Best WRB/drainage plane setup for exterior walls 6″ – 12″ apart?
We are building our mudroom about 6″ away from our existing garage… not ideal, but it’s a very long story, and 6″ gives us seismic racking protection.
This will result in ~ a 20 ft run (~ 10 – 15 ft high) of exterior walls** separated by 6″ – 12″ (they’re not totally parallel). I live in the Seattle area (Climate Zone 4c), so am particularly aware of both bulk water and water vapor/condensation/humidity. As you can imagine, we are interested in reducing the longterm maintenance, as these walls will be virtually inaccessible.
I am wondering if a rain screen would be worthwhile on one or both walls to allow air drying and prevent capillary water retention between Hardie panel and WRB. There will be a 2′ roof eave overhanging the gap, and potentially flashing of the garage roof (which is lower than the mudroom roof) to the mudroom wall… so there is likely to be minimal bulk water entry, but Seattle air is wet… a lot.
Bonus query: are there any WRBs or WRB systems particularly well-suited for this sort of situation? For this situation, primarily, but we also have an exterior wall in the setback that we’re residing and cannot increase our encroachment, but the city may be fine with a *minimally* thickened WRB that incorporates drainage and/or air drying (like Obdyke Slicker).
Bonus-bonus query: anyone ever tried to perform exterior maintenance/repair on exterior walls 6″ apart? What was that like? Any tips for keeping the walls in good order?
** The garage wall is 2×4 -> 5/8″ shear-schedule-nailed ply wood -> WRB -> Hardie panel. The mudroom wall is 2×6 Bonfig walls (so ~ 2×8″ nominal stud bays at 16″ on center) -> sheathing nailed to shear wall schedule -> WRB -> Hardie panel.
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Replies
finePNW,
In the PNW it's always a good idea to include a full rain-screen gap - especially with an air-tight reservoir cladding like Hardi-panels.
Given that it doesn't much matter what WRB you choose. I prefer Tyvek Commerical, which has a good but not too high perm rating, and is hard to tear. The WRBs that inculde a drainage plane are only minimally better than regular ones, and won't save a wall that needed a rain-screen to stay safe.
How to maintain it? I'm intrigued to hear how you plan to built it!
Thanks Malcolm!
Build it horizontally on the ground all at once and lift it into place!
The plan originally was Tyvek Commercial, but I saw they make Tyvek Drainwrap… which sounds good, but maybe it’s not any better given that you recommend a full air gap.
Speaking of, what exactly do you mean by “full” air gap? Strapping of >= 3/8”, or simply something beyond a “drainage” WRB? Are there any good low profile products under 3/8” that still do a fine job of allowing air-gap-based drying?
finePNW,
Yes strapping of at least 3/8". Nothing under that thickness will be effective enough. The propitiatory products - mesh, dimpled WRBs - give you a drainage plane, but don't provide enough drying.
The Hardi will be fine. Any surface mold should just be cosmetic, and can be removed every now and then with a light pressure washing with a very mild solution of bleach and de-greaser. I'd be temped to clad the wall in corrugated galvalum, but it probably doesn't match the aesthetic you want.
That's a fine suggestion, re: corrugated galvalum. The aesthetic doesn't quite matter since the walls won't be seen, but we did already order and receive the Hardie panel so it's probably not worth backtracking my contractor on it if there are other ways around it :).
Thanks for the clarification re: 'full rain-screen gap' and 3/8"!
@Malcolm I've unfortunately been advised this AM that there's no room for a proper air gap and no space at this last minute to redesign with bug screens etc. -- I should have been on top of this months ago. Bummer city.
With no full air gap possible, do you think replacing the Tyvek Commercial in my air-gapless detail with a "drainable" WRB like HydroGap or RainDrop 3D (or similar) would provide any benefit as basically a capillary break?
I am not expecting bulk water / rain exposure on these walls as the 6" gap between them is covered by a 2' eave and might even get flashed (garage roof flashed to mudroom wall, which is taller), so my concern is Seattle water vapor and condensation.
finePNW,
These things happen. I'd go with a dimpled WRB and make sure to include a good interior air-barrier.
Thanks Malcolm. A dimpled wrap is what we'll do, then. Is any 3D WRB -- like Tyvek's Commercial "D" or DrainWrap -- comparable to "dimpled" wrap in this regard?
I did some looking and was unable to find much on this topic or quantifying the improvement in capillary vapor drive in a non-bulk-water situation when using a 3D vs 2D WRB directly against cement fiber board. I did find some on stucco, but this seems different enough to not rely on for my situation. If any articles or resources come to mind, I'd be forever grateful! Thanks again!
finePNW,
I'm sorry - they aren't something would use, so I haven't spent any time investigating the difference between them. Hopefully other posters will chime in.
Good luck with your build!
Thanks Malcolm. Entirely understandable! I appreciate all of the help you've given me -- both in this thread and all of the historical threads I've been mining for this project.
Thanks again, Malcolm! Ended up going with plywood (seams sealed with Prosoco joint/seam) -> Tyvek Commercial "D" -> Hardie board -> Sherwin Williams Emerald paint. I'll try to get a video of them raising this 2600 lb wall into place for ya :).
finePNW,
Great. I'd like to see that!