Hello everyone, I have an old house built in 1875, which has a rock and mortar foundation about 2.5 feet thick. Since I plan to dig it up to have it sealed properly and add a platon membrane, I also plan to have it insulated on the outside at the same time. I'll be using 2" of rigid pink polystyrene. My view is that insulating from the OUTSIDE with a thick foundation like this has a list of benefits, since it will allow the heat of the house to soak into the thick foundation, as opposed to insulating on the inside, which would block the heat from the house from soaking in; 1. It will keep the floors over the thick foundation from getting cold 2. It will protect the foundation from thermal cycles and thus extend its life 3. Since the foundation itself will stay warm, there is less likelihood of condensation Here's where it gets tough. 1. To cover the polystyrene above grade, I'll have a mason cover it with a steel mesh first, then bond a stone veneer 0.5" thick. The stones will extend a little below grade. Is this strong enough? With nothing to support the stones underneath except the backfill, will there be too much of a shear force on the mesh/foam? 2. Will the foam be OK below grade, or should I protect it? Note: I live in Ottawa, Canada where the winters can be below freezing for 5.5 months of the year.

I would worry about frost getting under the stone and heaving it.

Foam is stronger than most people think and of course there are different densities if you need more. Regarding frost heave near the veneer - you can install horizontal foam wings to create a frost proof shallow foundation (ie, no freezing below the veneer edge). Cover with some non frost susceptible fill material.

Insulating the outside but not underneath a foundation/floor will mean that you have a thermal bridge to the earth. Not nearly as bad as a thermal bridge to the exterior air, but not ideal.

Frost heaves propogate in the direction of heat migration. For the foundation to frost heave would require at least 2 things:

1: There has to be significant moisture in the soil under the foundation, or between stones.

2: The temperature of the soil or stones has to be below freezing, so the direction of heat loss is from the deep subsoil up.

In a heated house when the soil next to the foundation is below freezing, the direction of heat is always from the house out to the soil, not conversely. By insulating from the exterior the foundation is also drier than in full contact with the soil, but even if insulated on the interior the frost-heave potential does not increase.

Fieldstone & rubble foundations inherently drain better than poured concrete in the first place, and by putting drainage & insulation on the exterior it's going to be even drier. If it hasn't frost heaved in it's current condition after 140 years, it'll be even less likely to frost heave after the upgrades.

Frost heaving foundation in unheated barns are common if the foundations are well drained, but only rarely an issue with houses, unless they insulate heavily between the first floor and basement, the drainage at the foundation is poor, and the basement experiences cycling temps well below freezing for sustained periods.

By insulating the foundation from the exterior the foundation-sill (usually a heavy beam in houses of that vintage) stays warmer, and thus drier, which gives the building even better moisture resilience. It's farm more common to insulate from the interior though, simply because it's easier.

FWIW: Using EPS rather than XPS is both cheaper and much greener at any R-value due to the envrionmentally damaging HFC blowing agents used for XPS.

Wow, thanks guys. That's awesome.

Does anyone else worry about excavating around a foundation that old? I guess replacing a foundation like this isn't cheap so this may be the only option. I've heard of closed cell spray directly against the interior but I don't think the owner wants to do that.

don't excavate straight down next to the building. The foundation stones are typically in a pyramid shape with a wide base; do not disturb any of the foundation stones or base, but you can dig out the dirt that's on top of the sloping foundation 3-4' down & install washed stone & drainage pipe . As long as water can drain off the wall, your drainage ditch & pipe can be away from the wall. The best job I've seen was where they exposed the foundation stones, coated them with closed cell spray foam, and covered that with Thoroseal plaster mix to protect the foam. In that case they left the granite capstones exposed and insulated them from the interior.

The proper way to insulate an existing rock or rubble foundation in cold climates is from the inside with 2# foam.

Hi LT56,
We did what your thinking 2 years ago to an 1850's banked barn in Prince Edward County ON, but used 5" EPS foam somewhere about R20.
To support a stone rim like you describe we lag bolted L channel to the wall to aid in supporting the stone.

PM me if you'd like some pictures or more info