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Half insulating a stone basement?

user-2423385 | Posted in Energy Efficiency and Durability on

It’s been a long road and we are almost ready to move in to our project. I tackled the two part plaster myself with nice results but now it’s getting cold. My basement is typical fieldstone 1880 style and the house is in Zone 6b. The foundation is about 4′ underground and 2′ exposed with a 10×10″ fir sill plate beam. I have read the building science articles about completely modifying the basement but that’s not in the cards and is another massively expensive project I cannot tackle at the moment. My other plan on getting the basement a bit more under control to increase the efficiency of the home is to insulate the rock wall only in the above ground area and the sill plate areas. I was going to place an inch or so of xps against the rubble foundation (again, only in the above ground top 2′ of foundation) and run some 2×2’s vertically and stuff some batts over the top of the xps, then enclose the 2×2 studs with drywall. The sill area will be cut XPS and spray foam to seal the edges. In the future I could simply spray foam the lower parts of the foundation when budget affords.
My Question is: Is this feasible? What precautions should I look out for when installing this system? Thanks much

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  1. GBA Editor
    Martin Holladay | | #1

    It's hard to install rigid foam against a bumpy stone wall. The irregularities in the stone surface are very hard to air seal without lots and lots of spray foam, and most attempts at using this technique end up with air leaks that undermine the performance of the rigid foam.

    If you can possibly afford it, I suggest that you buy one or more two-component spray foam kits (or hire a spray foam contractor) to insulate your rubble wall with spray foam.

  2. STEPHEN SHEEHY | | #2

    Martin: what about his plan to only insulate the above grade portion of the foundation? Does that make sense? I have the same situation in my existing house and I'm looking into how to insulate the rubble foundation without spending any more than I must.

  3. GBA Editor
    Martin Holladay | | #3

    Unless one's budget is very tight, it's a good idea to insulate 100% of your foundation wall, all the way down to the footing. (However, if your live in an area that is plagued by termites, you'll probably have to leave an uninsulated inspection strip near the top of your wall.)

    If your budget is constrained, and you can't afford the cost of the needed insulation, then it makes sense to insulate just the upper half of the basement wall -- because that is the coldest part of the wall in the winter. But remember that concrete, stone, and mortar are pretty good conductors. If your foundation wall is only half-insulated, you have provided a massive thermal bridge that allows heat to bypass your insulation. The insulation that only covers half your wall still helps -- but it doesn't do as good a job as full-height insulation.

  4. gusfhb | | #4

    Seeing as the largest energy [money] savings are in the first few 'Rs' would it not make more sense to fully insulate the wall at 1 inch of foam rather than half insulate the wall at 2 inches of foam?

    this would tend to allow full air sealing while depending on how finished you wish to make it may make future improvements more difficult

  5. Richard Beyer | | #5

    Considering budget is a road block, leave the foundation alone and focus on insulating the underside of the floor assembly. The risk outweighs any immediate benefit to apply foam to the foundation wall without spending many dollars on other preventative measures first.

    The proper way would be to shotcrete the walls to a smooth finish and then install rigid foam, drainage system to manage water and apply a fire barrier over the foam. This comes with tremendous cost. Another option is to install a moisture management system typically @ $10,000.00 and only insulate the underside of the floor assembly.

    Spray foam alone without moisture management will eventually result in significant insect damage as shown here with this older Canadian home....

  6. user-2423385 | | #6

    OK...I've decided to bite the bullet and just spray the's too cold down there and the heat above is sucking the cold up from the basement. Couple Questions:
    Can I just spray 1" then put my wall up and use foam or fiberglass between the studs or should I spray more than 1"??
    I've never seen water running out of the fieldstone in over two years but I plan on putting in a perimeter drain tile in the future so I'm not too worried about the moisture BUT:
    Some brilliant person installed a concrete type coating in the past and the moisture transference in the below grade portion of the wall has chipped and deteriorated along with a good deal of the original mortar. Should I take the time to tuck point the wall before spraying?
    And Finally...does anyone have a favorite or positive experience with certain brands of spray foam kits? I will need one 600 kit for each inch of foam for the square feet I need to cover. This is one case where low basement height is a good thing :)

    As far as the termite thing...the foundation is 2 feet off the ground and I just completed as much as a rebuild of the house as you can possibly do, replacing most of the structure and found no past termite thanks for the info though. If the home was closer to the earth I would take precautions.

    thanks so much

  7. GBA Editor
    Martin Holladay | | #7

    If your budget is tight, 1 inch of spray foam is probably enough. But make sure that 1 inch is the minimum thickness, not the average thickness.

    You don't have to tuck-point your fieldstone wall before insulating it, but you should certainly remove any loose mortar or flaking plaster.

  8. gusfhb | | #8

    and free shipping!

    buy spare nozzles, the fan type, only the fan type, they want to send you cone type i dunno why

    the linked place has the nozzle separate which is handy because they don't wear out but you keep buying them from the other places.

    lots of nozzles if you need to start and stop, they last about 10 seconds once you let go of the trigger.


    If you only need one, look to a local real lumber yard, mine carries them for not too bad a punishment

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