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Are two WRB’S a good idea or a bad one? Guess what the building scientist told me…

TUBER1DER | Posted in Green Building Techniques on



I’m a homeowner starting my build this week using Zip system with a ventilated rainscreen. Here’s the details: 

  1. location is North Carolina barrier island, coastal hurricane zone, over 50 inches annual rainfall
  2. zero overhangs (modern design architectural decision, please spare me the judgement on this point, thanks!) 
  3. vertical board on board 1×6 cypress siding with boards overlapping 1.25” 
  4. rainscreen detail is MTIdry 3/16” corrugated lathe strip 16oc vertically hung on sheathing to get the battens off the Zip, then treated 1×4 horizontal battens to receive the vertical cypress siding. This results is a nearly 1 inch variable to 1.75 inch air barrier. 
  5. Rough openings will have sloped sills and fully flashed on all sides with Zip 2.0 liquid flash

My builder is suggesting to add a layer of #15 felt over the Zip sheathing for good measure – as added protection against wind driven rain, reducing the amount of water the Zip sees. On the surface this seems like a good belt and suspenders approach. 


But I’m not so sure this is a good idea. I realize I may be testing the Zip system to the max, and I have no problem spending extra money for more robust protection and more peace of mind against water intrusion – yet the wall is designed to be optimized for drainage and drying, and ZIP is a WRB already.  I’m worried that the #15 felt, which tends to warp and buckle with moisture, would just inhibit drying by blocking airflow and actually holding moisture in the wall. It also makes the windows more difficult to detail, possibly introducing a negative for water management. 


I guess I’m torn, because I can definitely make an argument for both sides, so would love to hear from some of the experts found here at GBA. Should I add another mechanically attached WRB over the Zip? 

Many thanks! 


  1. Expert Member


    I think adding a high-perm sheet WRB like Tyvek would definitely add robustness to the wall and make flashing easier, but I'm not sure it's necessary. - especially since your wall includes a rain-screen cavity.

    I agree about using building paper. It's s too easy for it to wrinkle and bridge the capillary break the cavity provides. Fine under siding, bur not my choice in a rain-screen assembly.

    1. TUBER1DER | | #3

      Thanks Malcolm.

      I had not considered Tyvek, as it kind of has a bad rap in my area - just a regional bias against it as is typical with a lot of this stuff.

      Building Scientist Christine Williamson was kind enough to consult with me back when I was designing the house, and she expressed full confidence in the Zip system with rainscreen as described - no additional layers. I remember she specifically said "no tarpaper" on top, but in all the technical talk I missed the reason - whether she considered it just not necessary, or whether it might introduce problems. I've reached out to her several times since for clarification, but never got another response. (I totally understand as she's kind of blown up on the education aspect of all this stuff and can't respond to every homeowner with a question)!

      From a logical standpoint a belt and suspenders approach with two layers makes sense to me. Whether I "need" it or not is still a question. Ultimately it may come down to an issue of what will help me sleep better at night.

      One point by Christine that made an impression on me was that from a building science standpoint and the hierarchy of importance in wall design - the ability to drain and dry is higher than absolute waterproofness, because no wall can be totally waterproof, or something to that effect.

      OK, I've nerded out on this stuff way too much!

  2. NotYourFwiendGuy | | #2

    Bryan, Huber has a tech note on adding additional house wrap like Tyvek, might be worth a read:

    I'm currently adding an Tyvek layer to a Zip-sheathed house with a rain screen (Cor-A-Vent siding vent system). The main reason I added the additional WRB is because the Zip was exposed to weather for 6+ months longer than Huber allows and you're required to add an additional WRB layer in this case to stay within the warranty. This suggests that adding an additional WRB is beneficial in general to guard against Zip panel degradation... not clear though if for UV damage or excessive moisture (probably both).

    However, I disagree with Malcom that adding Tyvek will make flashing easier. Zip is such a nice easy system with the tape, but when you are doing BOTH the Zip tape plus Tyvek, you're right it does complicate the installation depending at which point you're at. Easier if rigid flashing aren't yet installed. Got to pay close attention to not accidentally create bladders and use the right proper tapes to adhere Tyvek to Zip and elsewhere. Just gets kinda fussy.

    I have noted that the Tyvek stays quite taught with the furring strips on it, so I'm not worried about any bridging of the capillary break.

    I'm not an expert, but I'm presently deep into this process and I would have loved to skip the extra WRB and installing Hardie right against the Zip with the rainscreen. However, the end result with the extra WRP does seem much more robust. Feels right in my circumstance with Zip that's been exposed for probs too long.

  3. TUBER1DER | | #4

    Thanks for the link Ian.
    Yes, I agree with your point. Reading between the lines with Huber would suggest an additional layer is beneficial.
    For sure I plan to caulk the windows directly to the sheathing, so any extra wrap would have to fall on top.

  4. bigred | | #5

    You need to think about your window details. Assuming you are using Zip Stretch tape for window pan (and also sloping your window sills 5 degrees by installing a beveled board or installing the framing on a 5 degree angle) you will be adding a seal on three sides of your windows with silicone, and then adding zip flashing tape over that. So then where does you tyvek go? I would also recommend for additional protection to liquid flash each and every nail hole and make sure all wall penetrations are liquid flashed with adequate blocking installed over the liquid-flashed surface.

    1. TUBER1DER | | #7

      I did slope my sills and Zip liquid flashed all my rough openings on all sides. Then the window flanges were nailed and “caulked” directly to the opening with liquid flash (excluding sill) and then liquid flashed on top of the window flanges (excluding sill). So basically the entire window install is a monolithic membrane - a rubberized one piece gasket. Then Zip tape on top of the window head and jamb flanges for good measure.
      They say every window leaks someday, but honestly I feel pretty good about my windows.

      My roof will be fully covered with Grace Ice and Water shield and lap about 4 inches down the sides.

      So, if I feel confident in my strategy at my rough openings, should I be concerned about any water over time that makes its way to the sheathing? I have about a 1.5inch rainscreen gap, so ample drainage and drying. But my cypress siding will definitely leak, through knots etc...Enough to worry about? I guess I would not be asking about houswrap if I wasn’t a little concerned. Still not sure it would do anything for me.

      Just as a gauge of my WRB, Zip sheathing has an exposure shelf life of 180 days before it needs to be covered up. I bet Tyvek and other house wraps are similar. I’ll need to check.

      But it leads to the question of how I would detail a house wrap at the roof and windows, when it’s just acting like an extra raincoat. Slipping a non-adhered houswrap under the perimeter of the Grace doesn’t seem like a good idea. I definitely did not want to include it into my window detail.


  5. JesterBlackDog | | #6

    another thing to consider in applying Tyvek are all the penetrations of the ZIP surface coat (the green stuff) you will be making with staples or nails.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      Tyvek, and most other sheet WRBs have to be installed with Cap-nails or Cap-staples, which limit water intrusion.

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