Furring Strips vs. Plastic Mesh for Rainscreens
For my addition project (porch-to-room conversion), I am using ZIP R6 sheathing. I am also going to have a rainscreen. I agreed with the contractor to have furring strips applied along the studs behind the sheathing. We are going to go for 1/2″ strips (rather than 3/4″), as thickening the exterior too much is not an option: on the sides of the new room, the roof does not have overhangs, so, we have to be careful that the siding stays well under the roofline, that is, the drip edge. For that reason, to the extent possible, we are building towards the interior, to the point that, at the lateral rafters––in a lean-to roof––I suggested using a strip of regular ZIP sheathing, with the ZIP R6 starting right under the rafter’s line, so that the 1″ foam part of the ZIP R6 is towards the interior, and the walls do not have to protrude for that one inch towards the outside. That way, the use of ZIP R6 does not thicken the lateral walls beyond the current state. What WILL thicken the walls outward, though, is the rainscreen. Hence the question: Should I consider NOT installing furring strips and, instead, have some premade rainscreen, made of mesh, to be applied? Is that going to be thinner? And is the application of that over the ZIP sheathing a good idea, or will it require all sorts of stapling into the sheathing, which I would prefer to avoid? Also, as the furring strips are already part of the contract, if there is no real advantage (if not drawbacks) to switching to a pre-made rainscreen, I’d prefer to stay with what agreed.
Any thoughts or recommentations on this? Thanks!
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Replies
Some of this depends on your siding. Many siding manufacturers require at least 1x3/1x4 strapping, so they want to see 3/4" thick (minimum) furring in order to honor their warranty. It's common to use 1x4s or ripped strips of 3/4" plywood here (I prefer strips of plywood since those are less prone to splitting). Keep that in mind when moving forward with your project.
If you want a minimal depth drainage plane, then some type of "crinkled housewrap" type of material like Tyvek Drainwrap is your next best option. These allow for some amount of water migration and drying, but they are not nearly as robust as a "real" rainscreen.
Bill
Bill, thank you! A very helpful response, as always. My siding will be wood lap siding, cedar lap siding to be precise. I'm not sure there will even be an issue of warranty. I suspect that the 1/2" gap, if not ideal, will be sufficient. (I saw a response by Martin Holladay, in a past discussion, in which he said many people stay with the 1/2" gap.). Do you think it is OK when using cedar wood lap siding?
Excellent point regarding the strips splitting. Maybe I should mention the use of plywood to the contractor (though you know how these things go: I don''t want to appear to be as if I am trying to control his work too closely, if you see what I mean.)
The primary thing a rain screen gap needs to do is to be open enough that water can't bridge the gap by capillary action. That means anything over around 1/8-3/16" or so will start to show some benefit. I wouldn't go that narrow for the gap though, since such a small gap is likely to get blocked with insect debris or other crud in a relatively short period of time. 1/2" would be much, much better.
I would try using plywood. I'm not sure you can easily get furring strips thinner than 3/4" in actual dimensions anyway. I know you can get 1/2" and 3/8" plywood pretty much anywhere. I would use plywood rated for use as exterior sheathing (which means it will have exterior rated glue holding the plys together), and I'd try to get at least 4 ply construction, not the cheaper 3 ply stuff. 3 ply plywood is much more likely to bow, flex, and have individual plys split compared with the better grades of plywood that use more plys. I would not use OSB here. I would also suggest getting plywood from a real lumberyard. I've had issues with plywood from box stores delaminating in the past, the lumberyards seem to generally have better quality materials. The reason this is more of an issue here is that if you cut the plywood into relatively narrow strips for use as furring, all the weaknesses of the material become more critical simply because there is less of that material present in each narrow furring strip.
Bill
Thanks Bill! I will definitely pass these bits of advice to my contractor. I don't think it will take long to cut those strips, not for just one room, that is, three walls. And the cost difference between good and better plywood should be rather insignificant.