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Radiant-Floor Heating

Why radiant-floor heating systems don’t make sense for new, energy-efficient houses

Posted on Mar 31 2009 by Alex Wilson

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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  1. Wirsbo
1.
Fri, 05/08/2009 - 01:47

hydronics
by Shawn Bowman

Helpful? 1

I can understand the argument set forth, however I believe that hydronics is the most versatile and adaptable system in a super efficient home. It (water) can provide and eliminate heat energy in so many configurations that I simply call it "hydronics" and consider it to be a mandatory element of any designed structure.


2.
Mon, 08/10/2009 - 09:24

My uncle installed one of
by Timm

Helpful? -1

My uncle installed one of these systems the last year, he didn't need any help for that, he was remodeling his house and the system came with clear installation instructions. Now, after one year there are some technical difficulties, the floor is too hot in certain spots and he need to fix that.


3.
Mon, 08/10/2009 - 09:45

Floor heat
by carlos - radiant floor heating

Helpful? 0

However radiant floor heating systems have disadvantages, they have advantages also, like the one explained above. It feels comfort while people walk on barefoot in winter.


4.
Fri, 11/06/2009 - 08:34

Designer Radiators
by Steve

Helpful? -1

Wow! Fantastic points.

I never thought about it before but yes, I can see how radiant-floor heating only makes sense in remodeling! I recently installed underfloor heating in a remodeled basement, and it worked out quite well, though I can see how it would have been a useless addition to my well-insulated and eco-friendly upstairs rooms. Sure this article might not have made you too many friends in the the radiant-flooring industry, but I sure do appreciate it. Thanks.


5.
Fri, 11/06/2009 - 14:27

I disagree
by Riversong

Helpful? 1

Alex, you suggest radiant floor heating for a high heat-load, drafty house, but a floor at a maximum 85° surface temperature can deliver only 34 btu/hr·sf, which would not be nearly enough for most poorly-insulated (let alone drafty) homes.

In a tight house, it's true that the low heat demand would require a lower floor surface temperature, and that might mitigate against the "warm toes" effect. But any surface that is above room air temperature increases the mean radiant temperature (MRT) of a space, and the MRT is at least as important as air temperature in determining human comfort. Human skin has an absorptivity and emissivity of 0.97 - higher than almost any know substance, including matte-black metal - so we are very sensitive to slight variations in MRT.

A truly green house will not only be super-insulated but also small, and a small house (or any house,for that matter) benefits from the unobstructed wall area of a radiant floor heating system. While the initial installation costs will be higher for a central hydronic radiant system (compared to wall-mounted space heaters such as direct-vent or mini-splits), the uniformity of heat, the lack of room obstructions, and the higher MRT and human comfort can easily justify the additional expense, and some of that incremental cost difference can be made up in increased heating efficiency over the life of the system.

And a high-efficiency (95%) direct-vent modulating boiler can also efficienlty provide unlimited hot water through an indirect tank, reducing the cost of the domestic hot water system and somewhat offsetting the heating system cost.


6.
Thu, 12/17/2009 - 18:33

Agree with Robert
by Austin Jackson

Helpful? 1

We have a new construction kitchen with radiant that loses too much heat and requires that our floors are hotter than 85°. Nice to have warm feet, not hot feet. We're looking into a supplemental heat source for the room. Wish we had been working with an experienced contractor who had used radiant before - would have maxed out on insulation and done better windows!


7.
Sat, 02/06/2010 - 18:19

would love numbers!
by jklingel

Helpful? 0

Alex, you've caused me to think; I hate it when that happens. I was 100% sure I was going to install radiant in my slab, but now I am hesitant. I NEED NUMBERS. Does anyone have temperature data for slab-on-grade floors heated via baseboard heat? (Choice two for me.) It would be interesting to see how it compares to a fairly steady, say, 75 degree floor w/ radiant-in-slab. I will tell you that my present house, though well insulated and burning a fairly small amount of fuel, does NOT have a comfortable slab temp w baseboard heat. I am guessing that 75 F would feel relatively good; I'll have to rig up some way to measure my floor temps. Floor temp and under-slab temps can be sent to jolinak@gci.net. Thanks.


8.
Sun, 02/07/2010 - 05:37

Insulate!
by Martin Holladay, GBA Advisor

Helpful? -1

J. Klingel,
The reason many people like in-floor hydronic heat is that they are comparing these floors to uninsulated slab floors. Most of the benefits of warm floors can be achieved by installing thick (R-20 or so — but thickness depends on climate) insulation under the slab — a continuous horizontal layer of subslab foam, plus good perimeter insulation.

Even with good insulation, however, a slab or ceramic tile floor will often feel cool to bare feet. That's because our bodies operate at 98.6°F. The solution? Wood floors or area rugs — without in-floor radiant heat.


9.
Mon, 02/08/2010 - 01:59

THANK YOU.
by jklingel

Helpful? 0

Very good to hear. I am trying to recall what we did 35 yrs ago, and I think we only put 2" of XPS under the peripheral 8' of the slab where it was exposed to the air (daylight basement). I don't think there is any XPS on the other three walls. The outer edge of the slab has 4" of XPS, down 2', where it sees air. I was planning on 3" of XPS under the radiant-floor slab and fake wood floors anyway, so maybe 4" (6??) under the slab and standard baseboard heat will suffice, and be cheaper. Food for thought. Thanks again. john


10.
Mon, 02/08/2010 - 16:43

a final note....
by jklingel

Helpful? -1

Anyone have any experience w/ this hair-brained idea I had last night? How about 4-6" of XPS under a non-heated slab, house heating w/ base board hot water, and then running electric radiant under fake wood flooring? The electric radiant would then only serve as a floor warmer for the boss's toes. I have no idea how to regulate the electric radiant independently of the air temp, so that will be yet another thing to learn about. Any links/opinions/thoughts are welcome. j


11.
Mon, 02/08/2010 - 16:55

If you want to warm your floor, use hot water
by Martin Holladay, GBA Advisor

Helpful? 0

J. Klingel,
If you want to warm your floor, and if you have a boiler, why use expensive electricity?

Just put a few PEX loops in your slab. Forget the baseboard units.


12.
Tue, 02/09/2010 - 01:50

ignorance, for one....
by jklingel

Helpful? 0

Martin: The idea of using electricity was a stab in the dark. I don't yet know what to expect for floor temps in a SOG w/out radiant heat, nor do I know what my floor temp profile would look like if I installed radiant w/ pex XX inches O.C. and water YY degrees F, etc. If the heat loss analysis indicates, say, pex 16" O.C. and 90 degree F water, that may mean a "cool" floor, so no big benefit from radiant floor heat. If the heat loss analysis indicates pex 30" O.C. and 100 F water, I may have a thermal Zebra for a floor. ("If you take big steps, and start here, you'll step on the warm sections...."). Siegenthaler has some floor temp profiles for a thin-slab installation in his book, but I don't know the PEX spacing in that one figure, and that is not a SOG anyway. So, until I learn a BUNCH of stuff, I am just groping for ideas. I neither want to install radiant and have really cool floors, nor do I want to install it and find I have cool-warm-cool-warm.... floors; neither being a great result. I am searching for data that will help me get a grip on what it will take to have warm floors but no open windows, or data that says "won't happen w/ radiant, give up on the warm floor idea and wear socks." I have no lust for heating w/ electricity, believe me, but I thought maybe a tiny shot now and then would keep my wife's toes happy. Surely there is somebody who has installed radiant in a SOG and very well insulated house/slab that can tell me it works, or doesn't. If I had not read the above article, I'd be blissfully ignorant and planning radiant, full steam ahead (in a word...) Much to learn. Thanks again for the input. j


13.
Tue, 02/09/2010 - 03:39

may have found it
by jklingel

Helpful? 0

Great info here. Very helpful, and thanks again, John Siegenthaler. http://www.pmengineer.com/Articles/Cover_Story/30dacb1d7dff8010VgnVCM100...


14.
Sun, 04/18/2010 - 03:59

Passive thermal home design and radiant heating
by Greenhorn

Helpful? 0

Lot's great information here and much food thought! Does anyone know how radiant floors could play into a passive themal two story /daylight basement house plan? Ideally couldn't pex tubing in the floors/ thermal mass be to use help warm or augment the radiant floor system as it moves South to North across the rooms reducing the demand on the boiler during peak demand hours. Also if a radiant heat system was coupled with a passive solar design wouldn't it mitigate the temperature swings by tempering the entire mass (North & South sides) to a common temperature. The other question is with a passive solar home design windows are low lead and typically two pains of glass max. and the glazing of the south wall is at or above 8% of the total square foot of floor space reducing the ability to have the South side walls super insulated.
Your thoughts are greatly appreciated.


15.
Mon, 04/26/2010 - 11:07

radiant floor heating and concrete floors
by Canan

Helpful? 0

We are renovating an old house and will be using radiant floor heating. One of our options is to use concrete floor as our final pavement (as opposed to tiles, stone or so) Our contractor, who is used to pouring industrial concrete floors, told us that the thinnest he could pour the concrete would be 8 cm. Would that be too thick? He does not have any experience with residential concrete floors. Is there a big difference? forgive my ignorance on this issue.

your thoughts will be helpful


16.
Mon, 04/26/2010 - 11:23

That sounds about right
by Martin Holladay, GBA Advisor

Helpful? 0

Canan,
Since 8 cm. is 3.15 inches, that sounds right.

If you want a thinner poured floor, you can use a gypsum product like Gypcrete. However, Gypcrete needs to be covered with a finished floor; it's not intended to be stepped on.


17.
Mon, 04/26/2010 - 11:28

Thickness of radiant slab
by Alex Wilson

Helpful? 0

In my opinion, 8 cm (about 3") would not be too thick to work well as a radiant slab. Sometimes used in thin radiant slabs is a lighter-weight gypsum concrete. Ask your contractor about these systems, which can be thinner than 3" as long as the subfloor is rigid enough. You can learn more here: http://www.maxxon.com/therma-floor/data.


18.
Mon, 04/26/2010 - 12:09

thickness of radiant slab
by Canan

Helpful? 0

Thank you Martin and Alex for your quick responses. Our problem is with having enough space once the concrete is poured. He (the contractor) would actually prefer to pour at least a10 cm slab which is the minumum he uses in his industrial jobs (like factory floors). The original plan we have allowed for about 3 cm above the tubes (if we have tile floors). Therefore the levels of doors, windows, etc have been drawn to those specs :-(.

We just learned that we could have the concrete as an alternative and it will help save us a great deal of money. but how to fit the rest with the added 5 cm in height? back to the drawing board


19.
Wed, 08/25/2010 - 21:55

radiant floor heating & cooling for super insulated new home
by Mike

Helpful? 0

I am near completion on a prototype hew home in Tucson, AZ.- the hot side of the sun belt section of the country. Radiant floor heating was initially considered to be too expensive for our relatively short heating season. However, if radiant floor cooling could also work, well, then Bingo! So, after MUCH research and considerable thought, I have design and built a 1900 sq. ft. home that has a thermal envelope (roof, walls and floor) that is so well insulated it is like being in a thermos. When it was 105 degrees a month ago, I ran cold water through the radiant tubing in a bedroom wing zone cooling the floor to 69.8 degrees - the living space between 3 and 7 feet was a cool and comfortable 75 degrees. This is before drywall and finish stucco. The 75 degree temp lasted all day. It was like being in a cool cave. Walking on the floor barefoot initially was like stepping into a swimming pool-a few moments to acclimate. The S.E.E.D. Home (Super Energy Efficient Design) is nearing completion of the finishing touches. It will have no need for any forced air mechanical system for heating or cooling. It is so well insulated, passive solar orientated (designed without any direct solar gain)and tight that the only air issues are fresh air exchange (simple enough to accomplish) and air mixing-ceiling fans. DOE will be monitoring this home for verification of energy efficient expectations.


20.
Thu, 08/26/2010 - 04:20

Response to Mike
by Martin Holladay, GBA Advisor

Helpful? 1

Mike,
As I'm sure you know, the main problem with an in-floor hydronic cooling system is avoiding condensation on your cold floor. These systems can work in very dry climates, as long as the floor isn't too cold, and as long as indoor relative humidity is closely monitored and controlled.


21.
Sun, 11/07/2010 - 11:09

Underfloor radiant
by joel

Helpful? 0

Martin, do you know the limit for the floor thickness you'd want if using underfloor water radiant heat.
The application I'm considering is a 530 sq ft home with full basement. Current floor is 1 1/2" of yellow pine boards. I'm in Bloomington, Indiana. If underfloor heat would result in a warm basement and cold house, we could do baseboard hydronic heat.

Thanks for the all the info here.


22.
Sun, 11/07/2010 - 12:03

Response to Joel
by Martin Holladay, GBA Advisor

Helpful? 0

Joel,
Here's an article that will get you started:
http://www.radiant-floor-heating.com/hydronic/floor-covering-options/

There are lots of articles on the Web on this topic; do some Googling. Here's another article with information on flooring options:
http://www.boucherenergy.com/pdf/JLC.pdf


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