This episode was born out of a recent job one of our contractors did recently. They had to replace some insulation that was improperly installed around joist ends. This job wasn’t a proper vapour barrier, and rather than preventing moisture from making it to the joist ends, it wound up trapping and almost sucking the moisture towards the joist ends.
“Aha! Should’ve used spray foam” You say. Well that was my first response too. But it turns out they had used spray foam. Unfortunately it wasn’t dense enough spray foam and was improperly applied by an uncertified sprayer. Had they had it properly applied they likely wouldn’t have had an issue. But it raises a question about joist end insulation/vapour barrier details. They’re difficult to do! I’ve seen so many in new builds that are just vapour barrier loosely stapled to joists with fibreglass batts stuffed behind that. You can bet that’s going to leak!
What we’ve begun doing is a snazzy little detail that involves a minor amount of extra framer attention, but then saves a lot of worry. See the detail below.
Thanks to having minimal thermal bridging, this detail will give you an effective R-18 which is 10-15% better than the code requires. Best of all you’re completely assured of a lasting, non-cracking vapour barrier that can be visually inspected to confirm it is working as required.
Little details like this one go a long way in assuring a well performing, eco-friendly home! Come talk to us when you’re expecting to build your new home and we’ll help you design and build a better home.