Replies

1. GBA Editor
   Martin Holladay | Sep 27, 2017 01:30pm | #1

   Edward,
   It's not only OK -- it's recommended in several GBA articles.

   For example, here is information from "All About Water-Resistive Barriers":

   "Builders who switch to Zip System wall sheathing will have to change some of their flashing details. Those who usually install conventional housewrap are accustomed to lapping the housewrap over window Z-flashing and other penetration flashings, so that gravity helps keep their walls dry. With Zip System wall sheathing, on the other hand, keeping the wall dry depends on chemistry — that is, on the adhesive component of Zip System tape and AC148 flashing tape. ...

   "Many builders who appreciate this fact [the fact that Zip sheathing is a good air barrier] use Zip System sheathing as part of their air barrier — while still using housewrap, since housewrap can be lapped over window flashings and may be more dependable than Zip System tape."

2. 
   EdSe | Sep 27, 2017 05:15pm | #2

   Thank you for answer on housewrap over Zip sheathing.

   Should the housewrap be cut tight to windows and doors and taped to all four sides? Do I also need metal flashing at tops of doors and windows?
   Thanks,
   Ed Serafin

3. 
   Tyler_LeClear_Vachta | Sep 27, 2017 05:45pm | #3

   Ed,
   Zip has an install guide that you should reference at http://www.huberwood.com/assets/user/library/ZIP_Sheathing_Install_Manual-2013.pdf . It doesn't cover your question specifically but should give you some guidance.

   The question depends on which layer are you using as your water resistive barrier? If the housewrap, then install and flash penetrations to housewrap specifications. If the Zip is your WRB, follow their recommendations. Remember to let the bottom of the window drain.

   Are your concerns about overdriven fasteners? Tape quality? If you are really concerned about moisture the best practice would be to create an air gap between your Zip sheathing or WRB and the exterior cladding. A ventilated rainscreen design prevents the buildup of hydrostatic pressure, allowing bulk water to drain, and uses airflow to dry out the wall. Adding an extra layer of housewrap won't help the wall dry out. Look at using a rainscreen product like Sure Cavity to create the predictable air gap, and make sure you detail how the water will exit the wall.

4. GBA Editor
   Martin Holladay | Sep 28, 2017 04:27am | #4

   Ed,
   Tyler's answer brought up a key point: You need to decide which layer is your water-resistive barrier (WRB). If you don't trust the Zip sheathing, I'm guessing that you intend to make the housewrap your WRB -- but I might be guessing wrong.

   If the housewrap is your WRB, then you need to follow the installation instructions provided by the housewrap manufacturer. In most cases, the housewrap is folded into the window rough openings, after the housewrap is cut in a certain way. (The illustration reproduced below comes from the Tyvek instructions. You can click on the image to enlarge it.)

   Concerning the need for metal Z-flashing above your window trim and door trim: The answer depends on your exterior trim decisions. If you are planning a traditional house with exterior head trim and exterior casing, you will certainly need Z-flashing above the head casing. Some modern-looking houses skip the exterior window trim, however, and you may be able to get away without Z-flashing, depending on the installation instructions provided by the window manufacturer.

   .