Proper installation of replacement windows requires
that if the old opening (with or without the old frame)
be sealed in such a way that any of the degradation
or irregularities in the old frame are covered and the
new installation is adequately integrated with the
water management system of the wall.
In order to accomplish this, it is also important that
whatever is done to the old opening melds with the
new window to prevent water, moisture and air infil-
tration while allowing the opening to “breathe”, to
avoid the trapping of condensation in the old cavity.
When removing the old window, whether it is the 
old sash of a double hung, or the removal of a fin-
mounted sliding window, or even an embedded steel
casement window. often the opening needs some
modification to cover where the old window parts
were, and/or to adequately prepare the opening for 
a properly installed new window.
A perfect alternative installation method involves 
an extruded piece called an “F-Channel”. This
extruded piece has two “legs” which fit precisely over
the frame of the new vinyl window, and the width of
the F-Channel extends past the second leg to cover
the full 3-1/2” opening, as shown in the diagrams.
For sash replacement, the two sash and parting 
stops are removed. The exterior blind stop is cut off
as in an “outside-in installation (described elsewhere
in this book). The F-Channel is affixed to the old
opening on the sides and top and the new window 
is “snapped-in” the channels from the outside.  
While the fit should be fine, if there is extra play, 
the window can be centered and anchored to the 
F-Channels using pop rivets.
If the “F”Channel is made with a “Nailing Fin”, it 
can be slipped up under old siding, and with the “F”
Channel turned upside down, and one leg trimmed
off, the “fin” can be modified to create a level “sill”
while providing a functional sill pan for drainage.
Perhaps the best part of this method of installation 
is that the capping can be placed over the extended
portion of the F-Channel as shown.
This protects the installation from
water penetration without excessive
caulking, allowing the opening to
breathe. This is important as warm
moist air on the interior migrates
towards the outside during colder
Window Fitters Guide to Window Installation 
88                ©2022 AWDI, LLC Do not reproduce without permission
F-Channel
Installation 
F-Channel Installation can be very effective where the
old opening is degraded.
Capping
Capping
F-Channel Legs
Caulk,
Seal and
Insulate as
Necessary
Trimmed
Blindstop
BEFORE
Fin can be
used to “level”
Channel for
use as Sill Pan
with one leg
trimmed
SAMPLE MANUAL

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