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Community and Q&A

Insulating interior basement wall of 150 year old foundation

mzuern | Posted in General Questions on

Hi all,

Just finished sistering and supporting some of the old beams in the basement of our 150 year old post and beam farm house and now we are on to the fun part: Insulation and Encapsulation!

Some Background: We are in Nova Scotia. Basement is old sandstone rock foundation parged over with cement. The floor is open dirt and gravel. Half of it is a full basement and a proper reinforced frost wall (goes at least 4 ft down below grade) and the other half is a crawl space where the foundation wall does not go down the full 4 ft below grade. The basement is quite dry for an old house and we are going to do some swaling and are planning on putting in a french drain to direct water away from the outside.

Our plan: Efficiency Nova Scotia is offering us a rebate if we bring the basement wall insulation up to r23. We are planning on layering XPS foam to reach r 23 and either framing it against our basement wall or adhering it with PL 300 or a combo of both.

Our worry: We heat with wood and when our wood stove is cranking all winter it definitely means that our basement walls freeze less (since the heat is reaching it). I’m worried that if we do as Efficiency Nova scotia suggests (heavily insulate the interior wall) that our foundation will go through some more serious freeze-thaw cycles and start shifting a lot. Thoughts?

We could insulate underneath the floor instead, but then we would need to add a secondary heat source to the basement to a) keep our plumbing from freezing and b) keep our concrete wall from freezing. This may be the best option though I’m not sure.

Encapsulation: Although our basement is quite dry there is still a fair bit of moisture coming up from the open dirt floor and slowly rotting the beams. We have been told that how people deal with this is by adding a poly layer between the open dirt floor and the structural beams. The way I see it we have two options here:
a) get a heavy duty 10-20mil poly and cover the floor and the walls making a seal that we can walk on that prevents the moisture from the floor from making it to the beams.
b) take regular 6 mil vapour barrier and staple and tape it underneath the main floor (to the exposed beams in the basement). This would I’m imagining do the same thing (prevent moisture from reaching the beams) but would be cheaper because we would not need a heavy duty poly. Would this cause any moisture trapping problems though?

Please let me know what you all think the best way is to insulate and encapsulate without lifting the house up and putting in a new foundation! haha



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  1. GBA Editor
    Martin Holladay | | #1

    For information on insulating the walls of your basement / crawl space, see these two articles:

    How to Insulate a Basement Wall

    Building an Unvented Crawl Space

    You definitely don't want to staple polyethylene to the underside of your floor joists. That could lead to moisture problems and rot. The polyethylene belongs on the dirt floor. The two articles above (especially the crawl space article) have the information you need.

    If you anticipate walking on the polyethylene, you might consider installing a rat slab (a thin concrete slab) over the poly. Again, the crawl space article has the information you need.

  2. Expert Member
    Dana Dorsett | | #2

    All XPS in north America is blown with extremely high global warming potential HFCs (>1000x CO2). As the HFC diffuse out over a few decades the performance drops, eventually dropping to R4.2/inch from it's (US-labeled) R5/inch. At 4.75" it would hit R23.75 from a labeled-R, but over time it would eventually drop to ~R20.

    But 5.5" of Type II (1.5lbs per cubic foot) or denser EPS would be R23 now, and still performing at the same level 50 years from now, and would usually be 15-25% cheaper than 4.75" of XPS.

  3. mzuern | | #3

    Hi Martin,

    Thanks for your response. I have read the above articles. The reason I asked specifically about our house is because of how old it is and how the foundation was not built with insulation in mind. I see how this would be done with a newer house with a proper foundation (that went down 4 ft on all sides). But I'm worried that applying the same methodology in this context will cause shifting. Is this not a legitimate concern? Have you come across situations like this before? Is insulating the interior wall heavily still the best option?

    Thank you for the clarification about the poly I will make sure to put 20 mil on the floor.


  4. GBA Editor
    Martin Holladay | | #4

    Installing interior insulation won't generally put a foundation at risk. (After all, barn foundations last for 150 years without any interior heat escaping through the foundation walls.)

    But every foundation is different. I'm not quite sure what you mean when you write that your foundation doesn't "go down 4 feet on all sides." I guess it would be useful to know:

    1. Do your foundation walls have footings?

    2. Are the stone walls only 4 feet high or less? If so, are there wood-framed walls above the stone walls to provide enough room to stand up in your basement?

    3. What is the distance from the dirt floor to the underside of the floor joists?

    4. How far below grade do your stone walls extend?

    5. What is the frost depth in your climate?

  5. mzuern | | #5

    Hi Martin,

    Half the basement is dug out completely so you can stand. The foundation in this section is an old sandstone rock foundation that has been parged over with concrete and reinforced with a secondary concrete wall. This section of the basement goes down 4 ft below grade (which is below our frost line in this area). The other half of the basement is not fully dug out and so is more of a crawl space. It is also the old sandstone parged over with cement. It only goes maximum 2 ft below grade (not below the frost line). I'm mainly worried about this section as it can definitely frost heave. What do you think? Is it still a good idea to insulate the walls? What do you think of the insulating the floor idea?



  6. mzuern | | #6

    Also I don't think the foundation walls have footings no.

  7. mzuern | | #7

    The section where you can stand is around 5 ft from dirt floor to the joists (beams). The crawl space section is roughly 2.5 ft from dirt floor to the joists.
    There are no wood framed walls on the foundation that have the purpose of simply increasing basement height. The sill plates sit directly on the foundation and the joists span across from there.

    I hope that answers all your questions.

  8. Expert Member
    Michael Maines | | #8

    Mzuern, I have a similar situation at my farmhouse in Maine. For the portion of the foundation that extends below the frost line, if you insulate the interior you probably don't have to worry about frost heaving the foundation, as dry-laid foundations usually get thicker as they go down and there will still be some heat getting through the wall to keep the exterior surface from freezing.

    At the portion that does not go below the frost line, if you don't want the building to move with frost, I would not insulate the interior--I would create a foundation system that extends below the frost line to stop movement first. You might be able to use a pier system instead of a full foundation. Do you have plumbing at this section?

  9. Expert Member
    Dana Dorsett | | #9

    The heat loss through the uninsulated floor in the shallower foundation section should be sufficient for keeping the foundation from frost heaving.

    But if there is room for burying 4' of wing insulation on the exterior for additional frost protection it won't hurt, if you're the type that would otherwise lose sleep over it.

  10. mzuern | | #10

    @Michael Maines: We unfortunately do have plumbing in the crawl space section. Our sewage pipe goes through it. We also don't have the money right now unfortunately to be doing major foundation work. Perhaps we can simply tightly insulate the sill plate/rim joist on the crawlspace section and do the entire wall on the full basement section? Then when we have the money to do some foundation work on the crawlspace section we can add more insulation then?

    @Dana Dorsett: Thank you for this information. I would definitely prefer to do as your suggesting and insulate from the exterior, however several people with older houses in the area have warned us not to. These old sandstone foundations have been known to shift drastically when any serious digging occurs right next to the exterior of the foundation walls.

    So Dana says go for it and Michael says fix the foundation first. Hmmmm....

  11. GBA Editor
    Martin Holladay | | #11

    You need to take advice from an engineer who can make a site visit, or from a local foundation contractor who can make a site visit, rather than taking your advice from a free web forum like this one. There's a lot at stake if you make the wrong decision.

    That said, performing air sealing work in the region of the crawl space rim joist doesn't carry any risk.

  12. Expert Member
    Michael Maines | | #12

    Dana's idea of wing insulation would work. I hadn't thought of it because it's not an option for my somewhat-similar situation, but if you have the space and inclination, it's a solid concept.

  13. Jon_R | | #13

    Just to be clear, wing insulation is horizontal and near the surface - so little impact to a foundation.

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