Plywood sheathing as rainscreen – GreenBuildingAdvisor

We’re building our house in North Idaho zone 6b (ish). We’ve noticed condensation issues on the plywood sheathing. We even made a test box with 6 mil poly, R30 rockwool (more than we have, but we still seem to have issues with R23), right on the sheathing. In a few days, it was a little damp, so through the winter would be a real problem. For the curious, R15 rockwool did the same thing without any cover, bare drywall, or plywood.

Our options seem to be adding a ton of insulation to the outside at this point, removing sheathing and doing alternative shear strategies, or… adding vertical girts on top of the horizontal girts (as rain standoffs) and using out sheathing as a rainscreen. We’ll put wood siding on top of the sheathing. Probably will use 2x4s for the vertical girts that will create air space and let moisture find its way to the ground, and should prevent vapor drive in the summer.

This is not an airconditioned structure. We use a wood stove. Part of the moisture issues are due to it being a daylight basement where the foundation contractor did not put a vapor barrier under the slab! We have a fix for this in the works. Currently, the humidity is around 60%, so it’s a good testbed for condensation issues. That said, our testing has made us think that there may be issues even in drier houses without damp slabs.

I have not heard of anyone turning sheathing into a rainscreen. I don’t see why it won’t work. It seems like the easiest option. Certainly, exterior insulation plus a rainscreen is preferable, but it’d be quite the expense. R23, especially with so little thermal bridging as a postframe, is plenty for us.

Hopefully someone can chime in and tell me if this is crazy or not. Hopefully someone else has already tried it?

On a stud wall, one could use 2x4s on the wide end mounted to the studs, to then mounting the sheathing to. You could put a WRB between the studs and 2x4s on the wide end as well, or use furring strips, but I feel a little more comfortable having a particularly solid connection to the sheathing.