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Under slab insulation in tropics

J2V | Posted in General Questions on

We have a house in the Florida Keys, where the house is build on a concrete slab 9 feet above grade, supported by concrete columns. There is a separate slab at grade where we park vehicles etc. The summer sun reflects off the slab at grade and heats the underside of the raised slab, where the parts getting this reflected heat are noticibly hotter than other parts away from the sun. This is placing a large load on the airconditioner on top of the normal very high ambient temperature. Is there a simple way to substantionally reduce this heat input into the house in summer? If this could also potentially reduce the loss of heat in winter, that would be an added bonus?

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  1. GBA Editor
    Martin Holladay | | #1

    In your Climate Zone (Zone 1), floors are required by code to have a minimum of R-13 insulation. Two inches of closed-cell spray foam, or 2 inches of polyiso, should do it.

    Remember that foam insulation should be protected with a durable layer on the exterior.

  2. NormanWB | | #2

    Get polyiso with foil to get the benefit of a radiant barrier.

  3. Jon_R | | #3

    More insulation and to a lesser extent, high reflectance/high emissivity paint (on the overhead portion, the opposite on the parking slab).

  4. GBA Editor
    Martin Holladay | | #4

    Unlike Norman and Jon, I think that you should focus on R-value, not emissivity. If you protect the polyiso with a plywood ceiling, the radiant barrier foil won't add any R-value. (A radiant barrier provides no benefit unless it faces an air space.)

  5. Expert Member
    Dana Dorsett | | #5

    If it's the ceiling of an open drive-under space big enough to park a car, it's definitely a big enough air space that radiant barriers or high SRI paints would have a measurable effect.

    On a U-factor basis code-max is U0.064 for floors, which is R15.6, which includes the top and bottom side air films (adding up to more than R1) , the R-value of the slab itself, as well as the finish floor' material layers. Depending on your stackup you might get there with 1.5" of foil faced polyiso or 1.5" of 2lb or 3lb spray polyurethane rather than a full 2" or more. If the floor is something like ceramic tile applied do the slab with thinset it'll need the 2". At 3lbs density spray polyurethane is rugged enough to function as a walkable roof, and would only need to be painted with something to block UV degradation.

  6. J2V | | #6

    I appreciate these answers. The issue is the elevated slab at 9’ above the floor under the ground is getting hot to the point where the underside is really hot to the touch. This means the 9” of reinforced slab with tiles above in the airconditioned room above has a really high heat load.

    I guess it is feasible to spray the underside but that leaves how to seal and make what is the roof of the unfer-slab garage look attractive. Putting panels onto the existing slab underside means they need to be sealed and painted to be atteactive. I guess I understand the concepts, but would like some thoughts on how to do thid on the underside of the slab where it is all completely exposed to the garage area?.

  7. Expert Member


    if the only (or major) source of the suspended-slab overheating is reflected heat from the slab below, why not make a dropped-ceiling in the carport, much as you would do on a basement ceiling, using panels of low-emissivity materials, and leaving a ventilated gap above?

    If this turned out to be insufficient without additional insulation, you could then spray-foam the underside of the slab, and already have an attractive finished surface in place.

  8. GBA Editor
    Martin Holladay | | #8

    Glue the rigid foam in place (wedging it in place with plywood and 9-foot poles until the glue sets). Then install panels (plywood, for example) through the rigid foam to the slab with TapCon screws.

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