Rainscreen with Mineral Wool Insulation
I am planning out my wall construction for a home in zone 6. Not much rain (15″ annually), but plenty of snow in winter (56″ annually). Hot (and mostly dry) in summer (80sF to low 100s), cold in winter (mostly lows in the single digits, but sometimes minus teens F). My house will not have much direct sun due to northern exposure and tree shading.
All that said, here is the wall I am considering:
– wall board
– 2×6 fir studs on 16″ centers, Roxul Comfortbatt (R23) between the studs
– OSB, taped and sealed for my air barrier
– house wrap for vapor barrier
– 3″ of Roxul Comfortbatt IS (R12)
– 1×4 verticle furring strips, plus rainscreen vents at top and bottom between furring strips
– cement board exterior
A few concerns I have are: the rainscreen causing a chimney effect during a fire (wildfire is a big issue out here), the exterior only being R12 in my zone, and the work required to keep the furring strips coplanar for good looking exterior siding.
This got me thinking about the following modification:
– change the 1×4 furring strips to 2x4s (still laid flat over the 3″ of Roxul)
– fill the gap between furring strips with an additional 1.5″ of Roxul (R6)
This would add a combined R4.5 to R5 to the exterior, avoid any potential chimney effect, get rid of the vents top and bottom between furring strips, and should make it easier to get the furring strips coplanar since the 2x will deflect a lot less than 1x. In addition, this still allows me to use 6″ screws (probably Timberloks or CTX lags) for attaching the furring strips because I can countersink them a bit when I am predrilling
Of course, this loses the rainscreen gap. My understanding of Roxul is it should a drain fairly well when water/water vapor gets behind the cement board. Remember I don’t get much rain, I have 2′ overhangs (for snow issues), and wind driven rain is unusual. The only concern I can come up with is the cement board will not be able to dry out as quickly, which could effect durability of it’s painted surface (I’ve seen some issues with paint peeling off of cement board). And of course I have a fair amount of Roxul to custom cut to size.
My questions are:
– has anybody else out there tried something like this
– does anybody see significant issues to this approach
Note: I have also considered just going to 4″ of Roxul on the exterior and only venting the bottom of the rainscreen (using horizontal furring to block the top gap). This might be simpler, would at least partially address the chimney effect and be a better R factor. Unfortunately, I have seen concerns about using more than 3″ of Roxul when hanging heavy siding (like cement board) on it, and of course the 6″ screws become 7″ or 8″ (depending on what I can get).
Thanks in advance for your thoughts and advice.
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