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We have a 150-year-old home that has an original-construction bay window whose brick & field stone foundation has sunk away from the exterior wall above (anywhere from 3/8" to 7/8" separation).

First, it's a structural problem because the bay window is just cantilevered off of the house without any actual support underneath. Second, it's a thermal problem because the cold New England air is just blowing through the gap and across the underside of the uninsulated bay window floor, so we have approximately 20 sqft of incredibly cold floor in the winter, and you can really feel what it does to the room.

While I'm an avid DIY'er, this is something I'd prefer to hire out, but for the past few years it's been hard to find people to fix small things like this - especially qualified people. Some have suggested I "just fill it with mortar" which feels wrong because it would contact the untreated wood sills and pose issues with expansion/contraction... right?

Any advice on how to support the sill and insulate/air seal as best as possible? Only the exterior is accessible.

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Some additional pictures would help.

How far does the bay window cantilever from the foundation wall? Can you see the joists inside the basement? The picture makes it look like it is 6" cantilevered?

I'd make some pt shims and jamb them between the foundation wall and the cantilevered joists ends. I'd jamb some roxul insulation into the gaps and then I'd get a cellular pvc facia board and cut it to match the settled taper. I'd use backer rod and caulking for any large gaps.

Lots of 100+ year old houses didn't do footings in their walls and / or built them on top of top soil that settles. I'd expect the house has settled in the distant past. If it does continue to settle then you'll see that in the future. This piece of work isn't so expensive that you can't just throw it away and try again. You'd really have to mark it and watch it to determine if settling is occurring anyway without spending some ludicrous amount of money on a specialist if it was even possible maybe some with ground penetrating radar could map the soil densities but I suspect this would be unheard of for a problem of this scale.

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  • Thanks! It projects about 30" out actually (and it's almost 7' wide). It proved really hard for me to get clear pictures (and focal length of my cell phone cam makes it worse) but I'll try for more. You can almost see the joists from outside, but you can't see anything from inside the basement - there's no break in the main foundation where the bay's foundation comes out from. The rest of our house is surprisingly level so this seems like the only part that's really settled worse
    – kevlarr
    Nov 8, 2022 at 17:56
  • Thanks for the recommendations, they are very clear :)
    – kevlarr
    Nov 8, 2022 at 17:57
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If you want to fix it right, you need to understand what the problem is.

Has the foundation been steadily shifting over the last 150 years, or has it been stable and not moving since the initial settling back before 1900? Has it only recently (in the last 15-20 years) started settling?

Shoving in some insulation and filling the gap might get you through the winter, but if the house was stable for more than 100 years but now has started to settle, that could be a sign of more serious problems that should be addressed.

Has there been a change in the water table or drainage due to nearby construction or a river or stream? Has the house been added onto that changed the weight?

It would be good to get an expert in foundation repair to take a look, if things change from here on out.

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  • Thanks. I can't answer regarding when/how it's been shifting - we've lived here for 3 years now and found this only because this year we cut away old hedges and bushes that had obstructed this part of the foundation. The previous owner had not really maintained or even paid attention to any part of the exterior for probably 30 years minimum, as far as we can tell based on problems elsewhere and implied from what he's said. We aren't by a river (we're on fairly high/level ground with few drainage issues) and there's no nearby construction or recent additions to the house.
    – kevlarr
    Nov 8, 2022 at 16:57
  • "It would be good to get expert in foundation repair to take a look" is a part of the problem; there are a lot of people with bigger, higher-paying jobs in our area, so it's still really tough to find qualified help. While we wait on access to an expert, what would be the best "stuff insulation in for now and reasonably air-seal" approach given the exposure to elements?
    – kevlarr
    Nov 8, 2022 at 17:00
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    If it's been stable and lasted 150+ years, I would take a "wait and see" approach: if it's lasted this long it probably won't fail in the next five, either. I would follow the advice in the other answer and get it sealed for winter. Watch it for the next couple of years for any change here or signs of settling elsewhere in the house (doors sticking, cracks in plaster, etc).
    – spuck
    Nov 8, 2022 at 18:19
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    How close are the nearest trees, & how big?
    – Tetsujin
    Nov 8, 2022 at 19:01
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    Yeah, a large tree that close could cause land shrinkage, especially noticeable on older properties before modern foundations. The 'fix' is to get rid of it [which has been done] then… hope. it's rare that it will come back, but it might stop moving. As spuck says. I'd get a pro in to see if it's still moving.
    – Tetsujin
    Nov 9, 2022 at 7:43

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