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Community and Q&A

Lightweight concrete floors on upper levels?

user-1012653 | Posted in General Questions on

I am exploring concrete floors on the main level (over basement) to get some mass into the space. They would be stained and sealed to create the finished floor.

I am researching the use of ICF style forms that would allow a free span from wall to wall and eliminate the wood floor structure all together. An example is Lite-Deck, which costs around $3.25/sqft of floor surface for the form. (plus rebar and concrete).

The other option is a wood structure with (assumed) 2 layers of 3/4″ subfloor, vapor barrier, and then 2-3″ of light weight concrete.

The questions are:

First off, is getting mass on the floors really worth it? It seems to not be overly discussed on this forum, at least search results tend to resemble that. I have about 9% of high (+.50 SHGC) glass on the south that would be in direct contact with the floor. My solar design books lead me to believe I am on the high side of solar tempering and mass may not be overly effective without more glass. However I would also like the mass to absorb the air temp some and help to radiate heat to keep the space at a more consistent temp after the sun goes down.

If so, I am leaning towards the wood floor instead of the ICF form due to the space below the floor to run things such as electrical and 4″ HRV lines. Also install can lighting in the basement. I think cost wise it may be a wash anyway, perhaps the wood floor would be cheaper.

However an advantage to the concrete form would be a thinner floor system, and also a higher level of sound isolation between the basement and main level (in the basement would be the tv/movie area and pool table, etc). The wood structure would need to be 14″ i-joists min. where as the forms would be about 8″ total thickness, allowing me to drop a ceiling below it if I had too.

Anyone have any thoughts/comments on this topic?

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  1. GBA Editor
    Martin Holladay | | #1

    Interior thermal mass will only prove beneficial if you are willing to allow your indoor temperature to have a wide range. That may require a special thermostat. If your heating equipment always keeps your house at 70°F, then the thermal mass can't store heat.

    But if you allow your indoor temperatures to drop to 60°F at night and to range up to 80°F during the day, some of the passive solar heat that enters your house can be stored in the interior thermal mass. However, some Americans aren't comfortable with such a wide range of indoor temperatures.

    it's easy to install a concrete slab on top of wood framed floors if that's what you want to do. Just consult an engineer to be sure that the joists can take the load.

  2. user-651098 | | #2

    Conrad Nobert has built an interesting Passive Solar home in Edmonton, Alberta. He named the project the Mill Creek Net Zero Home. He has written an extensive blog about the building process and his philosophies on many topics. You will find info about thermal mass and concrete floors on his blog. You could start here.

    Merry Christmas. - Jim

  3. user-1012653 | | #3

    great, thanks for the links. That blog looks very interesting.

  4. user-716970 | | #4

    Here is a link to some interesting discussion

    You did not mention how well your home will be sealed and insulated...the more airtight it is and the more it is insulated, the more important that mass becomes for regulating temperature swings.

  5. user-723121 | | #5

    Extra interior thermal mass will reduce the temperature swings as the solar energy will be stored in the mass. The heat loss will be lower because the mass is storing the energy and not overheating the air and surfaces inside the building raising the Delta T. A very well insulated home will only lose a few degrees F even on the coldest evenings.

    The first priority should be a very well insulated shell as Garth has mentioned, gypcrete has been used as additional thermal mass over wood subfloors and for embedding hydronic tubing.

  6. kevin_in_denver | | #6


    I like the idea of this floor system. I think it saves a lot of money if you can use stained/sealed concrete as the finished floor. Hardwood flooring alone costs $3-$6/ft2 plus decking and joists. Concrete is also still very trendy right now, and I don't think there is a more robust or durable surface.

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