Winnipeg, cold? A bit. But every time I've visited, locals assured me "it's a dry cold you hardly notice--until your nose falls off."
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I'd use foam board as it serves as an air barrier and can likely be fitted tighter than rockwool, which would require an air barrier to be effective.

However, I'd consider XPS rather than EPS, as it has a higher R Value per inch. The National Building Code, which serves as Manitoba's building code, requires a minimum whole wall R Value of R23.5 for abovegrade walls. Some of the diy type cannister foams could be used instead. I can't vouch for their ease of use.

Here is a link that might be useful: Martin Holladay on Insulating Rim Joists

This post was edited by worthy on Sat, Sep 20, 14 at 7:24

I think you may find getting a perfect seal with rigid foam a bit more difficult than you might think. If warm air can migrate around it, it will condense and then you have a sealed area that will not allow it to dry easily. I could be concerned about rot in this circumstance.

If you're not going to spray foam the whole thing, personally I'd use roxul. But not sure why you're removing the fiberglass in the first place. If you need to air seal - it's the joist itself that needs sealed.