cooling flux under 5 Btu/hr/ft2, mechanical cooling is not necessary. Use dedicated dehumidification and elevated air speeds (ceiling fans), and ventilate for IAQ.
“For hot, humid climates and good-to-great buildings, get the sensible flux down below 13 Btu/hr/ft2 with enclosure design and external shading, ventilate for IAQ, and install dedicated dehumidifiers. If there are peak loads that occasionally exceed 13 Btu/hr/ft2, elevate the air speed with ceiling fans to boost the floor performance and enhance occupant cooling. If you can’t get the sensible loads down below 13 Btu/hr/ft2 then increase the cooling surface area by adding radiant walls and ceilings or switch to ceilings and/or walls.
“For hot, humid climates and bad-to-good buildings use radiant with a dedicated outdoor air system (DOAS); the radiant panel capacity is determined by deducting the DOAS coils’ sensible capacity from the total sensible load. If the radiant load exceeds 13 Btu/hr/ft2, increase the cooling surface area by adding radiant walls and ceilings or switch to ceilings and/or walls. Add dedicated dehumidifiers if necessary.
“In short, all high-performance buildings should include moisture control, which is essential to any radiant floor cooling system. For an overview, see the Radiant Flooring Guide.”
When I say that Robert Bean is a colleague and friend, I mean that Robert is a consummate building professional, passionate about his work and the building industry, and a genuinely great person to boot. His online not-for-profit, Healthy Heating, is a resource any high-performance building professional should use.
Other resources on radiant floor cooling you might find these useful:
- “Cooling with Radiant”
- Uponor Radiant Cooling Design Manual
- California Energy Commission Project 4.3: Residential Radiant Cooling and Heating Assessment Deliverable 4.3.6a, Final Report (March 2003)