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Community and Q&A

Insulating a Stone Foundation

RRSFDV | Posted in Energy Efficiency and Durability on

Hello,

We’re buying an old house (circa ~1850) in zone 5b (Hudson Valley, NY), and are considering some level of energy retro-fit as the first part of our renovations.  We’re weighing our options (and budget) on how far to go with the retrofit.   Because the roof and the roof deck need to be completely replaced, we have the opportunity to do a substantial retrofit with insulation on the outside and inside, and are planning to start there.  However, my question is about our stone foundation basement, and what we should consider doing there.   Currently, the house has fiberglass insulation on the ceiling of the basement in between the joists.  However, most of it is deteriorating and/or missing, and there is no air sealing.   At some point, the dirt floor was covered with a layer of concrete (and I’m fairly sure, because there are a few places where holes have been cut, that there is no insulation or vapor barrier below).  Is it worthwhile to insulate the walls and rim joist (say with closed-cell spray foam), but not insulate the floor?  The basement is damp, and we have no plans of finishing it, but we’re thinking we’d like to at least keep it somewhat conditioned since our current furnace (oil, forced air), duct work, and electric water heater are down there.

Additional info: I believe the house had some cellulose insulation blown into the wall cavities at some point (these walls have no exterior sheathing, just the siding.  Interior walls are mostly plaster—we have less options here because both the interior and exterior have historic elements.)  Many details are to be worked out, and the articles on the site are extremely useful, but I’d love some high level thoughts on how far to go with our retrofit.   Many thanks in advance.  -Curtis

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Replies

  1. DCContrarian | | #1

    In an old house the air infiltration is going to be a big energy loss. Sealing the walls will really help with that. You also need to seal the walls and floor of the basement against moisture.

    Do a good job and you may have to provide makeup air for the boiler.

    Once you get below ground the temperature stays pretty steady year round at about the average yearly temperature, which I imagine is around 50F where you are. If you can get even a little insulation that won't result in much heat loss.

    This is a good article:
    https://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations#P08

  2. Expert Member
    Zephyr7 | | #2

    Yes, it's worth insulating the walls even if you don't, or can't, insulate the floor. The walls lose more thermal energy than the floor so there is more benefit to insulating the walls. It's common in energy upgrades to insulate basement and crawlspace walls even when the floor has no insulation.

    A rough stone foundation wall makes spray foam the best option due to the irregular surface of the wall. You may have other options for the rim joist (if you have one), but spray foam works too. I'd try to get a capillary break installed if at all possible though, especially if you seal the rim joist area with spray foam.

    Bill

  3. Expert Member
    Dana Dorsett | | #3

    If you have the budget for it, 2" of HFO blown closed cell spray polyurethane on the full wall from the slab up to and onto the foundation sill beam up to the subflooring will do a great job of both air sealing and insulating to nearly code-min in your area. (Continuous R15 = IRC code min.) It would also do a good job of controlling ground moisture. In most areas intumescent paint would be all that's required to meet fire code for exposed foam, but check with the local inspectors first- don't assume.

    In my area 2" of HFO blown foam would run about $2.50-$3 per square foot- it'd definitely not a cheap solution.

    Going with 2" of HFC blown closed cell foam would run closer to $2 - $2.50 per square foot, but would be slightly lower performance, and has a ~75% greater environmental footprint due to the extremely high global warming footprint of the HFC245fa blowing agent. See:

    https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

    If it's fairly flat quarried stone rather than fieldstone ,or rubble it's possible to get there with just 1" of spray foam on the foundation for air sealing & moisture control, plus a 2x4/R13 non-structural studwall snugged right up to the wall foam, which is potentially cheaper as a DIY.

  4. RRSFDV | | #4

    Thank you all for this great information - it's incredibly helpful as we try to put our budget together. I am happy to learn that it still would be beneficial to insulate the walls without insulating the floor, as we might not have the budget to do all of that. And thank you for the options as well, so we can price out various scenarios. We'll definitely do our best to get a capillary break in there too. - Curtis

  5. Gayle Anderson | | #5

    This topic is timely for me as well. Our house is 1790 1 story Cape in southern coastal ME. I am deep in trying to define,budget and schedule upgrades. So far we've replaced sills on one side (and insulated with vapor barrier over that dirt shallow foundation opening it to the stone bassement) And a new roof which sadly didn't include adding insulataion to the deck.
    For the basement we have old mortared stone (rubble though some stones are huge at 2x3') and an arched brick central chimney base. The house is on clay with high water table and we've managed almost all bulk water now by adding new gutters, french drains and a stone patio with liner. This leave me with the question of wether to spray foam the wall of the basement, all of it , parts of it, the sill, or none. I've read a lot on this site including the ones on using foam for moisture barrier, using dimple cloth up the walls etc. My concern with foaming the walls (beside costs) are that this will not allow a drying of the mortar even if there is management for bulk water like dimple cloth. I've read that these old foundations have lasted due to them being able to dry to the inside. Spray foam being so new relative to these very old foundations I wonder what info we can base a decision on. I definitely won't live to see the outcome 50-100 years from now but would rather not be the source of that problem. And if spray foam is a good idea still should the walls be pointed and parged first? Thoughts?

  6. William Morse | | #6

    For an un-occupied basement I don't understand the logic behind insulating the walls. I my case the basement is storage and workshop, and I don't spend much time there, especially in the winter. Temperatures range from 50-ish in the winter to 70-ish in the summer.

    The big problem with a cold basement is a cold 1st fl0or, requiring a higher temp for comfort. The other big problem is infiltration of cold air from the basement to the house. In this case, low density foam insulation in the basement joists, along with the rim joists will both warm up the 1st floor as well as get rid of a major infiltration source. In my case with an old house there's no subfloor, just the floor boards, so there's ALOT of gaps! Unless you heat the basement, even if you insulate the walls it's going to be 50° there all winter.

    The other big problem with alot of basements, especially in the Northeast, is water and humidity. If you have any sense that you ever have water in the basement, internal french drains (perimeter drains) connected to a good sump with back-up are a must. For humidity control I first installed a Santa Fe dehumidifier. It works well but in the wet months uses ALOT of electricity. (I live next to a stream, and before installing the french drains regularly had wet walls and floors in the basement, even with a sump pump.) Thanks to a suggestion here (I think it was Martin) I now have an HRV in the basement; during the winter months it dries the basement out with a 60 watt fan motor.

    Bill

  7. Hammer 🔨 | | #7

    Hi Bill, what is the HRV you are talking about??

    1. William Morse | | #8

      Joe, are you asking what is an HRV, or what make/model I have?

  8. Hammer 🔨 | | #9

    Both without looking stupid. I looked at the acronym and humidifier came up. I have a humidifier if that’s the same thing

  9. William Morse | | #10

    Joe, no it's not the same thing at all. A Heat Recovery Ventilator (HRV) is a two-way ventilator that passes warm interior air past cold exterior air as it exchanges them (bad explanation, I know! ;^) The cold air from outside has a low moisture content, and so lowers the humidity inside.

    My model is a Broan HRV 150. There are other higher rated ones, but I got mine cheap on Craigslist! It does the job.

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