Rim joists without exterior insulation, condensation and mold – GreenBuildingAdvisor

It is possible to retrofit a capillary break onto an older structure, but it’s a big(ish) project. Sometimes you’re OK without it though, if you either have a foundation that tends not to wick water (stone), or have no issues with moisture in/on your foundation walls. Either way, you NEED to use an insulating material on the inside that is an air barrier, which means some type of rigid foam or spray foam. I like to use EPS since it’s a little bit vapor open, which results in a little bit of interior drying ability. Also, if the rim joist is open on the exterior in some way (and I mean open for ventillation, so covered only by something that doesn’t inhibit drying, such as typical paint, vinyl siding, etc.), then you are much less likely to have problems compared to if you had some type of vapor barrier on the exterior of the rim joist.

Bill