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Air entrained concrete reduce heat conduction?

coz24 | Posted in Green Building Techniques on

I am building a net-zero village in zone 6. My houses are slab on grade. I would love to save my customers the cost of wood floors on top of the slabs by somehow making the slabs acceptable to them. As I understand the most repellent aspect of the concrete floors is the highly heat conductive quality of concrete such that no matter how much insulation ( I am currently planning on 4″ ) you put below the slab the concrete still aggressively steals heat out of the feet of the occupants, resulting in a rather significant loss of comfort. Although, I am intending to give a nice pair of slippers with home purchase; my question is: does anyone have any experience with air entrainment reducing the heat conduction of concrete? Thank you in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    Air-entrained concrete is standard in cold climates. Its density doesn't differ much from the density of concrete that isn't air-entrained -- certainly not enough to raise its R-value per inch.

    Four inches of horizontal insulation under the slab will bring the temperature of the slab to room temperature. If occupants object -- and some occupants probably will -- they can install area rugs.

  2. coz24 | | #2

    Thanks Martin, It appears the slab temperature does not have much of an impact on the sensation of coldness. Although the R-value of air entrained concrete may not change I wonder if its conductivity will. See:

  3. Expert Member
    Michael Maines | | #3

    Susan, R-value is the inverse of thermal conductivity--in effect, a material's resistance to heat flowing through it. Concrete has very high thermal conductivity, and the inverse, very low R-value.

    If the concrete was the same temperature as your skin, roughly 90°F, it would not feel cold. Because even with a super-insulated slab the concrete will be room temperature, there is going to be roughly 20°F difference so the concrete will feel cool.

  4. coz24 | | #4

    ok got it. you think wood flooring will rise to a temperature higher than room temperature? as people report that wood installed over concrete feels much warmer to their feet. I guess I will offer an engineered perimeter adhered wood flooring over the slab as an option.

  5. Expert Member
    Dana Dorsett | | #5

    Wood has about 1/10th the thermal conductivity of concrete, so it FEELS warmer to bare feet than concrete at the same temperature, since it's conducting heat away from your feet more slowly. A rug is even less conductive than wood, and feels even warmer to bare feet.

    Wood will not be warmer than room temperature unless heated (by the sun, or a heating system embedded in the slab).

  6. coz24 | | #6

    Indeed, and you do not think that the air entrainment will induce the same effect ? I wonder if I run some PEX in the slab at less of a frequency (and cost) if it were the primary heat system, (primary will be ASHPs) and then pull water off of the dhw storage tank intermittently when a sensor detects the temperature low enough to do that, to simply take the chill off of the slab --and that may be less than wood floors and more durable.

  7. Jon_R | | #7

    A problem with radiant heat is that even heating the floor to ~75F (optimal for non-bare feet) will produce 10 btu/sqft. Consider polished/stained concrete plus many rugs.

  8. Expert Member
    Michael Maines | | #8

    In the sense that most insulation is based on trapping air into small pockets, air entraining concrete indeed would raise the R-value of concrete. However, a limitation with all insulation is the conductivity and quantity of whatever material is forming those pockets. The best common insulation, foamed plastic, has very little solid material and is mostly air bubbles--only about 5% of the volume is made up of solid matter. Concrete, on the other hand, has very little air. Air entrained concrete contains only 3% to 7% air; the rest is highly conductive mineral content.

    Let's be generous and say that air entrainment reduces the thermal conduction of concrete by 10%, taking it from roughly R-0.1 ft²•°F•hr/Btu to R-0.11. Still a miserably low value.

    Highly foamed concrete is available, with R-values approaching 2 per inch. It's just not used for floors.

    Wood is around R-1.25 per inch--as Dana said, ten times more insulating than concrete.

  9. Dana1 | | #9

    Seriously, if you foam the concrete to the point that it's R-value is comparable to that of wood (or even twice as conductive as wood), it won't be sufficiently durable to use as a flooring material. Think of how fragile pumice stone is.

  10. coz24 | | #10

    Jon R are indicating that 10btu/sqft is too much heat for a high performing home? Any of you guys have any luck with a simple/inexpensive means of staining concrete?

  11. Expert Member
    Michael Maines | | #11

    Susan, it depends on the house, but in an efficient home, 10 Btu/ft² is likely to be about what you would need on the coldest few days of the year. The rest of the time you will be generating excess heat. Like running a furnace on high when it's 50° out.

    Radiant floors feel luxurious when you have a poorly insulated or drafty home. Unfortunately, they simply do not make sense for affordable, low-energy homes.

    Staining concrete is fairly straightforward. Assuming you want the organic, mottled look, just be careful to protect the slab from joint compound and other acidic or alkaline substances, as well as oils. It would be smart to find someone who has done it before. Sometimes it's the concrete guy, but more often the GC or painter who takes it on. You can also get solid stains for concrete, which are basically paint, and more forgiving but less interesting than the transparent stains.

  12. GBA Editor
    Martin Holladay | | #12

    For a discussion of all of the reasons why in-floor radiant heating systems are a poor choice for well-insulated homes, see All About Radiant Floors.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    You can also stain concrete by getting the batch plant to add pigment to the concrete mix.

  14. coz24 | | #14

    thank you yes I know radiant is bad idea for these homes. I was thinking that a less intensive installation of the pex on bigger centers might take the tactile "chill'" off of the concrete and thereby eliminate the apparently quite common compulsion on the part of the owner to add wood flooring over the concrete.

  15. charlie_sullivan | | #15

    Let's say the room temperature is 70 and the foot temperature is 90. If the concrete has 10 times the thermal conductivity of wood, to make the concrete feel the same as wood, you'd need to reduce that 20 degree difference down to 2 degrees--so you'd need to heat the floor to 88. With the floor that hot, it would quickly overheat the room.

    It would be great to have some other options for flooring materials that have thermal conductivity more like wood than like concrete. But then again, wood works pretty well, so maybe we should just be thankful for having the option of wood.

    Or rugs.

  16. onslow | | #16

    Six years ago when I set out on planning our retirement home, I also considered radiant floor heat and a stained slab to save flooring costs. When doing a floor rehab in the kitchen I decided to install a wire based electric radiant system under key areas of the traffic pattern to save costs. It was also a deliberate choice that would allow me to test my spouses feelings about what felt warm enough.

    I soon discovered all of the truths noted in the other comments. I would like to add that my test areas of half spaced wires clearly showed the lack of heat transfer sideways despite my expectations regarding the high thermal conductivity of the porcelain tile flooring. Only the few spots where I used closer than recommended spacing felt close to uniform. Thermal conductivity is uniform in all directions for materials, so the heat transfers quite quickly out of the tile to air and sub-floor even over long periods of run time. The wire heats to over 100 degrees so the delta T to air or sub-floor is substantial. The loss rate is high enough to dissipate heat well before it travels horizontally. I only wish I had a FLIR image of the results.

    We also discovered that the floor only felt neutral/pleasant to walk on bare foot if the surface of the tile measured 84 degrees. Over heating the room was not an issue in our kitchen which suffered from a near total lack of insulation. Beyond that, I was advised that all slabs crack, staining is in the eye of the beholder and the skill of the contractor, and setting the pex tubing into the slab is trickier than it may seem if the slab is in the four inch range. If the top side of the pex comes too close to the surface cracks may appear along the tubing path. The typical placement biases the tubing down into the slab. I did ultimately add pex tubing to my basement and garage slabs for future use or added sales value if I sell. I personally lashed the tubing to the mesh and set the lifts for height as the labor cost I faced from the contractor was severe. I don't have any cracks, but I have not needed the heat in the basement. The garage is so well insulated that heating the slab seems pointless as I work in the basement.

    Two last notes, make sure that you have some way to trace the tubing or fountains may be in your future and a blog writer on this site noted that cat barf stains concrete enough to show. Hope this helps you decide with eyes open.

  17. coz24 | | #17

    Thanks Charlie and Roger -- It seems definitive. The slippers win! and I am looking into a good sustainable/green wood floor to install over the slabs as an option. If anyone has any suggestions on that -- I am all ears. and I know I would have to factor in distance from source. Appreciate all the help.

  18. charlie_sullivan | | #18

    Susan, my experience in New England is that if you can find a mill within a reasonable distance, buying hardwood flooring directly from them is a good way to get quality and plausibility of sustainability at a very reasonable price. I would look for that possibility first.

  19. STEPHEN SHEEHY | | #19

    Susan: Charlie had a good suggestion. Try A.E. Sampson in Warren Maine for wood flooring. If you opt for concrete, we are happy with our polished concrete floors. Jon Meade of Portland Maine did our floors about 18 months ago. Cost was a bit less than $10,000 for about 1650 square feet.
    With a well insulated house and 4" of reclaimed xps under the slab, no complaints of cold floors from me, my wife, the dog or cat.

  20. coz24 | | #20

    That is good to hear Stephen. Do you always have your slippers on in the winter time? I think in order to not have to install sleepers and subfloor--we are talking engineered wood. but I think there are some that will meet our standards

  21. STEPHEN SHEEHY | | #21

    Susan- We usually wear slippers or socks in winter, but that's always been the case, even when we had wood floors. If you still can't decide, maybe you can find someone with concrete floors who'll let you walk around with your shoes off. Where are you building?

  22. Expert Member
    Michael Maines | | #22

    Another good option would be cork tiles; it's completely renewable resource, as the cork is typically stripped from trees without cutting them down, and they feel warm because their R-value is about 3 per inch, two to three times that of wood. Cork mostly comes from Portugal but it's not very carbon-intensive to ship it across the ocean, all things considered.

  23. coz24 | | #23

    I have asked people that live in a new neighborhood near me of very close to passive house homes and they complain of cold feet./floors. That is what precipitated this inquiry, The project is in zone 6 -- Ithaca NY. I will look into cork. thank you

  24. propeller | | #24

    Susan, we’re in zone 6 in southern New Brunswick. We designed our 1600 square foot slab on grade Passive House and have lived in it since Feb 2016. We have polished concrete floors (8 in. concrete over 8 in. EPS). The house is south facing and we’ve optimized the windows size to benefit from passive solar heating without overheating.
    We used air entrained concrete because the slab was poured in October 2014 and was left exposed to the elements until we started the build in the spring. We are very happy we did this as it was a very cold winter and we suffered no cracks in the slab. At the design stage we entertained the notion of embedding PEX tubing during the pour as a safeguard. However the PHPP calculations indicated the surface temperature of the slab should never be less than 1 degree Celsius (2 degree Fahrenheit) off the ambient air temperature. We decided to be confident with our calculations and did not include the PEX tubing. This has since proven to be true.
    One of the biggest obstacles to maintaining even floor temperature is the quality of the outside doors as this is the biggest source of air infiltration leading to cold floors. This is true for all floors but is more acutely perceived with concrete.
    We love our concrete floors as they are comfortable to the touch and we have no issue with walking around bare foot. The area rug that we had planned to lay down in the living room has since been moved to a wall as a decorative feature as it is absolutely not needed on the floor. We initially had concerns with the floor possibly being too hard on our backs as we had experience this with ceramic floors in a previous home. We are happy to report that this is not an issue.
    If we had advice to give it would be to ensure that the concrete is evenly poured and powered trowelled. Ensuring that the floor is perfectly level during the pour is essential in obtaining a nice floor finish. In order to reduce cost it is best to have them polished before the walls go up. They can be covered with rolls of cardboard during construction and a final polish and seal can be done once the house is finished.

  25. STEPHEN SHEEHY | | #25

    Marc's experience is similar to ours. While our house is just a pretty good house, not a certified passive house, we're in zone 6, with 4" of reclaimed xps under the slab. It was certainly easier for the floor polishing to be done before any interior partitions were in, but it can be done any time. We used Ramboard and some cheap osb for floor protection and that worked fine.
    I walked around in my bare feet this morning (30 F outside, 70 F inside) and was comfortable. I think the key is a good building envelope, with no drafts and enough subslab insulation that the floor stays at room temperature. Our interior temperature never varies more than a degree or so during heating season. And with PH levels of air tightness and insulation, our heating season is about 2 months shorter (mid-October to mid-April) than in our old house, where we often heated from September to May..

  26. coz24 | | #26

    thank you Marc and Stephen, 8" of foam under the slab is a lot, aka expensive. and ditto 8" of concrete. I would love to install more insulation but I have to be sensitive to the marketability of the homes. I have not added stain and polish to the costs yet. I imagine that will be equal to or more than installing engineered wood flooring. I do plan on a very tight envelope using 4" of 2# spf with solstice blowing agent on top of the subbase (and layer of polyethylene) right up into the walls (6.5") so it will be very tight, very contiguous envelope, Banking on that and highly draining soils to insure a fairly warm slab. It is good to know that with only 4" of xps you are comfortable. I think the homes that I checked out in my area and referenced above did not control slab edge infiltration and conduction. And regarding RAM board.-- I have been a remodeler for a long time. And I dumpster dive at a local furniture store and get way better floor protection material for free. nothing like the box that a lazy boy comes in for being thick and huge. :) I know what day their recycling goes out and I pilfer before then.

  27. Expert Member
    MALCOLM TAYLOR | | #27

    My floor and the floors in a half dozen of the houses I've built are finished concrete. They were not polished, but simply had a good power trowelling and were then coated with sealer. This represents a nominal additional cost over what you would incur if you were covering the slab. The floors have occasioned complements from both their owners and visitors.

  28. coz24 | | #28

    Thanks Malcolm until I started obsessing about this cold feet issue, that's exactly what I planned on doing, now will return to, after this wonderful and informative dialogue I am pleased that the wall system I'm using will allow us to pour under cover so we can further insure a nice finish.

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