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concrete 2nd floor slab in or out of thermal envelope?

MichaelWolf | Posted in Energy Efficiency and Durability on

Hello all!  I’ve just received a gift subscription to this site, and am very interested to read the various articles.

I’m planning new construction of a metal frame building.  It will be workshop/garage on the ground floor, residence on the 2nd floor.  The structure of the mezzanine is metal joists, a metal deck, and a concrete slab poured onto the deck.  The deck + slab is 6″ thick (not yet clear to me how thick the concrete is).  The building will be near Lake Tahoe California, climate zone 6.

My question is: is it better to put insulation on top of the concrete slab, and flooring on top of that?  Or better to insulate beneath the metal deck, so as to have that floor mass inside the heated envelope of the building?  The workshop will be heated enough to keep from freezing, but I’m expecting to go most of the winter with a thermostat set to 50 F.

My thought is that it might lend more thermal stability to the residence to have the mass of the floor structure inside the insulated envelope of the residence, and improve comfort.

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  1. Robert Opaluch | | #1

    Without a diagram with more details about the building framing and how much insulation you plan around the steel structure, its difficult to provide a good answer to your question.

    Yes agree normally you would want the concrete floor (thermal mass) inside the thermal envelope for temperature stability. Without a more detailed diagram of the building envelope (including the detailing of materials where slab edge meets exterior wall framing), maybe there's at least two possible answers:
    1. Since steel conducts heat VERY readily. (Steel R-value near zero, much worse than wood structures R-1.25/inch, and insulation at R-3 to R-7/inch), you need to have insulation installed BETWEEN the steel structure and the second floor residence concrete floor (including at the exterior wall steel framing). Otherwise, the steel shell will conduct heat readily from the concrete floor that is inside the conditioned space to the outdoors and the cooler spaces of the garage and workshop. Keeping the temperature at 50 in the garage and workshop is a lot better than outdoor winter temps, but still can conduct a lot of heat from the slab directly through steel and into the garage, workshop, or exterior wall framing, resulting in a floor that's probably a little too cold for comfort. Not to mention a source of significant heat losses from the residence. It would be somewhat cooler than your residence air temperature, depending upon specified insulation, how the residence is heated, and analysis of any thermal bridging. Just keeping the air temp of the garage and workshop at 50 doesn't stop heat loss from any thermal bridge from the second floor residence to steel framing. Concrete has almost no R-value (about R-1 per foot, or 0.1/inch).
    2. As you had planned, you could leave the concrete floor uninsulated, and add insulation on top of the concrete, then add a finish floor. Yes you lose the thermal mass and any additional costs for flooring over insulation layer.

    1. MichaelWolf | | #2

      Thermal bridging has definitely been on my mind. I'm still working through a lot of the structural details, but the building style is a metal post frame construction, with 6 posts for a ~1800 ft^2 building footprint. The walls are supported by widely spaced 8" metal girts, and I'm planning to have both insulation between the girts, and a complete wrap of 3" insulation to reduce the thermal bridging the girts cause. There's a very similar story for the roof, with 9.5" metal purlins, insulation in the bays between, and a complete layer of 3" rigid foam to limit the thermal bridging. It's not yet clear to me what my options are for limiting the thermal bridging between the mezzanine floor and the metal posts (and thus into the shop below). The good news is that the whole floor is only attached to the posts at 6 points, but I'm guessing that there may be no effective way to put a thermal barrier into those joints.

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