Why radiant-floor heating systems don’t make sense for new, energy-efficient houses
Occasionally I wonder if I have some sort of masochistic streak — somehow enjoying the grief I get when bursting people’s favorite bubbles. I’ll brace myself for such a response to this column, when I point out why radiant-floor heating systems don’t make sense for new, energy-efficient houses.
Radiant-floor heating is a way of delivering heat through the floor — usually with hot-water tubing embedded in a concrete slab. It’s a very popular heating system advanced by zealous proponents. If you want to pick a fight in the building industry, simply criticize such sacred cows as radiant-floor heating or ground-source heat pumps (stay tuned on that one).
Don’t get me wrong. Radiant-floor heating makes a lot of sense for the right applications. In fact, I think it’s a great heating system…for lousy houses. But with new construction, if the house is designed and built to be highly energy-efficient (something I always encourage as the number-one priority), it doesn’t make sense.
Before explaining why radiant-floor heating is a poor choice in new construction, let me describe what I like about it. The heat is distributed over a large surface area, so it is delivered at a relatively low temperature. It’s uniform, and it warms people directly, rather than having to heat the air. This means that radiant heat can provide comfort at a lower air temperature than is required with forced-warm-air or baseboard hot water heat. You might be able to keep your thermostat lower — say 65 degrees—and be perfectly comfortable with radiant-floor heating, while 68 or even 70 degrees would be required with other systems.
Most people with radiant-floor heating absolutely love the warmth underfoot; you can walk around barefoot even in the middle of winter. If we’re used to drafty old houses, there’s nothing nicer than a floor that’s warm underfoot and gently radiates heat upward. Radiant heat also tends to have less of a drying effect than does forced-air heat. And because there aren’t baseboard radiators, furniture can fit right up against the wall.
So, what’s wrong with radiant-floor heating?
I have two concerns, both of which apply only to very energy-efficient — superinsulated — houses. First, in a highly insulated house (and I’m talking about a really tight house with at least R-40 walls, an R-50 ceiling, and triple-glazed low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. windows), such a tiny amount of supplemental heat is needed that a radiant floor needs to be kept no more than a few degrees above the air temperature—or else overheating will occur. If a concrete-slab or tile floor surface is maintained at 72 or 75 degrees, it will likely feel cool underfoot—since it’s at a lower temperature than your feet. So you may not get that delightful benefit of a warm floor surface. And, if you’re delivering heat to the floor during the nighttime, and then have significant passive solar gain during the daytime, overheating is likely to occur. In short, radiant-floor heating just isn’t a good fit with superinsulated houses.
My second issue with radiant-floor heating has to do with economics. Radiant-floor heating systems, with tubing embedded in a concrete slab, multiple pumps for different zones, and sophisticated controls, will easily cost $10,000. I’d rather see someone spend that $10,000 on better windows, more insulation, and so forth — then recoup some of that extra cost by spending less on the heating system. Homes built to the rigorous German PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard (which I’ll describe in a future column) can be heated with, literally, a few incandescent light bulbs in each room. In a more typical superinsulated house, we can provide the desired comfort with one or two through-the-wall-vented gas space heaters or a few lengths of inexpensive electric baseboard heating element.
Again, these arguments apply to highly insulated houses — usually new construction — when you can pull out all the stops and far exceed typical insulation standards for New England. In existing houses or in new construction when fairly standard energy details are being used, radiant-floor heating makes lots of sense. In a house with a relatively large heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load., and especially in a drafty house, a radiant-floor heating system is a great option.
If you’re thinking about radiant-floor heat for your current house, the challenge is that these systems are not easy to install in existing houses. I like to benefit from the thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. of a slab floor with embedded radiant tubing, and that’s a difficult retrofit project. Some people use light-weight gypsum-concrete slabs that are poured over existing floors; others attach the radiant tubing underneath a wood floor or subfloor.
One last point: if you do install a radiant-floor heating system, be aware that it should be controlled differently than other central heating systems. The set-back thermostats I described in last week’s column aren’t effective. This is because these systems take a long time to warm up and cool down and are thus typically operated to maintain near-constant temperature day and night.
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