Whats the number of times the letter a appears in:
I was going to put Tyvek on the sheathing around my house but the local place has Typar and says it is much better.
A quick looks shows the Tyvek has far higher vapor permeability.
My walls have 3.5” of closed cell foam (I know, I know) and therefore are a good vapor barrier.
my thinking is that the membrane on the outside of the sheathing should have a high perm so that the sheathing can breathe or dry to the exterior.
My supplier says both products breath outward the same.
Is my analysis on target?
What to do with bottom flap of Typar + air seal edge of sheathing?
Foam over wall sheathing: Typar under foam?
BSC is a well respected source of information on the topic.
Every product needs to be considered in context of the design and other products being used. I’d suspect in most cases, Typar is a fine product to use.
Both are good products and will work, I wouldnt worry about using either one. If you can get access to tyvek commercial roll (costs more than regular stuff), but it is way stronger and can be left exposed for much longer before siding.
The only WRB you want to avoid is the woven stuff that looks like a tarp, those are best left for covering lumber at the lumber yard.
So I don’t have to worry about sandwiching the sheathing between two vapor impermeable materials?
What are the two vapor impermeable materials? Typar, at 11.7 perms is not anywhere near impermeable.
Ah! You found my mistake.
But you saying that the typar over the sheathing won’t prevent drying of the sheathing to the outside?
All the WRBs that have been mentioned in this discussion - Typar, Tyvek, Flat-Wrap, and Commercial Tyvek allow adequate drying to the outside.
Typar is not a vapor BARRIER, its a vapor RETARDER, which means it slows, but does not prevent, moisture migration -- drying, in this case. Latex paint, as an example, is often mentioned as allowing drying, but is only about 5-9 perms -- less than the Typars 11.7 perms. 11.7 perms is more open than even a class III vapor retarder (up to 10 perms).
A true vapor BARRIER like polyethylene sheet is typically under 0.1 perms.
The short answer is Typar allows drying, its not a vapor barrier.
That is great to hear and to learn.

user
The total number of "a"s in that passage is 151.
assistant