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Turning a previously enclosed porch into a bathroom- how to insulate slab?

HelenRein | Posted in Energy Efficiency and Durability on

I am renovating an old house in Climate Zone 3. A previous owner turned an enclosed porch into a study. It has a concrete slab on grade floor, stone columns, and a low brick wall at its perimeter. The exterior finish is a layer of Celotex (I am assuming this is an insulated board) and stucco and the wall cavity as constructed is about a foot thick. It is sort of framed up as a double wall. There is an almost 5 inch step up into the adjoining room, so I have plenty of space to build that floor up with insulation. I want to finish the floor with tile. I am trying to figure out the best way to air seal and insulate this space. I have read several GBA articles on insulating below grade slabs in basements, but haven’t seen one related to an above grade slab. Would it be treated the same way- maybe 2″ XPS, one or two layers of subfloor, backer board, and tile? Attached is a photo of the gutted space.

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  1. GBA Editor
    Martin Holladay | | #1

    You might want to try again to post a photo.

    The most important slab insulation you'll need to install is the vertical insulation at the perimeter of the slab. You need to dig a trench at the perimeter of the slab, to a depth of between 2 feet and 3 feet. Install between 2 and 4 inches of vertical rigid foam -- EPS is the most environmentally friendly option -- into the trench, and extend the foam to the top of the existing slab.

    Here is a link to an article that includes information on ways to protect the above-grade portion of the exterior rigid foam. (You want to protect it from physical abuse and UV rays).
    How to Insulate a Basement Wall

    You will also need to include flashing between the bottom of your existing siding or cladding and the top of the material you choose to protect the above-grade portion of the vertical rigid foam.

  2. HelenRein | | #2

    Thanks Martin. Sorry the attachment didn't work. The photo is not great, but it will give you a general sense. What about the interior slab insulation? Do I need a vapor barrier on top of the slab under the insulation? I am pretty positive there is no vapor barrier underneath the 1928 slab.

  3. HelenRein | | #3

    Let me try that one more time. Attachment.

  4. Expert Member
    Dana Dorsett | | #4

    Celotex is asphalted fiberboard, which is highly moisture tolerant and also permeable to water vapor, which is used because it won't rot under the moisture drives seen behind stucco or brick exteriors.

    But the thermal performance of 1/2" Celotex is ~R1.3, at 3/4" it's ~R2. That isn't sufficient insulation to come anywhere near code minimum performance. For a zone 3 climate to meet IRC 2015 minimums you would need another R20 of insulation between stud framing or another ~R15 if continuous, with no significant thermal bridging.

    With the highly permeable sheathing there can be wall condensation issues in the finished wall assembly if the house is going to be air conditioned, and you are in zone 3A rather than 3B. But before diving into that, tell us more details about the "...sort of framed up as a double wall..." assembly.

  5. Expert Member
    Dana Dorsett | | #5

    Now that I've seen the pictures...

    Is there any air gap at all between the interior studs and the brick?

    Is there proper window flashing?

    How deep are the roof overhangs?

  6. GBA Editor
    Martin Holladay | | #6

    Yes, you'll need a vapor barrier above the existing slab. After you've completed the perimeter insulation -- which, as I wrote, is by far the most important type of insulation for this slab -- you can add horizontal insulation above the slab if you want (although you may not need it if you install the vertical insulation at the perimeter as I described.)

    You can install horizontal rigid foam, followed by either cementitious backerboard plus tile, or by a new 3-inch-thick concrete slab and tile.

  7. HelenRein | | #7

    Thanks for the information on Celotex. I'm glad to know that it is vapor permeable, I have actually not seen the space in person since it was gutted a few days ago, but will send more information in a few days. It looks like there is framing just inside of the Celotex and also on the outside of the stone columns. We may have to remove the existing framing anyway since it may not be up to code and if we are going to raise the floor, we will likely have to reframe it.

  8. HelenRein | | #8

    Thanks guys. I will send some more information after I see the situation better in person.
    Just to clarify, if I do the perimeter slab insulation, do I need a layer of poly between the interior slab and the rigid foam or if I use XPS will that be a sufficient vapor barrier?

  9. user-2310254 | | #9

    Helen. Check the slab for level when you are onsite. You may need to level it.

  10. GBA Editor
    Martin Holladay | | #10

    Q. "If I do the perimeter slab insulation, do I need a layer of poly between the interior slab and the rigid foam or if I use XPS will that be a sufficient vapor barrier?"

    A. If you install vertical insulation at the slab perimeter, you don't need to install polyethylene between the edge of the slab and the vertical rigid foam.

    If you install horizontal XPS -- and by the way, EPS would be more environmentally friendly -- above the slab on the interior, then the rigid foam would be an adequate vapor retarder above the slab. That said, polyethylene is cheap -- so you might want to install polyethylene above the slab, followed by a layer of horizontal rigid foam.

  11. HelenRein | | #11

    That helps a lot. Thanks very much!

  12. wisjim | | #12

    This past summer, we raised the floor almost 24 inches in our sunroom so the floor is level with the bottom of the glass, and we used EPS that came from a roofing project , so we got the insulation for a fraction of new price. We filled the space solid with foam, topped with 2 staggered layers of t&g subflooring and Marmoleum tiles. We are thrilled with the space now. It was easier and cheaper than new floor joists, and a center beam, which would have left a very slim crawlspace.

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