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Community and Q&A

Foamboard under enclosed concrete porch floor

pcv21 | Posted in Energy Efficiency and Durability on

The back porch of my 1910 rowhouse in Washington DC was hastily converted about 10 years ago into a half bath. There seems to be little or no insulation for the half bath, which is getting quite cold despite being airtight with a newer window. Below the room is an open-air basement entrance way. A 4′ x’8 piece of 2″ rigid foamboard insulation would fit perfectly onto the bottom of the structure, which is concrete.

Given that the walls have little or no insulation, and can’t be accessed without ripping out the drywall, would insulating under the floor help at all, or just be wasted effort? Thanks for any advice.

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  1. Expert Member
    Dana Dorsett | | #1

    If the concrete exposed to outdoor air, HELL yes it'll make a difference, even if the wall insulation is next to nothing! The R-value of a 4" slab R-0.5, and the R- value of a tile or vinyl flooring is near zero. When it's 25F outside that floor is probably getting down to ~40F even at an average room temp of 70F. Empty-cavity studwalls would run maybe ~50F under the same outdoor conditions, as would triple-wythe no-cavity red-brick.

    You may also be able to blow insulation into the wall cavities without gutting the wallboard. What is the wall stackup, from the interior paint to the exterior paint?

  2. GBA Editor
    Martin Holladay | | #2

    If you are trying to insulate an exposed slab that has air underneath it, remember that you have to insulate the edges of the slab -- not just the underside of the slab.

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