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Insulating Old Stone and Mortar Basement Walls

thegiz | Posted in General Questions on

Plan on insulating basement walls within the next year, right now planning it out. I’m in climate zone 4 near nyc. House was built in 1903. I believe my basement walls are stone and mortar with a layer of concrete over them. I think this is true because I have an interior masonry wall that I can see stone and mortar because some concrete has broken away. I guess this could also mean that it is only stone and mortar on these interior walls. Can I still use rigid foam as insulation? I was reading article here that says not to insulate stone and mortar wall with rigid insulation but I have concrete over them making the wall somewhat flat in most areas. Would I need to cement over these areas that have broken away before applying rigid foam? I also have ceiling joists above at various heights. I think it would be easier to use 2×4 furring strips on the flat every 24 inches. Is a 2×4 wall with top and bottom plates necessary? The house is already sitting on a massive masonry walls.

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  1. b_coplin | | #1

    I'll respond to give your question a bump. My guess is you have a parge coat flaking off from moisture cycling, rather than a layer of concrete. Any pictures of the walls?

    If the walls are fairly flat you can use rigid foam; leveling off with another parge coat is a good idea. It seems to me it would be simplest/lowest cost to use 1" rigid foam held in place with a 2x4 wall, which could be insulated with batts.

    There is also some good information in the "related questions" on your question's landing page. See this one in particular:

  2. thegiz | | #2

    Bryan thank you for the response. You are correct that the wall is parged. I read the post you linked at

    Dana said:

    “You don't say where you're located, and both local climate and subsoil temperatures make a difference.

    But the solution is to air-seal and insulate the foundation walls, and at the very least put down ground vapor retarder. Air sealing the walls reduces summertime infiltration reaching the rest of the cool basement, and raises the average temp both winter & summer.

    Fiberboard/OSB/plywood are all mold-food, and have a very low insulating value. Forget about trying to air-seal the ceiling- even copious amount of spray foam would still fail. Instead, seal and insulate the band joist/foundation sill, along the foundation walls.

    Use air-impermeable rigid foam (EPS is the most cost effective, and is highly tolerant of moisture) or closed cell spray polyurethane against the foundation, sealing the edges with can-foam (or FrothPak) and the seams with duct-mastic. You can cut'n'cobble the rigid foam (again, sealed with can-foam) at the band joist & foundation sill, sealing that to your wall-foam. You can then put a non-structural studwall butted up against the wall foam, insulated with unfaced batts. Put at least an inch of EPS under the bottom plate of the studwall & slab as a thermal break.

    The thickness of the wall-foam required to do an insulated interior side studwall approach varies with climate, but if you stick with the IRC prescriptive values for your area you'll have plenty of margin:

    Even an inch of EPS against the foundation wall will slow the migration of ground moisture through the foundation considerably, as well as stopping the air leakage if you seal it well. With the interior side of the studwall as unpainted or primered wallboard it won't trap any ground moisture that does find it's way in, and the average monthly temps of interior face of the foam would stay above the dew point of the interior air year-round, avoiding an accumulating moisture inside the studwall.

    The foamy mats for the floor are somewhat insulating but not air tight, so if you took that approach you'd have a mold-farm under the mat. You can put a dent in the ground moisture load by using an acrylic or silane masonry sealer on the existing slab, but don't put ANY floor covering down until you can insulate it with an air-impermeable water-tolerant rigid board insulation (EPS or XPS- again EPS is cheaper.)

    When you're ready to take that step, dig down sufficiently to put 4-6" of clean gravel, and 1-3" of foam, and put a sheet of 6mil poly between the foam and the new concrete as a ground vapor retarder.“

    He said not to put any coverings on the floor until basically I have built a new slab. But couldn’t I put 1 inch of eps or XPS on the floor as a vapor barrier and build a new floor on top of that? Can I use eps on the floor too or do I need something more rigid like XPS? Also why is it better to build a 2x4 wall as opposed to using furring strips over the rigid foam?

    1. b_coplin | | #3

      You can use eps on the floor. It sounds like you might have read it already, but there is a good article on the topic:

      I think a 2x4 wall would be easier to build than installing furring strips into a stone wall, but my experience colors my suggestion. I have a fairly bumpy limestone basement wall and I think it would be difficult to get the furring strips coplanar. Certainly everything I've mounted to those walls has been considerably fussier than mounting to concrete.

      Using furring strips also implies (to me) that you intend to use only foam. If so, a 2x4 wall with batts is likely going to be less expensive to build. If you have thick reclaimed foam available to you at low cost, then a fiber insulated stud wall + board foam may be more expensive and/or more hassle than thick foam and furring strips.

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