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Community and Q&A

Floor over slab insulation?

curtis1512 | Posted in General Questions on

I live in Pittsburgh, PA. I’m turning an existing porch into a dinning room. There is an existing 6″ slab with 1 1/2″ thick pavers over it. The pavers are cemented to the slab and grouted. Dirt under the slab.

The local code calls for R10 insulation 2′ in around the perimeter walls. I wanted the entire floor insulated. My contractor only did the 2′ perimeter insulation. In your opinion is this OK?


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  1. charlie_sullivan | | #1

    I don't think it's OK.

    The three considerations that come to mind are energy efficiency (heat loss), moisture issues, and code compliance. It might comply with code--I don't know much about that--but I think you could do better as far as heat loss, and I also think you could have serious moisture problems.

    The main moisture problem is that warm, humid interior air could lead to condensation on the slab. That could happen in the winter, especially near the perimeter, and in the summer, especially near the middle. It can happen both in the open bays in the middle, and in the fiberglass-filled bays around the perimeter. Instead of a highly permeable insulation like fiberglass, the floor should be insulated with rigid foam board. You could perhaps do the "cut and cobble approach between the 2x4s that are there, especially if they are pressure treated, but that's a lot of work and worse than having continuous foam insulation across the whole floor, both from a moisture perspective and a energy performance perspective.

    It's the same as insulating a basement floor, except a little more critical near the perimeter because the winter temperature will be colder.

  2. GBA Editor
    Martin Holladay | | #2

    There are lots of risks here. You really don't want to be using any fiberglass. You also need to make sure that the existing slab is insulated with vertical rigid foam at the perimeter of the slab.

    In your climate zone (Zone 5), the code calls for a minimum of R-10 of vertical rigid foam at the slab perimeter (on the outside of your house). This R-10 insulation needs to extend 2 feet into the soil, measured vertically from the top of the slab. All exposed sides of the slab need to be insulated in this way. If you fail to do this, the slab perimeter will be cold enough for condensation to form.

    Ideally, you need to install horizontal rigid foam above the existing floor tiles, covering the entire room, followed by a new concrete slab (or, alternatively, by a layer of 3/4-inch plywood, screwed through the rigid foam to the slab below). You don't want joists, and you don't want fiberglass insulation.

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