I'm trying to figure out how to build the wall of a home addition. I am in Houston, hot and humid, climate zone 2. 2x6 framed wall with open cell spray foam insulation. OSB sheathing on the outside, Tyvek and Hardie siding. I was thinking of using seam tape on the OSB, to get it as air tight as possible, then creating a second air tight barrier by caulking top and bottom of the Tyvek to the sheathing and taping all the joints. Tyvek website says not to cover the sheathing seams though. What am I missing? Why not? Is it wrong to create 2 (hopefully) air tight barriers right on top of each other?

Actually that'd be a great way to capture moisture against the OSB sheathing.

Maybe look at Zip or Zip+R sheathing using the "Zip 2.0" system (using liquid flash over seams and nail holes instead of tape).

Also look at either using liquid flash on the sheathing-foundation interface or the taping detail shown here...

https://youtu.be/KiZ1COo27NQ?t=129

Essentially Tyvek is an imperfect barrier system as you're creating penetrations every place you have to staple it.
So what'll happen, with condensation, you'll wind up with rot around such staple penetrations unless PERFECTLY detailed.

Granted, another option would be to do a full-field peel-&-stick. And again, you definitely want to consider exterior insulation for thermally breaking the wall.
Otherwise you wind up with warm/cold spots coming through the wall.


One of the reasons for the Zip+R is the thermal break it provides.
Spray foam can be an air sealant, but it has to be detailed PERFECTLY. Otherwise, as it dries and sets, spray foam can shrink and pull away from stud bays. Hence using a system like Zip to handle your air sealing detail.

Additionally, you could then get away with using a flash-&-batt setup in the stud bays.
2" of foam. Pack the remainder with Rockwool/Fiberglass.