Replies

1. 
   Malcolm_Taylor | Apr 22, 2024 02:32pm | #1

   Brad,

   The problem with using permeable insulation on the rim-joists is that warm interior air can move though it and condense on the wood. The usual recommendation is to use spray foam, or better still foam board to avoid that. This article explains it in more detail:
   https://www.greenbuildingadvisor.com/article/insulating-rim-joists

2. 
   user-6445625 | Apr 22, 2024 03:36pm | #2

   Thanks, Malcolm, great article. Very helpful.

1. 
   Malcolm_Taylor | Apr 22, 2024 03:42pm | #3

   Brad,

   Installing and sealing foam board on the rim-joist in crawlspaces (especially short ones) is unpleasant and difficult work. What I've taken to doing is installing the board and sealing the bottom and sides before the sub-floor goes on. The all that is left to do is seal the top of each bay.

3. 
   user-6445625 | Apr 22, 2024 04:00pm | #4

   Thanks again, good point. One of my questions is about the spray foam the GC wants to use to encapsulate the first floor deck TJI's. I would think that might not be ideal and I was planning to just use rockwool batt in between the joists where the joists would be exposed at bottom. Any thoughts?
   Brad

1. 
   Malcolm_Taylor | Apr 22, 2024 04:42pm | #5

   Brad,

   Ah sorry - I thought you were just referring to the rim-joists, not spray foaming the floor above. If you are encapsulating, insulating and and conditioning the crawlspace there is no benefit to using any type of insulation on the TJIs.
   https://www.greenbuildingadvisor.com/article/building-an-unvented-crawl-space

4. 
   user-6445625 | Apr 22, 2024 04:55pm | #6

   Thanks again. One last clarification: we will only be able to accommodate 2" of rigid under the rat slab. I think the GC was considering the rockwool batt in the crawl ceiling to add additional R value between the crawl and living spaces. Any thoughts?

1. 
   Malcolm_Taylor | Apr 23, 2024 10:45am | #7

   Brad,

   Crawlspaces are best kept either completely inside or outside the house's conditioned area. otherwise it is hard to keep that intermediate space conditioned property to avoid moisture problems.

   However if you can't get sufficient insulation on the ground and have to add some to the floor above that's better than leaving the space with too little insulation. You may have to add some dehumidification to keep it dry.

   I don't see any advantage to that insulation being spray foam.

5. 
   user-6445625 | Apr 23, 2024 11:14am | #8

   Thanks, Malcolm I appreciate your input on this.

1. 
   Malcolm_Taylor | Apr 23, 2024 01:48pm | #9

   Brad,

   Good luck with your build!