Best practices for maintaining insulation vapor barrier on ERV intake through wall
What are the best practices for maintaining the continuity of the insulation vapor barrier on an ERV intake duct where it passes through a wall to the exterior? The duct is insulated with the typical fiberglass with metallized vapor barrier wrap. If the rim joist area is spray foamed, I’m unclear how to achieve a tight seal between the exterior vapor barrier of the duct insulation and the surface of the spray foam (OR the surface of the duct just prior to the rim area).
I really want the vapor barrier here to be as robust as possible to minimize any chances of condensation (the duct is above the suspended ceiling of a finished basement).
Suggestions are greatly appreciated!
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My approach to this is as follows, assuming you are going through the spray foamed rim joist with a 6" duct. This is a detail that is often overlooked and in cold climates can be quite problematic.
1. Install your 6" duct through the wall to the exterior hood (an 8" hole, if structurally possible, leaves some room for foam), and leave 4-6" of the 6" duct protruding inside. Seal it in place with spray foam.
2. Install an 8" (or 10", depending on duct insulation thickness) collar over the interior duct stub from step 1 and spray foam it in place, sealing the 8" flange to the rim joist spray foam, and insulating the interior gap between the 6" duct and 8" collar. This plastic collar is what I used on my setup. Just remove the damper: https://www.homedepot.ca/product/speedi-collar-8-inch-hvac-connection-collar-with-damper/1000738241
3. Attach your ERV ducting to the 6" duct stub. Pull the insulation over and tape to the 8" collar.
Thank you, Dennis, that's a great idea! Today I stumbled across this diagram in a Broan ERV installation manual which shows what appears to be a manufactured version of what you suggest - but unfortunately, Broan (Nutone) doesn't have any idea where one might buy such an item. I was thinking of using an 8" sheet metal cap, cutting a 6" diameter hole in it to slide over the duct from outside, roughly replicating the collar shown in the diagram; your idea seems like an easier approach.
I can't believe someone doesn't manufacture a collar like the one shown in the diagram.
Glad to help. That Speedicollar I linked to is the lowest cost flange/damper setup I could find. I used them also in my external wildfire smoke filter setup.
I think most installers just tape to the duct and call it done. At -35 C however, there is a fair bit of frost/condensation in this area which can accumulate, soaking into the insulation sleeve.