Hi all,

 

I’m requesting feedback on my plan to insulate & add air sealing to a shared wall between a 1st floor attic side wall and an adjoining 2nd floor wall on our split level home in Ottawa, Ontario (built 1968, 2×4 framing, lower and upper vented attic).

 

For context a few pictures attached below.

 

A racoon moved in the week before we did and tore down a fair bit of insulation as it found a way to move between the 2 attics (insert story about half the shared wall having a 24″ jog in it and a void above roof sheathing to allow for kitchen cabinets below). 

 

As a result, I replaced the old kraft faced fiberglass insulation from the ‘back side’ with Rockwool ComfortBatt (R14) leaving the kraft paper in place as it and old paint are the only vapour movement mitigations in place.  

 

What makes this tricky is that the top/bottom chords and webs of the roof trusses sit mostly flush with the back side of the framing.  Initially I had planned to add plywood sheathing, caulking/sealant & taping this, etc, but upon reflection this probably doesn’t add much value (or am I wrong here). 

 

Applying the KISS principal as best I can here’s how I propose to add insulation and air seal this shared side wall.  Suggestions, questions, critiques most welcome.

 

1) Cut/cobble ComfortBoard 80 (1.5″ – R6) around the diagonal truss members bringing everything roughly into the same plane (insulation face & truss chords/webs). 

2) Skip adding any kind of plywood or OSB sheathing (although it could be done at this point with fewer cuts if this would make a significant improvement to the air barrier).

3) Add a mechanically fastened WRB (Tyvek Homewrap is cheap, Tyvek commercial wrap not so much, but could splurge on Siga Majvest or equivalent)

4) Seal the WRB to the 2×6 side faces of the bottom and top chords with bead of sealant (type??) & tape (say 3M 3087 or Tyvek or Wigluv tapes).  

5) Add another layer of Comfortboard 80 (1.5″), then retain in place with a 3/4″ x 4″ plywood straps screwed through to the framing which I’d landmark ahead of time.

Questions:
-Does this seem reasonable given the constraints? could it be improved upon?
-Can I get away with skipping sheathing and placing the WRB (as an air sealing layer) between the two layers of Comfortboard 80 so that the WRB is in plane with the trusses to make it easier to seal.
-What sealant or type should I use between the 2x framing and the WRB to augment the tape and cap nails?
-Does it matter too much which WRB I select for this retrofit, I’m guessing not. Total area is ~150sq ft.

I plan to add lots more attic floor insulation once I get this and other air sealing projects done.

 

Thanks,

Owen