Are two WRB’S a good idea or a bad one? Guess what the building scientist told me…
I’m a homeowner starting my build this week using Zip system with a ventilated rainscreen. Here’s the details:
- location is North Carolina barrier island, coastal hurricane zone, over 50 inches annual rainfall
- zero overhangs (modern design architectural decision, please spare me the judgement on this point, thanks!)
- vertical board on board 1×6 cypress siding with boards overlapping 1.25”
- 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.
- 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?
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