This resource 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. There were some issues with the application, granted, but this serves as a reminder that there is no silver bullet in detailing your home envelope. Products, methods of installation and pre-existing conditions can all play a role in the success of your solution.
In my mind, the more simple and more redundant the detail the better. We have been workshopping for a while, but have come up with the following detail. After we run all our mechanicals and seal them with sealants or tape we spray a 2″ spray foam coat for redundancy.
Thanks to having minimal thermal bridging, this detail will give you an effective R-18 (~r-30 by the time the spray foam layer is in) 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.