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While reading about superinsulation, I came across the idea that a radiant concrete floor is a poor choice if you have really good insulation. The reasoning goes like this:

In a regular house you heat through walls, ceiling, windows, etc., so the floor is kept warm enough to keep bringing in more heat. This is one of the selling points of radiant floors: that warm floors feel good.

In a super-insulated house, if the thermostat is set for a comfortable air temperature, the radiant floor will rarely turn on, so the floor will always be cool. Walking barefoot on a cool concrete floor is unpleasant. On the other hand, if you turn up the thermostat to warm up the floor, then the air temperature will be too high.

Is this correct?

(I suspect this means that if you're going to super-insulate, especially in a mild climate, you should choose a floor material with low thermal mass and low heat conductivity, like wood or carpet.)

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Sounds reasonable - but you can always cover concrete with something like throw rugs. –  The Evil Greebo Mar 31 '12 at 0:37
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3 Answers 3

I suspect that if it is cold enough to require any heat, even if the furnace only runs a few times a day, that the slab would stay comfortable. If your climate is such that you will go extended times without the furnace, you may be correct that it will feel cool.

Also, if you have south-facing windows, you may be able to get some passive solar gain using the slab. We have an upstairs bathroom with a dark stone floor that gets afternoon sun (from a window that goes down to floor level) in the winter and the floor can be noticeably warm for a while after sunset.

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The more things change, the more they stay the same. The Romans used a heated floor system in which stone is the heat storage. –  Fiasco Labs Nov 19 '12 at 23:35
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You are mostly correct. The floor concrete won't necessarily be cool; in a super-insulated house, it will likely be the same temperature as the air. However, this may feel cool to your feet since it is lower than body temperature.

I have heard that radiant floor heating is not a good choice for super-insulated houses because of this reason.

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Your floor is a massive heat-sink, it will naturally cause a convection current in the room, gradually cooling more of the air in the room.

Installing radiant heat in the floor will help to cancel out the cooling affects of your floor.

In order to prevent your room from heating up too much, you have a few options:

  • proper (lower) placement of the thermostat
  • installation of a dual thermostat (one that can measure both floor and air temperatures, and control your radiant heat system to maintain your comfort levels)
  • open a window to let heated air to escape.
  • wear slippers to insulate your feet from the floor.
  • install area rugs to insulate your feet from the floor.

(not trying to be funny on the last 3, I think they are all valid options).

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Are you sure there will be convection currents? If the floor is cool & the warm air rises to the ceiling, things will be pretty still, right? –  Jay Bazuzi Apr 2 '12 at 21:24
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