Flashing Bottom of Window flange?
RICHARD EVANS
| Posted in Energy Efficiency and Durability on
I’ve noticed that a lot of builders will omit taping over the bottom flange of a window once it has been installed. Sides are taped/flashed and the top flanged covered by WRB flap. Yet, bottom flange is left exposed.
I presume this is to allow water to drain from the assembly. But is this really necessary- especially if weep holes are left uncovered? Will this result in air leakage at the bottom of window?
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Replies
https://hammerandhand.com/best-practices/manual/3-windows-doors/3-1-new-window-installation/
Are the omitting the flashing of the WRB at the bottom of the window?
You could get moisture below the window in the rough opening itself, which would not drain with the weeps. Your air seal should be accomplished on the interior side of the window. Joe Lstiburek has an excellent article on this at https://buildingscience.com/documents/insights/bsi-004-drainage-holes-and-moderation. The image is from the article.
John, Tyler- Thank you for your replies and excellent sources!
Looks like omitting bottom flange flashing is the way to go. (John, it appears Hammer in Hand agrees with BSC.)
Rick,
if you are using rain-screen strapping you can either mount the window outboard so there is a clear drainage plane for water to escape from the sloped sill, or if mounting the window on the sheathing drill several holes in the bottom flange to get the same result.
I always find your answers helpful, Malcom- thank you! Never thought about attaching flanges to the strapping. That is a great idea!
Malcolm* sorry! (Typed with thumbs...)
Rick,
There is sort of a push/pull between the demands of dealing with draining bulk-water intrusions that end up on the sill, and those of insulating the window opening.
When you open the area under the window to allow water to escape, as Tyler pointed out, you move the air-sealing boundary to the inside of the window frame. That means the sill, unlike the jambs and head, doesn't get a layer of insulation along it's depth.
I don't see any way round that, but hope someone comes up with a detail that provides adequate drainage and also integrates insulation.
The continuous insulation piece always adds a new level of complexity - it depends on where you are flashing the window to and which layer is the WRB. If you use something like Zip-R sheathing you reduce the complexity. Another good article from BSC https://buildingscience.com/documents/insights/bsi-085-windows-can-be-a-pain describes some techniques. If you are are putting continuous insulaiton on top of your wrb ensure there is a small gap between the insulation and the WRB for drainage. 1/8" is enough to drain and dry, but maintains most of the thermal performance of your insulation.