Wall sheating on top of the rainscreen
This is something I’ve been wondering for a while.
Why on a vented roof we put the sheating directly on top of the vent channel but on a wall we put in behind it (rainscreen)? Wouldn’t it be smarter to just nail the sheating on top of the wall furring and then install your cladding on top of it?
You could always put some sort of paper (Tyvek for instance) between the two to slow water absorption like it’s done on a roof. And if your sheating get really wet, it can dry pretty quickly as long as the venting gap behind it is large enough. Plus, your house would have some good shear resistance without messing with your insulation plane. That would solve the double wall cold sheating problem. And you wouldn’t need to worry about where you nail your trims because you got wood everywhere.
Of course, you’ll still need to replace the sheating behind the furrings with some kind of windtight membrane to prevent wind washing through your insulation so there are some added costs. But that’s still a pretty cheap wall compare to using rigid foam or rockwool panels.
I am missing something because it’s seem like a simple and good solution to me?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Alex,
I think it is a clever idea. The only concern I have is that the structural sheathing would only be nailed along the left and right edges of the sheathing but not the top and bottom edges. To be a true shear wall, you must nail ALL four edges of the sheathing to solid blocking/studs. Doing so in this wall would effectively block vertical air movement and eliminate the benefits of the air gap (expelling moisture).
Perhaps you could get away with something like cora-vent strips as the blocking substrate between the furring strips. If Zip-R can have foam between the studs and the sheathing then I imagine the cora-vent has a chance to work. Obviously, you would need to run this by an engineer- especially if you live in a seismic/high wind area.
Your WRB under the furring strips will have to be your air barrier. It might be difficult (but not impossible) to seal this air barrier if you need to run plumbing wires to the exterior as you will have to do so behind the layer of sheathing.
Thanks for the answer. Didn't think about this structural issue. I'll have to think about it.
I think with this kind of wall you need to have two barriers : one on the outside under the furring strips acting as a water/wind barrier but vapor permeable and one on the inside acting as an air/vapor barrier. That's the way they do it in Europe and I think it's a smart detail.
Rain screen on the walls is mostly about providing a capillary break between the siding and sheathing, the fact the channel is there and helps with drying is mostly a bonus. With your suggestion, the siding is back directly against the plywood, which is what you want to avoid in the first place.