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Insulating a Rubble Foundation

Doug_A | Posted in General Questions on

I’m considering bringing my basement inside my house’s building envelope. I’m in an 1850s Greek Revival farmhouse on a rubble foundation in Upstate NY (Climate Zone 5A).

Two great articles that have guided my thinking are Steve Baczek’s How to Save and Old House (GBA, 2015) and Joe Lstiburek’s Rubble Foundations (BSC, 2010).

Steve and Joe have designed very similar basement-wall assemblies: both use a vapor barrier laid directly over the interior face of the stone foundation, with a 3-inch layer of closed-cell spray foam on top. (I believe both of these houses are also in Climate Zone 5A.)

Steve and Joe have different approaches to the vapor barrier where it meets the sill. Steve staples the top of his 10-mil cross-linked poly vapor barrier (VaporBright) to the inside face of the sill and lets it hang down the wall. Joe goes through the trouble of jacking up the entire perimeter of his house—in sections, by just an inch—in order to install a butyl membrane (Grace Ice & Water Shield) as a capillary break between the foundation and the sill. “The installation of a robust capillary break is a necessity,” he says, calling it “critical” for rubble foundations that don’t project far above grade. (As it happens, this describes my foundation.) Joe then extends that membrane all the way down the interior basement walls.

Question 1: Is Joe’s capillary break necessary? Or, if I don’t see any evidence of previous sill rot, can safely I get away with Steve’s approach? (Given how knowledgeable and thoughtful Steve is, I’m inclined to do it his way, but I just can’t shake Joe’s warnings.)

Question 2: Should I first repoint the inside of the foundation wall, now only patchily mortared, before hiding it forever behind a vapor barrier and insulation? (I’m inclined not to go through the trouble, but I’m open to hearing advice to the contrary. Either way, I’m counting on installing a continuous vapor barrier / dimple mat and perimeter drain to deal with inevitable water intrusion. I even wonder if more mortar will actually be counterproductive, causing hydrostatic pressure buildup behind the foundation and more of the upward capillary action that concerns Joe.)

Question 3: In the interest of minimizing global warming potential, I have to ask: is there *any* recommendable alternative to closed-cell spray foam for insulating a rubble foundation wall? (One local contractor, a very nice person who markets himself as a residential energy consultant and insulation contractor, proposed using furring strips to make a uniform vertical plane on which to mount polyiso sheets. However, given the irregularity and slight bowing of the rubble wall, I’m concerned about the huge cavities that this assembly would leave.

Thanks in advance for any help!

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Replies

  1. Expert Member
    Akos | | #1

    Spray foam is about the only way to insulate these without doing exterior insulation.

    The vapor barrier does not need to go up over the whole foundation, if you are doing interior perimeter drain, run the dimple mat up a foot or so on the foundation so any moisture that does make it behind there can drain.

    If your rim joists are bellow grade, you are really asking for trouble without a capillary break. I would add this in, if too difficult, insulate from the outside.

    1. Doug_A | | #2

      Thanks very much for that advice. The rim joists are above grade, but there are a couple parts of the house where there are only a couple of inches between wood and earth, and weeds grow up and hold moisture against the siding. In those spots, I’m planning to dig down a few inches at the foundation and regrade the surface to slope away from the house.

      I hear what you’re saying about not needing the vapor barrier to go up over the whole foundation wall since the spray foam itself acts as a vapor barrier when it’s sprayed directly on the stone. The one thing I like about having a membrane between the spray foam and stone—and between the spray foam and wood elements, for that matter—is that the spray foam will be easier to remove if anyone ever wants to do that.

      1. Expert Member
        Akos | | #4

        You want the spray foam directly applied to the stone. The reason is you don't want any air pockets. Any air pockets will have high humidity a good part of the year and mold can grow.

        I would rather have an assembly that works great right now than one that is questionable that might or might not make something down the road easier. Plus peel and stick is way harder to remove than spray foam.

        "regrade the surface to slope away from the house"

        This is the most important thing to get right. Most basement water issues with rubble foundation can be fixed by getting the grading right. Check the grade even away from the house, I've had issues where the grade close was good but than the whole backyard sloped towards towards the house. Not fun to regrade the whole thing.

        Make sure the top surface is either soil or compacted stone dust, you want something that will channel water away from the house. Definitely not river rock or gravel unless there is a sloped membrane under it.

  2. DCContrarian | | #3

    If you've read those two articles you're 90% of the way there.
    I believe Joe Lstiburek has relented and now believes that a capillary break isn't necessary with a stone foundation. For the most part stone doesn't wick the way concrete does, although I guess it would depend on the type of stone.

    Years ago I talked with a contractor who advocated building a stud wall on the inside of a rubble wall, offset as far as needed so it would be plumb. The wall side of the stud wall has dimple mat or similar to channel any seepage and block any vapor coming out of the rubble. If that is well sealed you can insulate and finish like any other basement wall. Where the wall meets the joists you need moisture sealing and fire blocking back to the sill. While this could work, spray foam is pretty foolproof and you don't lose as much basement space.

    There are low-GWP spray foams sold, I don't know if it's just a gimmick.

  3. NYNick | | #5

    I'm confused. I have an old house with old additions. Part of the foundation is rubble stone and part is 1 foot thick concrete. Am I to understand that I can apply closed cell directly to both, assuming no apparent water issues?

    1. Expert Member
      Akos | | #6

      Provided there are no bulk water leaks, closed cell spray foam directly over the foundation is the safest interior insulation no matter what your foundation is made from. It is a solid air and vapor barrier and since the foam is directly adhered to the substrate there are no air gaps where anything can grow.

      The reason to use rigid over poured concrete walls is simply that it costs less.

      1. DCContrarian | | #7

        Rigid also releases less greenhouse gases.

      2. NYNick | | #8

        Thank you Akos.

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