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How does water drain in a rainscreen with horizontal furring above/below innie windows? – GreenBuildingAdvisor
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.