Smart vapor retarder: Worth it? Do I even need a retarder in my case? Which one? – GreenBuildingAdvisor

I had asked about this before and it seemed the consensus in my case was a smart vapor retarder. As I’ve moved along with my build, a few things have changed.

My roof is compact R50, R26 walls, R20 under entire slab. The ceiling goes The house will be passively solar heated with a wood stove for supplemental heat. I’m in S Idaho…zone 5, dry. No AC.

My wall assembly is as follows?

Two 2×4 walls together, with staggered studs. 24″ o/c. They are filled with hemp batt insulation for a total of ~R26. The outside is sheathed with 1/2″  plywood (taped w/ SIGA Wigluv) and covered with building felt. Most of these walls will have wood clapboard siding…over a 3/8″ rain screen. Small sections of these walls has 3/4″ clay plaster applied over the felt with fiberglass lath.

Here’s my change: Inside, I was going to do what many natural builders do in Europe: Reed fencing stapled to the studs followed by clay plaster over. Instead…I’m going to do standard 1/2″ sheetrock with a 1/4″ clay plaster finish over that. No paint of any kind. Clay is super permeable. 

From what I have read, CertainTeeds MemBrain sounds too fragile for my tastes. DP+ supposedly offgasses fairly badly. As far as I know, that leaves Siga Marjex and Intello Plus. The Marjex is a bit less money. Though both are costly, I haven’t a lot of wall to cover since most of my walls are strawbale …and the house is small to begin with (1000 sq ft). Nothing goes on the straw except clay plaster inside and out.

I’m thinking a smart vapor retarder would be a good idea on those stud walls but want to confirm. Yes? No? Maybe? Also, do they really work…and how do we know besides taking the manufacturers word for it? Preference to which one between Intello Plus and Siga Marjex? I’m leaning towards Marjex…from what little I have read and the slight cost savings.