Smart vapor retarder: Worth it? Do I even need a retarder in my case? Which one?
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.
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It sounds like your wall assembly will have good drying in both directions, which should make it more forgiving. I would still use a smart vapor retarder as cheap insurance though. I use Certainteed's MemBrain myself, and I've never really found it to be fragile, even where stapled. Intello plus is more durable, but it's also a LOT more money. I think you'd find MemBrain would work fine if you're looking to save some money. If you have any doubts, buy one roll and try it. If you don't like it, use something else for the rest of the project.
Bill
Thanks, Bill. Odd question: Does MemBrain have any odor to it?
I've never noticed any smell. MemBrain is basically a nylon film, not much else to it. I've never noticed any smell, or any of the slight slimyness you sometimes get on crummy quality plastic products. I've never had any problems using the stuff.
The polyurethane sealant I like to use to seal the edges to the framing stinks though. If you're worried about smells, that's where you'll get smells. After a few days it goes away though -- I've never had any issues with long term stinkiness from polyurethane sealants.
Bill
Thanks. My next question is: Do I really need a retarder in my case...considering my locale and particular construction method?
Or is it something of an option....like cheap insurance that won't hurt or create another problem?
Or, would it be considered a "must"?
You don't have any exterior insulation to keep your sheathing above dew point. So interior generated moisture will migrate quickly through your permeable wall assembly, find the cold interior surface of the sheathing and condense into liquid water. Yes, that water can dry to the outside in your case, but you still have a condensation issue. As long as it always can dry quickly you shouldn't have mold growth or rot, but if something happens in the future that inhibits drying you will wish you installed the membrane. You're in a dry climate so the humidity is much more likely to come from inside the house than outside the house (bad rainstorms will cause a temporary reversal of this).
Dean,
The first answer to your question is in whatever code you are building under.
Practically, double-stud walls need an interior vapour-retarder. The typical inside finish of drywall and two coats of paint are about 5 perms. Your drywall with plaster would be around three times as permeable. MemBrain is probably the easiest option.
Thanks, everyone. I'll do the MemBrain.