It sounds like a good plan that I have used several times on homes if owned over the last 30 years. You will get some criticism from code reference citers and hair splitters that deal on .001's on perm ratings. It is just a small space, so do not worry about much because what you propose is estimated at a 98.7768365% improvement for a small area relatively.
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Cj, if you air sealed the rim joist (with a sealant) from the mud-sill plate and deck sheeting above before adding the foam board, you will be fine. The rim will expand/contract over the seasons allowing infiltrating air, sometimes moist; http://www.buildingscience.com/docum...d-but-strange/