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Community and Q&A

Radiant floor heating vs. the building enclosure

jbmoyer | Posted in Energy Efficiency and Durability on

I see a lot of fancy homes being built in my market that have complicated, expensive radiant floor heating systems. What confuses me is that these homes are built with 2×6 exterior walls with blown-in fiberglass insulation, no exterior insulated sheathing.
An R-20ish wall does not seem efficient enough to start introducing radiant floor systems. Shouldn’t these systems be treated like renewables? Meaning that they only should be specified after the appropriate efficiency levels in the foundation, walls, windows, and roof have been met.
I know that radiant floor heating is an efficient way to heat your home. Comfortable systems but they are not cheap. Shouldn’t the money be spent on the building enclosure first? Then if the budget allows, put in the radiant heating?
I figure its best to have a super tight, efficient envelope with a +90% efficient furnace, as opposed to a not-so-efficient envelope and radiant floor heating.

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  1. jbmoyer | | #1

    I forgot to point out the climate in our area....
    Cold- 8,000 heating degree days

  2. GBA Editor
    Martin Holladay | | #2

    I agree with you.

  3. Riversong | | #3

    There's nothing "fancy" about a new home with code-minimum insulation levels. A tight, efficient envelope is certainly the first priority. But we often fail to consider that the primary determinant of the energy consumption of a home is the size and the secondary determinant is the shape - simple, more cube-like shapes have less surface area to volume ratio, and elongated or complex shapes are inherently less efficient.

    But a house is a complex system, not simply a shell into which mechanical systems are installed as an afterthought, and the package needs to be designed as a whole. So what is cost-effective for HVAC depends on design goals. If AC is to be included, then a forced-air system may be the most cost-effective. If not, then a boiler with indirect hot water and comfortable and non-intrusive radiant heat may be the most cost-effective method of reaching design goals.

    If the house is designed with high, cathedral ceilings, then a forced-air system will result in heat stratification and a radiant floor will offer more uniform, comfortable and economical heat.

  4. Rick | | #4


    It looks like you should read a lot more around this site, to see how houses should be built as compared to how they have been and are being built by mainstream builders. (This site falls dramatically short in addressing the use of solar energy for electricity, space heating and domestic hot water for houses, however.)

    The following is from a Comcast homepage news story today. It's hard for me to give the American public this much credit, but is it just possible that word is getting around to potential homebuyers that it's impossible to buy a new house designed for the future from mainstream builders? Are mainstream builders trying to scramble to transition to building right at the same time they are stuck with unsold bad designs and inventory, so in the meantime aren't building anything?

    Obama warns of double dip recession
    WASHINGTON — The budding economic recovery is getting little help from the home building industry, which normally creates jobs and boosts growth as a recession ends.

    Construction of homes unexpectedly plunged last month to its lowest point since April, the Commerce Department said Wednesday. The weak figures show that builders fear there aren't enough buyers to soak up the glut of unsold homes already on the market — a supply magnified by record-high foreclosures.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for the feedback concerning GBA. I hear you -- you'd like more articles on solar energy.
    In case you missed them, here are some links:

    A case study about an Oregon house with solar energy features:

    An article about solar hot water systems:

    A discussion of the economic case for and against PV systems:


    Many homeowners don't choose radiant floor (or endless hot water) because they want to save energy. They generally buy it because it is perceived as the most luxurious type of heat available (and they buy demand water heaters so they can take 30 minute showers)

    Because radiant heating is perceived as a luxury item it is often sold at a ridiculous price and the systems tend to be over-engineered. Many builders provide less complicated radiant floor systems that work acceptably well at less cost. Any radiant or hydronic heating system will help eliminate the air pressure induced energy leakage associated with forced air heating systems.

    People often use concrete slabs as thermal inertia in passive solar home designs and adding radiant heat to that type of home really maximizes the benefit of the slab. My cost for a simplified radiant floor system is equivalent to my cost for a good quality wood stove (including metalbestos chimney and installation). I think many people will benefit from a radiant floor many more days out of the year than a wood stove.

    My point really is that a radiant heating system should not be treated as renewable unless tied into a drain-back solar water heater with an over-sized collector array. Like a demand water heater, they are more efficient than standard issue equipment but really are purchased as luxury items. Less of a guilty pleasure than a swimming pool or a big SUV but not all that deep green.

    They can be a part of a home with an efficient shell and viewed holistically they can be an important tool to use in building a high performance home but they are not something that can compensate for a code minimum building envelope.

  7. GBA Editor
    Martin Holladay | | #7

    I disagree with your statement, "People often use concrete slabs as thermal inertia in passive solar home designs and adding radiant heat to that type of home really maximizes the benefit of the slab."

    In many ways, in-floor radiant heat and passive solar design are incompatible. Consider a clear January night. The temperature drops. Of course, in the wee hours of the morning, as the house cools off, the boiler fires and the circulator sends hot water to the PEX tubing in the slab. By the time the sun comes through the windows at 9:00 a.m., the slab is already warmer than the air temperature, and therefore the slab is less able to absorb solar heat than it would have been in a passive solar house without in-floor radiant tubing.

    For good passive solar design, you need a cool slab on those winter mornings, so that the sun shining through the window has an available thermal mass to heat up. If the sun shines on a slab that's already warm, the value of the mass for passive solar purposes is greatly diminished.


    this has just not been my experience. The heat of the sunlight falling on the concrete is absorbed even if that slab surface is already room temp. We don't see the passive slab homes over heating in the mornings due to solar gain or thermal lag effect.

    If the slab were to reject the heating ability of the sun shining on it because it was already room temp that heat would have to bounce off and over heat the rest of the house. This has not been happening in our homes. I beleive the heat is being absorbed and moderated by th mass of the slab.

  9. gaspr | | #9

    I agree that most radiant systems are over engineered and ridiculously over priced. In a super insulated, super tight, small home, there is very likely no need for the temperature of the supply water to ever be above 80 degrees F. This alone eliminates the need for expensive controls that monitor outside temps and adjust accordingly (reset controls). There would also be no need for separate zones, eliminating the cost of zone valves and the electronics and wiring that go with them. Also no need for a large dedicated boiler. As Robert points out, a condensing water heater or a condensing wall mounted boiler with an indirect water heater, can easily do the job. It should be pointed out that these boilers need low temperature return water to actually condense the flue gases and achieve their 90 plus efficiencies, and are therefore a good fit with low temp radiant systems. All that should be required for good control of the system, would be one line voltage thermostat (with a low differential) to control a single circulating pump. Supply water temperature can be easily controlled using a good mixing valve. Add an expansion tank, and the needed check valves and ball valves, temperature gauges, the headers, and of course the pex in the floor and you are pretty much done.

    The thermostat could be upgraded to a setback type so that the owner could keep the slab temp a degree or two colder in the morning to take more advantage of passive solar gains. ( This could also be done manually of course). And as has been mentioned, solar thermal systems can be added at any time down the road if ROI's improve...

    If an "open direct" system is used, it is possible to valve the system to use the cold domestic water to cool the slab somewhat in the cooling season.

  10. Robert Riversong | | #10


    I agree with you on this point. Solar heat arrives in the near-infrared spectrum to which glass is transparent, and radiant heat transfer between sun and slab is proportional to the 4th power of the absolute temperature difference (which is virtually identical whether the slab is 65° or 85°). A concrete slab can easily absorb far more heat than a radiant system would impart to it, and without significantly raising its temperature (high specific heat and good thermal diffusivity).

    With the radiant system circulating hot water throughout a slab, the solar heated part would simply absorb less heat from the water, leaving more BTUs available to the cooler parts of the slab not in direct sunlight. This is an excellent way of "moving" direct-gain solar heat to the non-illuminated parts of the house in the north and behind walls and closed doors.

    While it's true that a very tight house will not allow a radiant slab to rise to sufficient surface temperatures to feel as warm as it would in a more poorly insulated house, the floor will still be warmer than it would be if unheated and the mean radiant temperature of the space will still be higher, which contributes more to human comfort than air temperature.

  11. Robert Riversong | | #11


    I agree with you that a radiant heating system in a tight house does not have to be excessively complex, and hence can be more easily justified.

    But I advise against any attempt at radiant cooling in a slab floor since that is an invitation to condensation during the more humid summer months and all the consequent problems that we strive to avoid in a healthy home.

  12. MICHAEL CHANDLER | | #12

    I do have one slab that has radiant cooling down here in NC. I've witnessed the condensation effect and it's pretty stunning esp in a stained concrete slab, It looks like all the pipes have sprung leaks!

    It can be managed but not something I'd do again in this climate, Might be workable in a really dry area though. the flow control and mixing need to be designed very carefully.

    "Open direct" systems have all kinds of unintended consequences. I really don't think anyone with that sort of system will be happy with it in the long run. It's one extra pump and a flat plate heat exchanger to switch to a closed system. Add an expansion tank if you are doing staple up, the savings of $500 or so is not worth the future headaches in my experience.

  13. gaspr | | #13

    I for one, would be interested in learning more about the "unintended consequences" regarding "open-direct" systems. Just curious...Thanks.

  14. Robert Riversong | | #14

    Besides the floor cooling and condensation issue, another problem with making the hydronic loops integral with domestic hot water supply is the significantly increased volume of stagnant water at ideal temperatures for pathogen growth (think Legionaire's Disease).

    To prevent that, the heating plant would have to be kept at 140° F, which is too hot for either domestic hot water or for radiant heat.

  15. MICHAEL CHANDLER | | #15


    The risk of bacteria growing in the stagnant water draws most of the attention with open direct systems but another concern is just general mineral build-up that happens when you take hot well water and mix it with the cool water circulating in the slab to bring it down to the 85 or 90 degree temp you would circulate in the concrete to warm the house. As the water cools the minerals drop out of suspension and accumulate in the tubes in the concrete. When I have gone back to old homes to purge air bubbles out of the pumps at the end of a long period of inactivity I often will flush out muddy water that has accumulated.

    I've also seen really high quality brass check valves with stainless steel pins and springs get gummed up in half open position and fail due to this sediment and corrosion. With the closed direct systems we install, the heat is transferred from the indirect water tank to the floor loops with a flat plate heat exchanger so the build-up of sludge, all check valves, and the potential risk of Legionnaires disease are all eliminated. It's worth it to me just to get rid of the risk of check valve failure but feeling secure on the Legionnaires disease is a good thing too.

    The theory is that many cases of Legionnaires are being mis-diagnosed as pneumonia and that older people with weakened immune systems are dying preventable deaths from water heater related infections, hard to prove but not worth the risk when the cost to switch to closed indirect is so low and the maintenance is so much less.

  16. gaspr | | #16

    Robert & Michael
    Thanks for the great answers....

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