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Rubble Stone and Dry Stack Foundations

Austin G | Posted in General Questions on

My home has areas of basement/cellar as well as areas of crawl space.  The cellar areas are rubble stone + mortar and the crawl space areas are composed of flat rocks, dry stacked with sill beam in top. 

I retucked the rubble with lime based mortar, as all the old timer masons insisted that the mortar needs to be softer than the stone.  I’ve resisted spray foaming the walls for the same reason, “old walls need to breathe” and I wanted to have access to point and tuck again if needed.

I realize now that I smack myself in the forehead whenever I hear the “old timers” say that a house needs to breathe, and my home is tight as can be with an erv for controlled ventilation.  Why am I listening to them about the masonry?!?

Looking for confirmation on the best way to manage (and insulate) rubble stone and dry stack, the modern and building science supported way.

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Replies

  1. Expert Member
    Michael Maines | | #1

    Austin, the old-timers are right about the mortar. You're lucky--that knowledge seems to be in short supply. The best approach I've found with foundations like that is to make sure there is good water management on the exterior (gutters or ground gutters, slope away from house, etc.) and a backup system on the interior. If you go a year without seeing liquid water on your walls, or just a dribble at most, it should be safe to install closed-cell foam.

    Be sure to insist on HFO-blown foam. Conventional HFC-blown foam comes with very large carbon emissions; HFO-blown foam has about 1/4 the climate impact on a 75-year scale.

  2. Karl B (Zone 6A) | | #2

    Hi Austin, I'm wrestling with the same question / concern, especially as the consensus seems to be that ccSPF is the way to go:

    Martin: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall

    Joe Lstiburek: https://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations

    Dana Dorsett: https://www.greenbuildingadvisor.com/question/advice-for-lowering-the-humidity-in-an-old-stone-basement#comment-127910

    Michael (comment #1) Dana (#3) Bill (#20): https://www.greenbuildingadvisor.com/question/finishing-a-room-in-an-old-basement-with-stone-walls

    I've been batting around a couple alternative schemes in that last thread, to no avail. If your walls are planar enough, there's no reason why you couldn't use rigid foam, which is where I'm pinning my hopes.

    1. Austin G | | #6

      I appreciate the multiple links! Building science is definitely an area in which the more I read, the more confused I become!

      1. Karl B (Zone 6A) | | #7

        Very welcome! Please let us know what you decide, and how it works out!

  3. Walter Ahlgrim | | #3

    I say you choose to live in a 100 year old home because you like the charm and part of that charm is that it cost a little more to heat. How much heat is really lost into the 55° ground from a semi conditioned unfinished basement at 62° I say not much. I do not feel the risk does not equal the reward.

    Understand this house survived 100 years because it found a way to dry itself before the wood could rot and you are messing with a that balance. I say if it works don’t try to fix it.

    Am I right in guessing you are dreaming of finishing this basement into living space? If so this is first step on the road of pain and misery stop now.

    Walta

    1. Austin G | | #4

      My primary objective is definitely in maintaining the integrity of the home. Sealing the rubble walls was more about keeping moisture and humidity low, I’ve already laid a vapor barrier.

      I have an enormous amount of quite literal blood, sweat, and tears in this house. Gutted to bare studs, new plumbing, new electrical, brought the floors back to life - 99% of everything done with my own two hands. I just want the house to last long enough that it’s still a good house when I die and pass it on.

  4. Walter Ahlgrim | | #5

    If your goal is to leave the basement unfinished I would, do what I could to keep the bulk water out and install interior perimeter drainage system to get rid of any that does get in and skip the spray foam.

    I worked on my friends 1890s house we hulled hundreds of buckets of dirt out installed the drain and vapor barrier under the new slab. It has been great but I would not want to live in the basement.

    Walta

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