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Community and Q&A

Experiences with unheated slabs in cool climates

Jason Hoetger | Posted in General Questions on
I’m planning to build a new construction 1000 sf ADU in Zone 4C (Seattle area). We’re planning to have a concrete slab as the finished floor, but I’m concerned about the slab being a bit cold. I’m looking for some advice based on other people’s experiences with unheated slabs in tight houses in cold(ish) climates. Do you consider your floor “too cold” in the winter? How do you mitigate the cool concrete feeling on your feet? Do you regret not heating it?
 
We’re planning to heat the ADU with mini-splits and I’d like to avoid heating the slab if possible. Radiant water heating is out of the question, but electric in-slab heat is an option. Our heating design temp is 28F, and our soil temps appear to be in the mid-30’s-low 40’s F in the winter, but I’m not sure if that’s at 2″ or 8″ depth.
 
Code min for the slab is R10, but I’d be happy to increase that if it would make a perceptible difference in slab temp. If we were to heat the slab, code min is R15, anyway.
 
What would be wonderful is a formula to calculate the expected temperature at the top of the slab given a soil temperature, slab insulation level, and indoor air temperature. Absent that, I’m willing to make do with anecdotes and sound advice. Similar questions have been asked before, but the responses seem to be mixed: Some people insist heating the slab is unnecessary, while others insist it is absolutely necessary. A few examples:
 
Appreciate any experiences you can share!

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Jason,

    I'm just north of you on Vancouver Island, and have an unheated slab with only perimeter insulation (as that was how we built here 25 years ago), and it's cold under foot. I'm fine is socks, but my wife needs hard soled slippers to be comfortable.

    In a well insulated and air-sealed house I don't think there is any way to heat a slab so that it will feel appreciably warm without over-heating the whole space. An in-slab radiant heat source would only be a couple of degrees warmer than the air in the house. A well insulated slab without heat, maybe a degree colder than the air - which is nowhere near the temperature of your feet. I have designed or built about a half dozen houses since my own with good sub-slab insulation, and have had no complaints.

    Akos, who posts here, suggested an interesting alternative: Include a smaller area of heated floor in areas where it would make the most difference. That would allow those parts of the slab to be much warmer without compromising the whole house.

    1. Jason Hoetger | | #2

      Thanks, Malcolm, this is good info. Glad to hear you haven't had complaints from homeowners with good sub-slab insulation. I share the concern about over-heating a tight house with in-slab electric resistance heat, though I have not seen any actual data on this.

      Have you ever measured the temperature at the top of your slab on a cold winter day? I'd be curious to know the difference between the air temp and the slab temp.

      In a tight house, you expect the slab to be ~1C cooler than the air temp? Our current kitchen has a ceramic tile floor with radiant heat, so I may try heating the floor up (or cooling it down) to ~20C (we keep our house at 21C under normal circumstances) and seeing how it feels under foot. Not a perfect comparison though.

      1. Erik_Brewster | | #16

        Anecdotally, I have in-slab hydronic heating in a house that is creeping up on well insulated. The floor is never "warm", but it sure isn't cold. It's like being on a second story, where the first story is heated, so the second story floor is room temp. I don't have much trouble regulating temperature, though I found that using an outdoor temp rest in the thermostat made a big difference. There are a few days in each shoulder season that get a couple of degrees overtemp, but that's it.

        FYI, I'm in the San Francisco Bay Area. On the coldest days here (40 degress F delta) the surface temp is maybe 74 F? With hot spots where the pipes double back on themselves a couple degrees hotter.

  2. Expert Member
    Malcolm Taylor | | #3

    Jason,

    No I haven't measured my slab temperature. Completely unscientifically, I'd say it doesn't vary that much throughout the winter. I don't notice swings based on the weather. Things are further complicated because we turn off the heat overnight, so the air temperature varies a lot.

    With both perimeter foundation and under-slab insulation I think 1C is a reasonable temperature difference to expect between the slab and air. Unfortunately that still leaves you with a difference of 17C between the slab and your feet. I guess the question is: Is that appreciably different that the (let's say) 14C you would have if the floor were heated? A lot of the problem isn't due to the temperature difference as much as the conductivity of the concrete. Any finished floor will be a similar temperature.

    1. Jason Hoetger | | #4

      Thank you, you've given me a lot to consider. What do you typically insulate to under the slab and on the perimeter now? R10, or more?

      1. Expert Member
        Malcolm Taylor | | #6

        Jason,

        I use 1 1/2" EPS on the interior of the walls, and 2" under the slab.

  3. Stephen Sheehy | | #5

    In zone 6 Maine, with four inches of reclaimed xps under the unheated slab, it's been just fine.

    1. Expert Member
      Malcolm Taylor | | #7

      Stephen,

      As I recall you have cats. They are the ultimate arbiters of comfort. Do they mind?

      1. Stephen Sheehy | | #9

        Down to one cat. Cats are way smarter than we are, when it comes to finding a comfortable spot. He loves to sleep on the floor when the sun heats it up.
        The dog goes back and forth all night from her warm bed to the cool floor.

  4. Brian P | | #8

    Zone 6, New Hampshire, tight/highly-insulated house, heated by a mini-split, 4" XPS under slab and slab perimeter.

    Do you consider your floor “too cold” in the winter?
    No.

    How do you mitigate the cool concrete feeling on your feet?
    It's not cold, we wear socks, nothing more.

    Do you regret not heating it?
    No.

  5. Expert Member
    Akos | | #10

    Thermal comfort falls into the same category as thermostat settings. No two people will ever agree on what is comfortable.

    For example, I keep my bathroom floor heat on well into the cooling season. I just don't like cold floors.

    I would try to find a house with similar construction and see if it works for you. Some things are very hard to change once the house is built.

  6. Jon R | | #11

    Lots of variables, but with 2" of EPS, expect the floor to be about 2F colder than the room air.

    You can get a pretty good idea of comfort vs temp from here:

    https://www.healthyheating.com/Thermal_Comfort_Working_Copy/Definitions/floor_temps.htm

  7. Jason Hoetger | | #12

    Thanks everyone for the advice. I'll let you know how it goes once we've been through a Fall/Winter in our ADU (so, about 18 months from now!).

  8. Mark Harrison | | #13

    In Virginia, Zone 4, with average ground temp of 64 deg F, with perimeter insulation but no sub-slab insulation: yes, the floor feels cold!

    And I would like to offer a comment on our own experience with a finished concrete floor. Given the chance, we wouldn't do finished / pigmented concrete again. In the end it didn't actually save much money, we had little control over the finished color, and we can never change the finished color. I would insulate under the slab, just as you plan to do, and budget for a floor covering.

    1. Jason Hoetger | | #15

      Were you able to put a floor on top of the concrete after you discovered you didn't like the temperature and aesthetics of it? I'm curious if you were able to find a material that you were happy with and didn't cause issues with door thresholds and cabinet heights.

      We're also not sure if we'll like the finished concrete look. Friends' homes with concrete floors seem fine, but you never know until you really inhabit the space. We're trying to decide how much to hedge our bets.

  9. Deleted | | #14

    Deleted

  10. Mark Harrison | | #17

    Hi Jason

    Perhaps I should explain how our house works before addressing the question of concrete flooring. The house is over/under, with the main level above and two bedrooms, a bath, and a family room below. The lower level is a glorified walkout basement, with lots of windows and a lake view.

    The concrete floor is on the lower level, not the main level. The floor is pigmented 4000 psi concrete, which was textured when placed and later sealed with a semi-gloss sealer. We spent somewhere in the range of $3 to $6 per sq ft to have the finishing done. It looks a lot like a natural stone floor, and we have received several compliments on the looks.

    The drawbacks, as I mentioned earlier, are that it feels cold and hard, plus poor control over the final color and no way to change the final color (short of painting or covering it). The color issue became apparent when my wife decorated the bedrooms - the final finished color turned out redder than our samples and limited what she could do.

    We are set now, and have no plans to cover over the concrete floor. It is "guest space" anyway. But I am glad we didn't try concrete for our main floor. We would probably have covered it over, and the money spent on the concrete finishing would have been wasted.

    On the main floor, btw, we used luxury vinyl plank, and I love the stuff. It passes for real wood, but is nearly indestructible - cat poops don't faze it, and you can drag furniture around on it!

    HTH

    Mark

  11. Bob Irving | | #18

    We use 6" of EPS in heated living spaces with concrete floors in NH - zone 6. One thing not mentioned is that the slab also has to be isolated from the exterior in the edges as well as isolated from any other concrete which has ground contact. The floors stay pretty much at room temperature; same as the walls and ceilings, and our clients do not find them cold, and do not miss radiant heat.

  12. stickerbush | | #19

    Our 1440sf home near Seattle has slab as finished floor throughout and it doesn't feel cold to us. Insulation under slab is R10. I have measured the slab temperature and it tends to be close to the air temperature in our home. We did put radiant in the bathrooms (electric, wires in the slab), it's nice in the winter but uses a fair amount of energy.

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