GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

  • |
  • |
  • |
  • |