How critical is my interior air barrier?
So, my father in the Bangor, ME area is building a new home. I know what I would suggest, but he’s only willing to do an interior vapor barrier (Siga Majvest, as I can get this for good prices) and/or rain screen behind the siding if it’s absolutely critical.
His wall assembly is lp lap siding, zip r-6, 2×6 wall with r-23 rockwool and then drywall. I am trying to tell him he needs the rain screen on the exterior, especially if he doesn’t use the siga on the interior. I don’t love his exterior insulation to interior insulation ratio, so I’m thinking the air barrier and rain screen are critical. Could he get away with one and not the other or neither without having a majorly risky wall? Am I overthinking/over building it?
He’s old school and definitely budget conscious. My builds in PA always include the interior air barrier and a rain screen behind the siding, but hes pretty resistant.
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No rain screen is just bad building practice.
The interior membrane you can forgo, if you must.
The insulation is fine.
That wall assembly is only code-compliant in climate zone 6 with a class 1 or class 2 interior vapor retarder. He may not care about codes but everyone in the state is supposed to follow them and although they get better every year, they still represent the worst house you can legally build.
I would prefer to see a variable permeance membrane but a vapor-retarding paint would be better than nothing. Of the two options, I would prioritize the rain screen, so the wall and the cladding can dry. Maybe you could convince him to attend a local BS+Beer event? https://www.instagram.com/bs_and_beer_orono_me/.
Note that it's the drywall that is generally the interior air barrier, not the vapor retarder/barrier, although I personally like to detail both as air barriers for some extra insurance.
I would say that with the light exterior:interior insulation ratio, the interior side vapor retarder is more important. Better would be to use more exterior insulation though, and even then the interior vapor retarder is extra insurance.
Bill
From looking at the way LP siding laps it does look like it has some inherent air gaps behind it.
I’m pushing him to install slicker max (easy detailing for him to have the carpenters do), but I also think the Siga Majrex is going to be more bang for his buck if he’s going to start pinching pennies. I would go install the Siga for him.
If you really want to keep costs down, CertainTeed MemBrain is probably the cheapest smart vapor retarder out there. While less durable and not quite as high performance as the Siga product, it is a good bit cheaper.
Bill