Two years ago we moved into a large, 100-year old brick house with a 2-foot thick stone and mortar foundation. The basement is damp in summer. The dehumidifier runs constantly and water wicks up the wooden posts that rest on the concrete floor. We also have significant amounts of cold air that make their way through the foundation in winter. Here on the Canadian prairies, temps are regularly below 0 degrees F. Unfortunately the basement was finished shortly before we bought the place so that makes interior insulation difficult. I'm now wondering about getting drain tile (French tile) installed on the outside and possibly applying spray foam insulation to the exterior of the foundation while it is exposed. An incentive program will likely cover the cost of the insulation. What are the pros and cons of using spray foam insulation on the exterior of a stone foundation? Thanks.

Downsides: 1. Can't see the pretty rocks. 2. Carpenter ants can tunnel & nest in it unless you cover it well with a cement based coating which you can get at a masonry supplies. It's not difficult to do.
Upsides: you will not believe the difference in comfort in your house.

You'll be excavating anyway so this is the perfect time to do this and the right thing to do.
Slope the trench but dig it as deep as possible.

You should also take care of the basement floor; finished or not. Take it up at least around the edges dig a deep trench, fill with stone & drain it off. Install EPS foam & Poly, taped & 2" concrete. Will go a long way in lowering humidity.

Thanks so much for the advice Bob. I don't entirely follow what you are suggesting we do with the basement floor. Are you suggesting interior drain tile and additional insulation and concrete on the floor?

That's the way it's done- insulate the floor from the much colder sub-soil temperatures- it makes a difference.

For the cool Canadian midwest it's reasonable from a long-term energy savings point of view to put 3-4" of EPS under the non-structural 2" slab and up to 4" of closed cell spray polyurethane foam on the foundation wall. In addition to providing powerful moisture barriers from ground water and vapor diffusion, insulating both the walls & the 2"rat-slab will raise the average temp of the basement, which lowers the moisture content of the wood structures (and any other moisture susceptible materials) stored in the basement. Hopefully you have the headroom for that much slab insulation (?), but even 1" EPS slab foundation is enough to make a substantial difference.

High relative humidity in the basement in summer is largely a function of outdoor air with a dew point infiltrating into a cool/cold basement, plus whatever ground moisture migration might be occurring. In most cases (barring very high water table) the air infiltration is the dominant factor. When you insulate the basement slab instead of a ~12C slab (at or below the outdoor dew point), you would then be looking at a ~18C slab.   Insulating on the exterior of the foundation wall isolates the wall a bit from the cooler sub-soil temps, but there will still be a thermal bridge at the bottom of the foundation. But the total area of un-insulated ground contact will have been reduced by an order of magnitude or more- the basement temps will track the first-floor temps much more closely.  The cc foam should effectively air-seal the crackage in the stone foundation, probably reducing the uncontrolled air leakage more than an order of magnitude.

thanks Dana for the detailed recommendation. Even if you get the exterior drains below floor level, there is plenty of moisture around the house which needs to be stopped from getting inside. The spray foam will work somewhat on the exterior walls and will at least help direct drainage water away, but it's often done on the interior. So you should make sure all wall is insulated one place or the other. Definitely put thick plastic (6-10 mil) on the floor over the foam; overlap and tape the joints, and run it up the walls a ways and spray foam it onto the wall.