HoW-To
Have you been installing windows the right way?
Nailing fins, sometimes called mounting flanges, are the thin strips installed on the exterior sides of a window. Unlike a “front flange” which is a decorative trim piece, a nailing fin is usually set back from the outer edge of the window frame and has fastener holes punched into it. The purpose of nailing fins is to secure the window to the wall sheathing and hold it in place while the shims and screws are being installed. Fins also work in conjunction with flashing and the weather resistant barrier (WRB) to prevent wind and water infiltration.
There are two types of window nailing fins, integral and non-integral. Integral nailing fins are called “integral” because the nailing fin and the window frame are extruded as one solid piece. When the windows are assembled, all four corners are melted/welded together creating a sealed shell around the entire perimeter of the window. Integral fins are only found on vinyl windows.
The back side of all nailing fins should be sealed to the wall with an exterior-grade, non-shrinking sealant that will remain flexible. Non-integral nailing fins also need to be sealed to the frames they’re connected to. This can be accomplished by running self-adhering flashing over the fin and up onto the window frame (above). Do not apply sealant or flashing to the bottom nailing fin. An unsealed bottom fin will create an escape route for any water that finds its way past the building envelope somewhere above the window.
The name “nailing fin” would suggest that nails would be the fastener of choice, however, screws work better and not just because they have more holding strength. Future adjustments are easier to make with screws and overtightening is easily avoided. Also, installing screws eliminates the possibility of damage caused by wayward hammer blows.
Windows with non-integral fins are shipped with gaskets to seal the corners. If the gaskets get lost or damaged, it is acceptable to create your own gaskets out of self-adhering window flashing.
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