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Anonymous /diy/2921972#2924049
6/14/2025, 2:25:30 PM
>>2924014
>>2924036

In English it's typically called "air barrier" "weather resistive barrier".

It is vapor permeable because the assumption is that water will get in somewhere and will need to dry out. As well, if you live in a cold climate the required interior side "vapor barrier" won't be perfectly sealed so moisture that leaks through the vapor barrier (blue membrane in >>2924036 pic) must be able to evaporate.

It's the "good enough" approach to building, but buildings intended to last a long time and have low operating cost tend to use pic related approach, as in below link.

https://buildingscience.com/documents/insights/bsi-001-the-perfect-wall