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

Proper WRB behind Boral Siding

user-7040235 | Posted in Energy Efficiency and Durability on

We’ve been contacted to replace beveled cedar siding that is rotting on a 1939 story and a half. The existing wood windows have been well maintained and appear to be in good condition, and will not be removed during the project. The existing walls have little to no insulation that we know of.

Our plan is to remove the existing siding(following lead safe practices) insulate the wall with dense pack cellulose, install a WRB, reside with beveled Boral siding with mitered outside corners, and paint.

The question is what WRB to use and how to incorporate the existing windows without removing them. I’m leaning toward BO HydroGap favoring it’s drain-ability.

Any help would be much appreciated. Thanks

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. Expert Member
    Michael Maines | | #1

    User-7040235, could you tell us your name? The software is buggy when it comes to usernames.

    As for your question, some of the answer depends on your climate zone--do you know what it is?

    In general, the more robust the rain screen, the better. In colder, damp climates I like a full 1/2" to 1 1/2" space behind the siding, especially with a vapor-permeable wall assembly. Obdyke's spun-woven polyolefin rain screen products are good but hard to use with lap siding; I prefer to install a smooth WRB and run vertical strips of wood over each stud.

    If that will push the siding beyond the window trim, you can add a backband, or stick with a thinner rainscreen. Once the siding is removed, assuming you have board sheathing, I recommend installing a self-adhering WRB such as Henry Blueskin VP100 or Siga Majvest SA to seal against both air and water.

    It's hard to get existing windows perfectly detailed without removing them. If you want to give it a shot anyway, try prying the casing off the wall so you can run the WRB behind it. Seal the head casing to the wall with a good tape, then cap flashing. Run the WRB over the cap flashing and tape that connection tight. At the sill, seal the interior portion with canned foam, but leave the outer portion open for drainage. This is the most vulnerable part of the whole assembly so if possible it's best to get a sill pan in there.

  2. user-7040235 | | #2

    Andrew Webster. I noticed it didn't show.

    We're is zone 5 here.

    Thanks for the help Michael

  3. Expert Member
    Michael Maines | | #3

    Andrew, my advice stands, then. If it's zone 5A (moist) the rain screen gap is more important. In zone 5B (dry) it's less important. But it also depends on exposure, roof overhangs, etc.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |