Replies

1. Expert Member
   MALCOLM TAYLOR | Mar 24, 2022 12:15pm | #1

   Andy,

   There aren't too many circumstances where the relative permeance of a WRB matters all that much, because the other materials in the wall assembly are usually lower, so they govern how much drying will occur. One instance where a lower perm WRB might be preferable is when you have a reservoir cladding and want to avoid excessive solar vapour drive.

   As you say: in walls designed to dry to the outside (and not all are), it's the vapour-openness relative to the inside that matters most.

   The big advantage of self-adhered WRBs is when you want to use them as an air-barrier. it's a lot more difficult to detail regular ones for that purpose.

   Wind is only a factor during construction. Once it is protected by the cladding it doesn't make much difference if it's self adhered or lapped.

1. 
   andyfrog | Mar 24, 2022 02:16pm | #2

   Thanks Malcolm.

   Interesting, this makes sense. Do you happen to know what the vapor permeance of dense packed cellulose insulation is? Searching just returns a lot of results about poly vapor barriers.

1. Expert Member
   MALCOLM TAYLOR | Mar 24, 2022 04:23pm | #3

   Andy,

   Martin included a really useful table in one of his blogs. It looks like the permeance of 4" of cellulose is 29.
   https://www.greenbuildingadvisor.com/article/all-about-vapor-diffusion

2. 
   andyfrog | Jun 28, 2022 11:34pm | #4

   "Wind is only a factor during construction. Once it is protected by the cladding it doesn't make much difference if it's self adhered or lapped."

   So I was thinking about this more; assuming your WRB was also your air barrier, wouldn't the mechanically fastened membrane balloon and collapse slightly when the wind gusted and lulled?

1. Expert Member
   MALCOLM TAYLOR | Jun 29, 2022 11:03am | #5

   Andy,

   Relying on the WRB as your primary air-barrier only makes sense in retrofit situations where there it isn't possible to use other options. I agree (as I said in my first post), in those circumstances a self-adhered one is easier to install and performs better.

2. 
   andyfrog | Sep 05, 2022 02:45pm | #6

   So I'm replying to this in case someone finds it with a web search later on. I did some research and there are a variety of opinions on this matter, from varying perspectives.

   From a construction perspective:

   -certain crews vastly prefer one over the other. Both say that the one they prefer is faster and easier to install.
   -self adhered is more sensitive to environmental conditions and substrate prep during installation and may be tricky to get right in less ideal conditions e.g. higher winds, extreme temperatures, airborne dust and sand, mud, etc
   -mechanically fastened is probably trickier to detail around openings, and as Malcolm mentioned, especially tricky to detail as a primary air barrier

   From a building science perspective:

   -when the walls start to get thicker, there's relatively low energy flux, so even if a mechanically fastened option could theoretically dry faster, there probably won't be enough energy flux to meet that capacity
   -when the walls start to get thicker and especially if the insulation is moisture sensitive e.g. double stud walls with cellulose, bulk water is the main concern. In this sense, a self adhered option may make more sense as long as the crew can install it properly.
   -self adhered options are better in high wind event areas e.g. hurricane areas, tree fall areas, because they hold up better during high winds and also when directly exposed e.g. if the siding or roofing is ripped off. In the aftermath of these events, it may be several weeks to months before substantial repairs can be performed, so the self-adhered option is more forgiving during this layover