Insulation in Shallow Crawl Space Zone 6
Dear GBA Team,
We are building a new house on an existing foundation in Zone 6. We inherited a foundation & crawl space that is super shallow at 14″ – we suspect because the ground is ledge and they chipped away as much as they could. That said, we are framing a floor which will bring the crawl to around 18″. Planning a rat slab with 2″ rigid foam underneath, R 15 inside crawl walls, and R30 rockwool batt between floor joists of 1st floor deck. (We will also loop the crawl into our ERV ducting) Contractor is asking to fill cavity and encapsulate the joists with spray foam instead of the rockwool batt. I am against this but wanted your feedback. Thanks!
Brad
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Replies
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
Thanks, Malcolm, great article. Very helpful.
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.
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
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
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?
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.
Thanks, Malcolm I appreciate your input on this.
Brad,
Good luck with your build!