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Wrapped Rimjoist Q’s w/ Exterior Polyiso Insulation

jaccen | Posted in Green Building Techniques on

Hello all,

I was wondering if you could help me.  Thanks in advance for any insight.

I have some framers who are a little set in their ways.  What with the shortage of skilled trades here in SW Ontario, I’m pretty happy to have them.  My first framer “ghosted” me and the 2nd said that he was making furniture this winter as it’s cold.  They’re used to building to R2000 standards.  I try to not let “perfect be the enemy of good” when it comes to my builds.  The plan was to do the following:
-place acoustic caulk on the top of the foundation
-place sill sealer on top of that
-place Tyvek rimjoist wrap on top of that
-bolt the sill
-place the rimjoist and joists on the sill
-wrap the rimjoist w/ the Tyvek and bring it inside the house (Membrain to be used in the interior to satisfy local building inspector; Membrain will be acoustic sealed/taped to the interior vapor barrier)
-frame the walls
-install the plywood sheathing over top of the Tyvek on the rimjoist and the walls.

This house has 5″ of exterior foam insulation (polyiso), outie windows, and the water barrier will be Tvek on the outside of the foam.  Taped plywood is the primary air barrier for the house.  Tyvek the primary water barrier and taped polyiso the “suspenders” in regards to water and air barriers.

As construction supplies seem to becoming scarce in my area, the crews picked up “standard” interior poly vapor barrier instead of the more permeable Tyvek rimjoist wrap.  I was away from the site and didn’t get back until it was all installed under the sill.

As I see it, I have 3 options:
1.  Just keep it.  We’re using plywood sheathing and if there is some moisture trapped in plywood between the polyiso and the vapor barrier, my gut tells me it will migrate up and into the house.  The house will have a small dehumidifier (already planned for).
2.  Cut it and acoustically seal the Tyvek to the vapor barrier.  This has a high chance of what I’ve heard called “fish mouths” where there are gaps between the plastic, especially when you start wrapping it.  This defeats the purpose of wrapping the rimjoist for an air barrier.
3.  Raise the insulation 1/16″ to 1/4″ off the foundation to allow a drain path.  I was going to just rest it on the concrete so that there’s less chance of settling.

Removing the sill and redoing it w/ Tyvek is not an option as the beams and joists have already begun.  I could likely get away w/ no barrier at all wrapping the joist, but the crew is used to it and I like the “belt” and “suspenders” approach to things.

Thoughts on the options?  My main concern is trapping moisture in the plywood (“soggy diaper”) as it can only really diffuse up and into the house (ie. a 12″ vapor path instead of 1.5″ through the rimjoist).  Or am I just overthinking this?

Thanks in advance.

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