Rim Joist Information Overload – GreenBuildingAdvisor

Hello from Climate Zone 5a, 

I’m trying to decipher all of the information on insulating rim joists I’ve taken in from this site and Fine Homebuilding. We just bought a home built in 1900, basement + 2 stories, concrete block foundation – bottom 3/4s of the wall is a “Michigan  Wall”, the entire wall is painted with, what I think is, latex paint. No bubbles or peeling yet. 

These are facts I have straight. 

– Rim joists need insulating to a minimum of the R value of the walls above. Understood.

– You want to make sure they are air sealed as well. I get this.  

– While you want them air sealed, you do not want vapor impermeable because that would prevent drying when it inevitably gets wet. Don’t fight nature, got it. 

This article from BSC states crawlspaces are mini-basements. So I can treat this as a guide for my basement – a tall crawlspace, right? Sorry, cant post links evidently – it’s “Info-512: Crawlspace Insulation” from 2015. 

Here’s my confusion: 

1. The article states that you want to essentially cover all of the cold basement surfaces (rim joists, concrete structural elements, etc). It goes on to say that you want to use a foam product but you want the vapor permeability to be able to dry inwards (“The best insulations to use are foam based…). I thought in a climate that has 4x as many Heating Degree Days as Cooling Degree Days, you want to plan for vapor flow to be outwards due to heat moving to cold.

2. We insulate the rim joists to prevent the relatively warm air in the basement from making contact with the cold rim joists and causing condensation because of the dew point. Won’t condensation still happen on the warm side of a foam product? Is this better than on the wood simply because foam doesn’t rot and is cheaper to remediate?

3. Won’t spray foam move and ‘crack’ via warm and cold cycles? With its vapor permeability won’t moisture end up on the rim joists anyway when it passes through the foam and to the outside condensing on the rim joists?

I’m having a hard time bringing all of this together into a solid action plan because I don’t want to re-do all of it after I find out I misinterpreted something. 

Any help is welcome! Thanks for reading