GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

‘Smart’: s-m-r-t…

user-1105327 | Posted in General Questions on

Hi again. i have heard of OSB panels, et al, as being a ‘smart’ vapour barrier/retarder in that it becomes more vapour permeable the wetter it gets. assuming ‘smart’ is a good thing, why is it a good thing that a vapour barrier becomes more permeable the wetter it gets?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    A smart vapor retarder isn't always desirable. For example, you don't want to install a smart vapor retarder under a concrete slab or on the bare dirt of a crawl space floor. In these applications, you want a stupid vapor barrier like polyethylene.

    For some wall and ceiling assemblies, however, a smart retarder makes sense. In these applications, a buildup of moisture is a warning sign that something bad is happening. The wall is getting wet. When that happens, the smart retarder absorbs moisture, thereby becoming more vapor-open. As the smart retarder becomes more vapor-open, it allows inward drying to occur. This drying helps prevent the wall from becoming so wet that mold or rot develops.

  2. user-1105327 | | #2

    i buy that. thanks again, martin

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |