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Hardwood (cherry) flooring on a heated slab?

Avasa Kreismann | Posted in Green Building Techniques on

I am constructing a home on a heated monolithic slab and would like to avoid having concrete, ceramic, or engineered wood floor. I would REALLY like to install cherry hardwood and am wondering about the (real!) pros/cons of 2 installation methods… 1) gluing 5/8″ plywood to the slab and nailing the cherry to this. (how much heating efficiency is lost?) or 2) putting 2×2″ sleepers in the concrete and installing the cherry directly on this.  Does anyone have experience with these methods OR another idea?  thanks!!!

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  1. Trevor Lambert | | #1

    5/8" plywood is R0.8. The efficiency lost depends on how much insulation you have below the slab. If you have 2" of foam, then it will be in the ballpark of 8-12%. The more insulation you add below the slab, the less you will lose. The sleepers would result in more energy efficiency, though I imagine there might be other issues. At the least I think you'd want an underlay between the slab at the hardwood. If it were me, I'd be trying to think up a way to make the hardwood a floating floor. Then you don't have to worry about a nailing substrate.

    1. Avasa Kreismann | | #3

      the current design has 12" of expanded foam under the floor (6" under the footing), for approx r-40. would that mean a 2% efficiency loss (plus that of the hardwood?)? Just to understand... the efficiency loss implies what exactly? the wood boiler (froeling) will still be producing the same btus... and the heat isnt actually lost, it would just take longer to penetrate the floor?
      regarding the sleepers... assuming that humidity isnt an issue (maybe thats a pour assumption), i dont really come up with downsides...

      1. Trevor Lambert | | #5

        The fact that it takes longer to penetrate the floor is what is causing the heat to be lost. The insulation under the slab is no different than the insulation above it; it just slows down the heat loss. So slowing it down on one side (adding insulation, e.g. plywood), increases the proportional amount that will go out the other side.

        With the amount of insulation you have, I wouldn't worry too much about R0.8 of plywood. I'd still be looking at making it a floating floor. My house is pretty much the same as yours, slab on 12" of foam (but no footing). I'm not opposed to engineered wood flooring, so that's probably what I'll end up doing. But if I was really keen on hardwood, I'm sure there's a way to make that a floating floor.

  2. Jon R | | #2

    There is also heating system efficiency, which depends on your heat source and occurs even if there is no heat loss to the backside of the radiator. It's a bigger factor with a heat pump, moderate with a condensing boiler, a non-issue with electric resistance heat.

    Also note that soil acts as insulation, meaning that heat that makes it through your foam isn't completely lost.

    1. Avasa Kreismann | | #4

      froeling wood gasification boiler with electric backup. in a very efficient construction (in theory!)

    2. Trevor Lambert | | #6

      I think you could only consider the soil as insulation if the ground temperature in the heating season was very near the target indoor temperature. That is definitely not the case where I live. I would call it a heat sink rather than insulation.

      1. Jon R | | #7

        It's both. The ground temperature just below the insulation will rise to something greater than normal ground temperature.

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