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Community and Q&A

Unheated slab on grade

Graham Fisher | Posted in General Questions on

I’m planning to build a house in Southern Ontario with a slab on grade (actually about 2′ above grade) foundation. The slab won’t be heated, but will be very well insulated. The house itself will also be very well insulated. There will be a moderate amount of south facing glass with a reasonably high SHGC (around .6).

I’m weighing two different flooring options for the main floor. The first is polished concrete, with the use of some area rugs. The second option is an engineered wood floor, also with the use of some area rugs.

Although I could always switch from one flooring to another, it would be great if that didn’t have to happen (changing flooring could affect door thresholds, trim, and the rise for the bottom stair, not to mention cost, waste, and hassle).

Any thoughts out there?

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  1. Kevin Dickson, MSME | | #1

    Polishing or finishing the top of the slab should be less expensive than installing a wood floor over the slab, and the exposed slab should perform better thermally. In other words, winter sunshine hitting a concrete floor will absorb and release heat better than wood, and will cause smaller temperature fluctuations.

  2. Graham Fisher | | #2

    I agree with you on the thermal performance and cost Kevin. One concern I have is that concrete will feel cold underfoot, even at, say 70F/21C. Area rugs can go a long way to managing this I'm sure, but there are places where rugs aren't the best choice, such as the kitchen. Doing cork flooring in the kitchen could work... but there is (possibly) a change in floor height to deal with too.

    Does anyone have experience with an unheated slab in a colder climate?

  3. Malcolm Taylor | | #3

    "Does anyone have experience with an unheated slab in a colder climate?"

    Yes, and with adequate insulation, such as you are installing, they are fine. The unforgiving hardness of the floor may pose a problem for some people. Rugs or work mats go some way to mitigating it.
    The choice between a finished slab and flooring isn't as stark as it appears. You have to power trowel the slab (that is polish it) whether you cover it or not, so the only cost that would be eaten should you decide to install another flooring over it is the inexpensive sealer.
    In our own house we sealed the slab with the idea that we would later put down flooring in areas where we found we might want a more forgiving surface. We never have.

  4. User avatar
    James Morgan | | #4

    Kevin is right that the slab will be cheaper, probably wrong about the thermal benefits, at least in your climate. You'll be cranking the thermostat up a notch or two to compensate for the cold surface underfoot. Still, if you feel like experimenting it's not a terrible idea to go with the polished slab - some people like cold feet - but be sure to at least set your kitchen cabinets and your entry door thresholds 3/4" high so that when you get tired of that cold hard concrete in a couple of years you'll have room to install a comfortable hardwood, bamboo or cork floating floor and the doors will still swing and the dishwasher will still fit under the countertop. The bottom rise on your stair will be short but hey, lots of older houses are like that and you'll soon get used to it. Body memory is a wonderful thing.

  5. Flitch Plate | | #5

    The whole idea is silly. A huge hulking bulk of expensive, high embodied energy concrete that has a mind of its own and a microclimate to fight against most of the year. Out of date technology.

    Cheaper, more comfortable, more control and responsive and greener would be a superinsulated wood frame floor using pressure treated piers, piles or sills on a rubble trench.

  6. Malcolm Taylor | | #6

    Are there still areas whose codes allow the construction you describe? Ours sure don't.

  7. Graham Fisher | | #7

    This is a good discussion - thanks for your thoughts. James, have you lived with a polished slab floor? Not that your opinion isn't still helpful if you haven't.

  8. User avatar
    Stephen Sheehy | | #8

    Polished concrete isn't just power trowelled. After the slab is placed, it should be power trowelled at least three times. Then, at least two weeks later,(a month is better) it gets polished with machines similar to floor Sanders, using ever finer grits. In the Portland Maine area, figure about six bucks a square foot. Of course that's in addition to the cost of the slab.

    People claim a concrete floor is harder than other materials, but I am sceptical that in a controlled test, the difference between concrete (or tile) and a typical oak floor would be all that noticeable. I have a slate floor now and it isn't a big problem for my aging body.

    James- I have to disagree about not worrying if the first step is 3/4 " higher than the rest. People would be tripping forever.

  9. Malcolm Taylor | | #9

    The overwhelming majority of exposed concrete floors aren't finished as you describe. There is a sliding scale of concrete finish from power troweling to using terrazzo machines. The look that has recently gained popularity you see in architectural periodicals and design magazines is almost always just power troweling finished with a concrete sealer, often saw cut with or without grouted joints. I'm standing on one right now and every house I've done in the last decade has had them.

  10. Flitch Plate | | #10

    Malcolm ... A grade beam perimeter and bearing wall foundation is code complaint; an engineer can sign off on a rubble trench. Saving thousands of dollars and improving interior climate control as we talk. I like the look of a polished and tinted concrete floor but slabs in southern Ontario are not climate practical in residential applications. I guess the idea comes from the concept of cyclical passive solar daytime heat gain followed by releasing it during the night. But what we know now and with modern materials, in northern climates, slabs should be relegated to the way of the dodo bird.

    I think the poster's raving about slabs are trying to make a silk purse out of a sows ear. It had its era.

    Here is a grade beam foundation that meets code – still lots cheaper, easier and greener than a full slab.

    From there a 12" -14" wood I-beam or truss joist floor, sealed and super insulated is also code compliant. A bullet proof vapor barrier is needed underneath. A mechanicals' well can be poured in the middle or mechanicals can be done above.

    There are many floor systems that pass muster that are cheaper and much more effective at enabling control over space conditioning.

  11. Peter L | | #11

    Why not just wear slippers? I have "house" footwear which is a soft rubber shoe that is almost like a beach shoe so the foot breathes but it protects your feet from the hard floors with a very flexible rubber sole. Keeps your feet warm and it provides support. Problem solved.

    Carpets are OK but the dust and cleanup are a pain. Not good for people with allergies or pets. Hard surfaces work best.

    Concrete floors, all the way!

  12. User avatar
    Stephen Sheehy | | #12

    Malcolm- here in Maine, what we call polished concrete floors are just that, I.e. polished with a succession of grits, done after the slab has mostly cured. Once my floor is done, I'll post a photo.
    Check out
    In Portland. Jon does new floors, but has also polished floors in old commercial structures.

  13. User avatar
    James Morgan | | #13

    Graham - I have. Never again, I hope.

    Stephen - tongue was in cheek. I don't really recommend this. Unless you really know for sure you and your s.o. are going to be eternally happy with concrete as a floor finish, do it right the first time.

    Peter - yes, you can 'solve' the problem. Or you can not create the problem in the first place.

  14. Malcolm Taylor | | #14

    Another regional variation I guess. Jon does really nice work. I hope you do post pics when he is done. when I built my place I saw cut the slab on three foot centres and grouted the joints.
    If you are ever in Ottawa try and go by the National Archives. In a cost-cutting measure they eliminated the terrazzo floors and instead used the grinding machines on the concrete slabs. the result is surprisingly beautiful.

  15. Malcolm Taylor | | #15

    I'm a bit confused. The grade beams you linked to are all integral to a concrete slab. I thought that was what you were against?
    Sure you can build a crawlspace and not have a slab, but there are minimum height restrictions. You can't build a wood frame structure at grade on a rubble trench, or a pier foundation either. It isn't simply a structural matter for an engineer to sign off on, it comes into conflict with too many other requirements of part 9. That's probably why you never see it done.

  16. User avatar
    Stephen Sheehy | | #16

    Malcolm: when you saw cut the joints, how did you avoid breaking the edges and leaving uneven ragged joints and chips? In my existing house we had a concrete floor installed in a garage that previously had a wood floor. The control joints were saw cut and looked pretty rough, OK for a garage, but not acceptable for a living room.

    I agree about ersatz terrazzo. It really looks nice. If I ever get to Ottowa, I'll check out the floor.

  17. Malcolm Taylor | | #17

    For avoiding cracks slabs are usually cut as soon as it has cured enough to get on it and that's when it tends to spall and chip. I wait a week or so. I also avoid using a large concrete saw. A good diamond blade in your oldest circular saw, without pushing the cut, and you get fairly nice results.
    Thank goodness sealers have advanced since the solvent based ones of the '90s. It was fine putting down the initial finish, but when it came time to re-seal you had to have the owners leave the house for several days. Nothing like a Xylene hangover to ruin your day!

  18. Debra Glauz | | #18

    Malcolm we also experimented with the nasty solvent based finishes. We then went to user friendly water based finishes that could be easily stripped if needed. These finishes were usually put over acid stained concrete. This worked ok for residential concrete but the restaurants and coffee houses found the finish and acid stain being worn off. Now the concrete is ground like terrazzo to the desired gloss. Liquid densifiers and penetrating sealers are then used requiring minimum maintenance.

  19. Malcolm Taylor | | #19

    That's a great idea for high traffic areas. Many commercial concrete floors here opt for epoxy finishes, but after a decade or so they are faced with a really messy and extensive re-coating. They would have been better served to do as you suggest.

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