Back of widow trim flashing: sheathing or furring strips for rainscreen
This may seem obvious, so please be kind. 😊
I will be adding 1/2″ PT furring strips for a rain screen.
The window nail fins are placed against furring strips and well-flashed with SIGA tapes.
I will add a 1×5 header above each window’s drip cap, which is placed against the windows nailing fins.
I will add an additional Z-flashing above the header.
QUESTION: Should that-flashing’s back section be installed a) against the sheathing (with the furring strips on top) or b) against the furring strips after the furring strips are installed on the sheathing?
While (a) would seem to “catch” bulk water that gets into the rain screen gap, it would also divert any water draining down the gap from above the window instead of just letting it flow out to the bottom opening. It might also impede depressurization from behind and below the window, although I think that could be addressed with small gaps.
Thank you for any insights.
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User...081,
It's not obvious. With rain-screens there are a few permutations based on whether the trim is mounted directly to the sheathing, is outboard of the furring strips, or is mounted on the cladding.
The rule of thumb is that wherever the rain-screen cavity is interrupted by penetrations like windows, doors, duct terminations, etc, the flashing should extend back to the sheathing. When the flashing is there as a transition between types of cladding or trim mounted on the rain-screen furring, it should be mounted on the rain-screen furring.
So if your 1"x5" head-trim is attached directly over the window flange, the metal head-flashing should extend back to the sheathing. That flashing should have a 1/8" gap to the cladding above, and include end-dams to stop water moving horizontally into the rain screen cavity.
You should not rely on the window frame to protect against water intrusion. If your head-trim is not deep enough to entirely cover the entire window frame at the head, you will need to install a second head flashing there to protect it.
Malcom, Thanks so much for the thoughtful advice.
The windows are Jeld-Wen, custom clad, pushout casements. They're really well made (In 2014, I actually was allowed to spend time on the factory floor in Bend, Oregon, to watch how the workers attended to the manufacturing. They were really focused, and the windows we installed then have performed flawlessly.)
These windows have an integral frame, and a substantial metal drip cap that snaps into a groove in the top of the frame.
We applied SIGA Majvest 500 SA as our WRB. We pre-flashed the window rough openings with SIGA Wygluv and Rissan welded corners, and flashed after installing the windows with SIGA Fentrim. The windows' frame metal cladding extends a little over an inch so the drip cap is well out from where the trim face will land. (But your note on the header flashing alerts me to check the edges for potential flow.)
We used 1/2" primed MDO plywood to furr the window frame to the same plane that the Windsor One trim and beveled lap cedar siding will be installed. (The Fentrim for the post-installation flashing lapped completely over the sides of the furring, and onto the Majvest. That means the windows are flashed to the sheating, not just to the furring.)
I'll have to mull over how to treat the slope and sides of flashing that would go back to the WRB because it wouldn't seem advisable to direct "captured" water in the direction of the window front. Better to let it flow down the rainscreen gap outside the width of the window.
As a final note, my assumption is that strong winds will inevitably push rain through gaps in lapped siding. But once the pressure drops in the rainscreen gap, it's not likely to get past well-applied WRB flashing and pre-flashing. We primed and painted one coat of high-mil Duration on the back sides of the beveled siding, so I'm considering the flashing as the belt to the suspenders.
User...081.
Sounds pretty bullet-proof to me.