I’m installing a rainscreen (furring over polyiso, which is over CMU), which made me wonder about whether it’s okay to have horizontal furring above and below the innie windows. From a carpentry perspective I’d want it, to have something secure to attach the siding to around the window opening. But from a rainscreen perspective, where one of the points is to allow a drainage plane, I wouldn’t.
I looked up Martin’s “All About Rainscreens” article which says:
“Some builders wonder whether they need to come up with a detail to provide openings under each window and at each window head. The short answer: no, you don’t. Just provide a gap at the top and bottom of your furring strips so that air can move sideways around the window.”
That’s not clear to me. If I don’t have to provide openings above/below the window (i.e., if it’s okay to install horizontal furring) then how can water drain, which is one of the points of a rainscreen?
Then again, even if I provided drainage somehow (like by drilling weep holes in the furring, or just using furring “blocks” with space between each block, rather than a continuous strip), any water trying to drain would be blocked by the window trim / sill.
I think maybe the answer is, “While drainage is an ideal benefit, you can’t do it at windows, so you get the drainage where you can (other parts of the wall), and for the window plane, you get just the ventilation.” Does that sound right?
Also, after I installed, I saw that Martin says to leave an air gap around the window, so at I minimum I do intend to cut the horizontal furring so it’s not as wide.
Replies
What about cutting furring strips into blocks above and below spaced to what ever distance you would want to secure the trim? This would allow openings for water to drain.
Michael,
There are two separate things at play around the windows. One is air-movement and the other is a path for water to drain.
To maintain continuous air movement you can either hold the furring back around the windows, or create an opening at the head much like you have at the bottom of the wall to introduce air.
Windows, whether there is a rain-screen cavity or not, need head-flashing. That flashing needs to extend back to the WRB and be lapped by it. The trim above the horizontal leg of the flashing gets help up 1/8" to provide a drainage gap.
As I just edited the question: I think maybe the answer is, "While drainage is an ideal benefit, you can't do it at windows, so you get the drainage where you can (other parts of the wall), and for the window plane, you get just the ventilation." Does that sound right?
Also, I'm sorry I wasn't specific: The polyiso is going over a finished CMU wall, so I think I don't need any special flashing at the top of the window around the polyiso, but maybe I'm wrong about that. Ditto for bottom of the window, though maybe it would be beneficial to flash-tape the top and front of the polyiso, I don't know.
i have been working out a similar detail, but wood frame with zip or similar sheathing. Using zip as the wrb, i want the windows flashed in plane with the sheathing (innie window). I have attached a diagram of my current thought. The drainage/air gap at the top and bottom of the window is achieved with ventilating batten strips attached to the furring strips. I think by current code, the horizontal furring strips are to be pressure treated.
Could you notch the strip above the window (on the back side) so to enable air flow up and some drainage down? Maybe some Coravent on top to keep critters out.
Below the window it is also possible but maybe not that important - drainage down there is not affected.
Both strips out of PT wood to reduce water risk?
I used vertical furring up to the top and bottom of the window, and landed the horizontal window trim across the ends of the furring strips. That's a pretty common method from what I've seen, if the windows are mounted to the sheathing plane and not a buck.
I had thinner furring strips (sized to match the horizontal Cor-A-Vent around the building) and filled between the furring strips with short sections of Cor-A-Vent as an insect barrier. At the head, all of that was attached over top of the taped leg of the head flashing.
Chris,
Yeah. The strapping over the foam should be just like a typical rain-screen, with all of them vertical. Picture framing the RO is for when you are installing bucks.