Replies

1. Expert Member
   Dana Dorsett | Sep 11, 2015 02:51pm | #1

   The wall doesn't need to "breathe" as in leaking air. It needs to be able to dry via vapor diffusion, and unpainted gypsum board is VERY vapor permeable (15+ perms), more vapor permeable than some housewraps, and more than 10x more vapor permeable than dry 3/4" plywood (less than 1-perm). With even the tiniest gap between the gypsum and metal siding the wall can dry toward the exterior.

   But even it's it's vapor-tight to the exterior (metal laminated to the gypsum), as long as you don't have an interior side vapor barrier it can dry toward the interior. In most of CA you don't need or want a vapor barrier anywhere in the stackup. The places in CA where tighter vapor retardency might be necessary in that climates is at altitude, in US climate zones 5B or higher. In 4B or lower it just doesn't matter.

   So, find yourself on this map, then take a big sigh of relief:

   https://www.greenbuildingadvisor.com/sites/default/files/images/DOE%20climate%20zone%20map.preview.jpg

   In zone 5B you could get there with a 1/4" rainscreen gap, but I'm not sure how to guarantee the gap with non-combustible materials. (Haven't thought about it very deeply.) In 6B you'd need a class-II vapor retarder on the interior. (Smart vapor retarders preferred.)

2. GBA Editor
   Martin Holladay | Sep 11, 2015 03:10pm | #2

   Peter,
   You are describing a wall that will be airtight on both sides. That is a very desirable wall. Every builder should always strive to build a wall that is airtight on both sides.

   Of course, any wall can be improved. If you have 2x4 or 2x6 studs filled with fluffy insulation, you'll end up with a wall R-value that is ho-hum or poor. If you want a higher R-value, that can be achieved by including a continuous layer of rigid foam sheathing on the exterior of your studs (presumably between the plywood sheathing and the exterior gypsum).

3. Expert Member
   Dana Dorsett | Sep 11, 2015 03:38pm | #3

   In a fire zone it's better if any foam in the wall stackup is rigid polyiso, which won't melt when heated, and has a higher kindling temp than polystyrene (EPS or XPS), which can melt and turn into a spreading puddle of flaming plastic. (Of course, the shed's probably gone by the time that happens if you're building with the thermal break, but still...)

   In almost all of CA a 2x6 / R20 wall would meet or beat IRC code minimums even for a house, and in much of the state 2x4 / R13 cuts it. But this is a shed, and SFAIK there is no code requirement to heat or insulate a shed. But if you're heating it & cooling it even on an intermittent basis some amount insulation (as indicated in the stackup description) is a good idea. If it's going to be heated & cooled most of the time, the IRC minimums or better would be a good idea, even if not required.

4. 
   ayyoitsp | Sep 11, 2015 03:40pm | #4

   Thanks for the answer. It looks like I conflated air permeability with vapor permeability. I understand the difference now. I'm in a coastal area, so it seems like I'll be fine without any extra interior barrier.

   Martin - Thank you for the insulation suggestion. I'll definitely take that into account.