frost protected shallow foundation retrofit for 100+ year old limestone foundati

I recently discovered that my foundation gets gradually shallower towards one corner of my house. At the corner it is only one course (12-18 in.) of kasota stone (dolomitic limestone) thick! I don't want to change the dynamics of the 115 year old house by excavating and replacing part of the foundation. The foundation is solid with no sagging floors and barely even any squeaking floorboards. But I live in Minnesota, and global cooling is beginning, so I'd like to avoid future frost problems.

Can I basically use EPS foam to insulate the foundation from the exterior like in the drawing?

 

Your frost problems will be form below the foundation, not next to it. If it has been like that for 100 years and everything is ok, perhaps it is best to leave it as is.

 

Hi degnem,
FPSF are a great concept IMO. But as drooplug says, if it has lasted this long, then frost certainly hasn't been a problem.

Frost requires three conditions, cold temperatures, moisture, and frost suseptable soils. The extra insulation below grade is an attempt to remove the temperature element, but you would need to be careful and not introduce new soils, or create a storage area for moisture. If your surface drainage is good, it needs to remain so and that can be difficult when excavating aroung the foundation.

Attention to details and a frost protected shallow foundation can be safe in your cold climate and provide a well insulated wall area to reduce heat loss.

Some reading, but you have probably already seen it: Frost-Protected Shallow Foundations (FPSF)

Bud

 

Well, the problem is... I only discovered the problem by excavating. So, it is already fully exposed. What should I do now? I know I need to repoint what little stone there is because the mortar has been turned to dust. But should I then just fill it back in with the same dirt?

 

Not a pro on limestone, but I have read you need to repair it in the same manner as it was built and with similar materials. Made sense at the time I read it.

Since it is open, I would proceed with the FPSF approach. My last building, a slab on grade commercial building specified 2" rigid foam one foot down, sloping to two and out 4' from the edge of the slab. That thinking is similar to a FPSF but has two advantages. It is directing the water farther away from the edge of the foundation and when the soil above the rigid freezes, it cannot push squarely on the wall. It also makes the heat loss path longer.

Since the soil is open, make sure there is good drainage below and cap with a soil type that will shed water away at the top.

You should be fine.

Bud