February 2010

Volume 19, Number 2

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Green Topics

Radiant Floor Heating: Wrong Choice for Green Homes?

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Illustration: Julia Jandrisits

Radiant-floor heating is popular for some good reasons. It provides very comfortable, uniform heat, owing to the relatively low temperature and the large surface area from which the heat is radiated. It does not interfere with furnishings in a home as most other heat distribution systems do. It’s quiet. And, according to proponents, it can save energy by warming people directly (rather than heating the air)—thus allowing occupants to keep the thermostat (air temperature) lower.

Indeed, people living in houses with radiant-floor heat are often effusive in their enthusiasm. Walking around with bare feet on warm floors is very appealing.

Radiant-floor heating systems commonly involve PEX (cross-linked polyethylene) tubing embedded in a concrete slab; hot water is pumped through the tubing. The slab warms up and slowly radiates heat into the room. Radiant-floor heating systems can also be achieved with tubing under wooden floors.

While radiant-floor heat makes sense in certain buildings, it is not well-suited to highly insulated green homes for a number of reasons. First, in a home with a tight envelope and a very small heating load, even a small amount of heat can cause overheating, and the thermal mass in a radiant floor system (especially with concrete-slab systems) increases the risk of overheating. This is particularly true in buildings with some level of passive solar gain—the radiant floor may still be delivering heat even after solar gain raises the air temperature.


Second, when the heating load is very small, the radiant slab has to be maintained at no more than a few degrees above room temperature to prevent overheating, and this means that the slab isn’t likely to be warm to the touch. A slab maintained at 74°F (23°C) will be cooler than an occupant’s skin, so bare feet will conduct heat into the slab.

Third, radiant floor slabs and the mechanical equipment needed to heat them are expensive. For a typical house, such systems often cost well over $10,000. Again, in a highly insulated house, that is a lot to spend for a few hundred dollars worth of heat per year.

Fourth, there is little if any evidence that radiant-floor heating actually saves energy. The argument that homeowners will keep their thermostats set lower with radiant heat is not supported by (admittedly limited) research. And with slab-on-grade homes with typical levels of insulation beneath the slab—rarely more than two inches (50 mm)—there may be significant heat going into the ground.

In short, radiant-floor heating is a great heating option for homes with average or below-average levels of insulation, but it is not well suited to highly insulated homes, especially such homes with moderate solar gain (passive solar heating). For more on these issues see “Radiant Floor Heating: When it Does—and Doesn’t—Make Sense” in EBN Jan. 2002.

Comments (8)

1 Energy Efficient Houses posted by Gary Pugh on 02/04/2010 at 08:55 am

I am a SIP builder in northern CA. My clients that put in radient floor systems report that it rarely comes on. Case 1 - 750 SF Granny in Santa Rosa. they have to leave the PILOT light out in the fireplace or house gets to warm, radiant on only occaisopnly & then only a few minutes at a time. Case 2 - Geyersville CA, SIP house, rearely gets below 64 degrees, hardly ever comes on even when house is left for 3 weeks in winter. Case 3 - Sebastopol, CA 850 SF guest house, gas for radiant boiler = less than $20/ month Case 4 - Lake county, 120 heat wave, house stays at 80 or below, no AC. When the fireplace was burned had to open all windows!! Simply stated, save the money for something else IF you are wise enough to build with SIPs. No matter where you live. Gary

2 Radiant Floor Heating: Wrong posted by A. George Beeler on 02/04/2010 at 01:10 pm

I have stopped using radiant heat for both residential, school & commercial buildings for all of the reasons that you state in the article plus: 1 The slab is not able to store heat for passive solar heating if it is already warm when the sun's heat is available. 2 During spring & fall when the days are warm but the nights are cool, the warm slab from morning radiant heating persists into the afternoon and interferers with passive cooling.

3 Energy Efficiency posted by John Swearingen on 02/04/2010 at 01:15 pm

We have similar results to Gary in his area with our strawbale homes. In a 2800SF home in Sonoma, they use the heat only 3-4 times each winter. However, these are mild winter climates that require minimal heat anyway. In the Sierra foothills and areas with little direct gain in winter, when heating is more continuously called for, the radiant floors are more practical. Our biggest concern in design is the balance of underslab insulation. Since we design primarily for cooling loads, which are generally greater than heating loads in most areas of California, we appreciate the significant boost that we can get from earth coupling with minimal or no underslab insulation, and that creates a thermal sink in winter.

4 Radiant floor heating in cold posted by L. Frederick Dure on 02/05/2010 at 05:07 am

We installed the Wirsbo floor tubing in a 3 in concrete slab elevated floor in our Alaska bush (off grid location), but have not deployed the system. The intent was to analyze options to power the system in the future. Anyway, I wonder if the potential negative effects mentioned in the article are pertinent to cold climate environments where there is minimal passive (thermal) gain during low sun winter months?

5 A general comment on how radi posted by Peter Yost on 02/05/2010 at 06:25 am

This is a great exchange of insights on radiant floor systems. I just wanted to add two:

1. On average, radiant floor heating systems deliver only about a maximum of 60% of their heat by radiation; the rest is mostly by convection.

2. The above ties into claims of reduced operating air temperatures as an energy efficiency element of radiant floors. You can't really KEEP air temperatures depressed if you have a slow-acting radiant system that is heating only 60% by radiation; the air temperature rises as the floor heats by convection. When radiant floor heat first comes on, there may be some higher mean radiant and lower air temperature but in steady state, the mean radiant and air temperature are going to be very nearly the same. The only way you can KEEP the air temperature depressed is if you have a really leaky home, but that seems overall counterintuitive and counterproductive.

There is no denying the thermal comfort of radiant floor heat, but Alex is right: if you have a limited number of dollars to spend on your heating system and building envelope, investing in a better envelope instead of an expensive heating system makes the most sense, just about regardless of climate.

6 What about IAQ and solar? posted by Maggie Carnevale on 02/08/2010 at 07:30 am

I do think there is a point here with super high performance smaller homes. If every home could be heated with a match we wouldn't even need to be having this discussion but unfortunately even many green homes still have a significant heating load, especially in colder climates.

1. I think the IAQ improvement with having no ductwork is a big plus that is not mentioned. After all, green building is not just about energy.

2. One of the biggest benefits for radiant floor is the ability to tie solar thermal in for heating climates. In North Carolina we have a 35% tax credit for solar space heating. This makes the finances a little different compared to a high efficiency heat pump with no tax credits available.

3. Our trick here in a mixed humid climate is that it doesn't offer dehumidification so you have to end up with an additional system if you want to dehumidify.

7 What about hydronic radiators posted by Helena Meryman on 03/19/2010 at 01:49 pm

I am currently looking into heating systems for my house renovation in San Francisco. Insulation and high performance windows and leak sealing are a big part of the effort. We'll be installing a HRV. But I *hate* being cold and want a comfortable level of heat (forced hot air is not an option for IAQ reasons). I'm attracted to the concept of hydronic radiators using solar thermal with pv elec or gas boiler backup. And I was considering a radiant slab on the ground floor because it gets zero sun and plenty of chill. But after reading the article above I'm wondering if even hydronic radiators are overkill. I could save a bundle of money by just installing a couple of gas hearths and using the solar thermal for the DHW only. Any thoughts? Thanks, Helena

8 Getting by without distribute posted by Alex Wilson on 03/20/2010 at 05:07 am

It's important to point out that our ideas about avoiding radiant-floor heat (or distributed heat altogether) apply when the envelope energy performance is dramatically improved. In a renovation, that means a "deep energy retrofit," which is really hard and really expensive (in cooler climates this means boosting wall R-values to about R-40, ceiling/roof to R-60, triple-glazed (double-low-e) windows, etc. If you can't go that far with the envelope retrofits, then hydronic heat (inluding radiant-floor heat) might be an excellent option.

Some readers think that we don't like radiant-floor heat; on the contrary--it's a great option when we can't go as far as we'd like with envelope energy performance. In most climates, unless you're able to superinsulate your envelope, it's hard to avoid distributed heat.

I say "in most climates" because coastal California may be an exception. You have a pretty easy climate for heating, and you might be able to get by without distributed heat even if you don't superinsulate.

I would contact a local energy expert, such as ReCurve (previously Sustainable Spaces). An expert will be able to look at your particular situation and advise accordingly.

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February 1, 2010