Best way to preserve stone foundation?
I am in the process of doing renovations to a 110 year old house with a stone foundation. The area is clay soil, climate zone 7a.
I am adding an attached garage and small addition to the back, and a covered porch to the front. The previous owners finished the basement in the late 90s-apparently with R14 fibreglass batts and vapour barrier. This has me of course nervous, but it has held up so far. The space is used as a gym. The basement air feels damp in the summer, but there has been no indication of water damage. I have a humidifier running to keep it at 50% humidity. I’d rather not redo the basement at this time…
I decided to ask the contractor to waterproof the foundation since it would be much more difficult after the addition, and also since the excavator was here anyway. They will repoint where required and then apply a blueskin membrane. New drain tile will be put in, and the trench back filled with 3/4 clean, top graded with mud.
I was curious if it’s worth adding rigid foam insulation over the waterproofing. It would be 2 inches of GPS. The irregularity of the stone would of course make it not sit flush, but would that in a way act as a dimpled membrane? Just how much less effective would the insulation be? I think spray foam would be cost prohibitive given the below quotes for GPS.
Trenching and waterproofing is quoted at 25k, which will go forward. The insulation would be an extra 25k, which seems steep if it’s not going to provide much added benefit. Perhaps saving the money for an eventual interior basement insulation overhaul may be the best course?
Ultimately, I just want to do whatever is best to preserve the foundation long term.
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Replies
Hi Birdhat.
I haven't ever looked into the question of how to best insulate a stone foundation for durability and I will be curious what others have to say.
A common recommendation for stone foundations is to use spray foam insulation from the interior and it is one location where closed-cell spray foam may be worth the economic and environmental impact.
If this were not a stone foundation, the situation would be less challenging, for sure.
My thought it that if you want to protect the stone foundation for longevity, insulate now from the outside. The warmer you keep that wall, the less susceptible the stone and mortar joints are to freeze and thaw cycles. This is similar to how exterior continuous insulation improves durability in a wood framed assembly by bringing more inside of the building envelope.
Again, these are my initial thoughts, so I would recommend you wait for some replies from GBA members who have experience with this situation or consult a local professional with experience preserving stone foundations.
While I typically like sheet goods for insulation and waterproofing, I'm not so sure about using them on a stone foundation. If the stones are well-dressed (meaning pretty flat), then pointing and plastering may provide a flat enough surface for the Blueskin, but that might be a challenge. Same with the insulation board. You should look into using liquid-applied waterproofing instead. You would still want to point and repair the foundation where necessary, but you probably wouldn't need to do any plastering/flattening, and that could save some $$. For insulation, you might want to try semi-rigid fiberglass or rockwool foundation boards. They'll squish a little to conform better to the rough surface, and they automatically act as a drainage layer as well as an insulation layer. Terminate the top with fabric and a termination bar to provide protection and keep soil from infiltrating the board. I absolutely agree with Brian that the warmer and dryer you keep the foundation, the better off you'll be.
Hi Birdhat,
I'm in a similar position with my house. Joe Lstiburek suggests a couple ways to waterproof and insulate from the inside. These mostly amount to installing a water control membrane along the floor and walls, and then interior insulation:
https://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations
That said, my instinct is to preserve interior access to the stone foundation walls (for inspection and maintenance). So I'm toying with the idea of sacrificing 3 feet along the interior perimeter of the basement, and building an insulated partition wall with air/vapor/moisture barrier *there*. Something akin to this (outside to inside):
backfill / foundation / (3' perimeter walk) / WRB / rigid foam insulation / studwall
Since you'd have access to both sides of the partition wall, I don't think it would be particularly difficult to build. The insulation just needs to be thick enough to control condensation on the conditioned-side of the WRB. I'd want to run the WRB to (and beneath) the sills, which might make for an awkward diagonal transition near the basement ceiling, but I think it could work. Not yet sure what type of "door" would be best to provide access between the newly enclosed basement and the 3 foot perimeter walk.