Turns out everything I have done in the past has been wrong. I’ve tried to be a good “energy saver” using expensive Poly-Iso foam sheets on both sides. Its all bad. Walls are either rotting on one side of the interior of wall or the other (or both!)

Actually I don’t know unless I tear it apart to look. If Joe L. can’t get it right who can!?

Latest problem is small 1950’s shack with “exterior Sheetrock” panels instead of real sheathing. Std. 1/2″ Drywall interior, 2×4 frame, Blown -In Cellulose (1970’s? add on) 24″ exterior 1/2″ drywall horizontily between strips of 8″ wide  “3/4 thick shiplap” Finally Vertical Fir T&G 3/4″ x 5 siding.  2×4 Frame Is Diagonal Braced with 3/4″ let-in braces.

I’d like to loose the entire exterior and the cellulose since the exterior drywall is ice cold and gets water droplets on the interior face of it that then soaks the cellulose (not good).

Think this would work?:  Replace interior drywall where needed (or all). Unfaced R11 F’Glass Batts, 1″ Poly-Iso foil foam sheets (on exterior) with carefully taped seems, NEW product (Tyvek “DrainWrap”) and finally 4×9 sheets of LP “Smart ply” for siding nailed or screwed carefully thru the 1″ foam to the studs. (Less work then re-using the Fir Board siding AND better shear strength). IF the vertical siding boards were cedar I’d be more inclined to re-use but the fir not so much due to paint peeling.

I thought about using 1/2″ Poly-Iso also on the interior under the drywall but am not sure if the Building Science website is saying “NO” or if it is saying that might be OK in Seattle (Marine Zone 4 Climate).

So confusing – maybe better to just re-use the fir boards with no wrap and let the wind blow thru the gaps to dry it all out!?

What would you do? (Besides not use LP Smart Side! – Its cheap & strong & fast) Thanks.