Replies

1. Expert Member
   Zephyr7|Feb 01, 2020 07:38pm|#1

   With 3” of EPS on the exterior, you already have pretty decent insulation over your rim joist. With air sealing done, you’ve done both things already, so you’re only looking for MORE insulation now :-)

   You shouldn’t have to worry about condensation issues due to the exterior insulation, so you could stuff in batts (mineral wool will be easiest since it will stay in place on its own), cut and cobble more EPS and hold it in place with canned foam, or apply how ever much open cell spray foam you need to reach your final R value target.

   Bill

2. GBA Editor
   Brian Pontolilo|Feb 03, 2020 11:10am|#2

   Hey Red Falcon.

   I agree with Bill's comment above. Try to get the rim area at least up to the same R-value of the walls above. In case you haven't seem this, check out:Insulating Rim Joists

3. Expert Member
   Dana Dorsett|Feb 03, 2020 02:42pm|#3

   The limits vary by location- the climate in Berlin (cool edge of zone 6 A) doesn't much resemble the climate in Portsmouth (cool edge of 5A.)

   Interpolating from the IRC prescriptives in R702.7.1 Chapter 7 , with ~R12 on the exterior in Berlin you'd be good for up to R21 of fiber insulation on the inside of the band joist (up to R30 in Portsmouth), as long as there is a class-III vapor retarder on the interior side of the assembly :

   https://up.codes/viewer/washington/irc-2015/chapter/7/wall-covering#R702.7.1

   Without an interior side vapor retarder dial it back to ~R20 max in Portsmouth, ~R15 in Berlin. Carefully fitted batts as long as they're high density, and trimmed snugly enough to limit convection around the batt.

4. 
   RedFalcon6|Feb 03, 2020 06:32pm|#4

   Thanks for responses! To clarify, I am in zone 5A. Basement is a walkout and will be fully finished, sheetrocked, and also with bathrooms. Have an ERV / dehumidifier (minotair).

   I guess my question is:
   1) is it easier just to spray foam this thing? Any issues with ensuring it can dry (must dry to outside?

   2) Or since I have my foam on the outside, am I fine just putting bats inside without going nutso on vapor barrier. From my understanding the vapor barrier "isn't that important" if sheetrocking and finishing everything off. (Also a good air circulation system in place too.)

   I'm inclined to just do #1 as I can spray and be done, but don't want to create a drying problem where there isn't one if I can avoid it.

   Thanks guys

1. 
   Fred Frasch|Feb 08, 2020 11:34am|#5

   Hi Red, Off topic, but I'm interested in your experience with the Minotair. Mind sharing?