[Greenbuilding] wall construction

[Topher Belknap]

Greetings,

I would be concerned about the permeability of OSB on the exterior.

I propose something more like:

Permeable membrane (water barrier)
5.5" Roxul
1/2" Air sealed, vapor barrier sheet board (e.g. Zip)
3.5" Roxul
1/2" Drywall

R-33  1" thicker, but plenty of space for electric boxes, etc.
No foam, so probably much cheaper.  Better drying characteristics. 
Better insulation (BTW, I get R-24 for your assembly, apples to apples. 
less if that 6" is actually 5.5)  And you aren't trying to tape up 1/2" 
foam board.

Or better yet:

Permeable membrane (water barrier)
9.5" Roxul
1/2" Air sealed, vapor barrier sheet board (e.g. Zip)
3.5" Roxul
1/2" Drywall

R-47  Getting into Passivhaus territory.

I haven't run WUFI simulations on any of these, but I am concerned about 
your cavity with OSB on one side and foam on the other.

Thank you kindly,

Topher

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.GreenFret.com/
(207) 882-7652

On 2020-02-14 2:25 PM, Clarke Olsen wrote:
Topher,
   Upstate NY, cold winters. If I used the 2x4s as another wall, I would 
fill it with more
   Roxul. I was tring to hold down the thickness, figuring thar 1.5” of 
dead air is as good
   as 3.5”, and running 2x4s horizontally as furring strips. Running the 
drywall vertically
   means no butt joints. Over windows and doors (3’+/-), 2/2” plywood 
can be substituted
   for the 1/2” foam to eliminate the header.

   1/2” OSB                         .5
   6” roxul                         21
   1/2” foil faced foam        4
   1.5” air                            1.
   1/2” drywall                       .5
    total                          R 27

    Clarke

clarkeolsendesign.com