After the opening is prepared for the replacement
windows, remove all sashes of the windows to be
mulled.  Note: Leave strap (if provided) on all win-
dows until fully installed.
Measure the opening height approximately where
units are to be mulled.  Cut “H” mullion 1/4” less
than opening height. Then measure opening width 
at top and cut continuous head expander 1/8” less
than the measured width. Also measure opening
width at bottom and cut continuous sill angle 1/8”
less than that width.
Starting at the top of the replacement windows, slide
the  “H” mullion onto windows side by side.  Liquid
soap on inside of mullion can make this easier.
Install the continuous head expander over top of  
the windows and “H” mullion.
Install full-length sill support if necessary and set
windows into openings.  Shimming the sides and
aligning the windows carefully, try to keep the 
units tight to the “H” mullion for maximum support.
Once mounted, check plumb and square. Then 
re-install and align all the sashes. Complete the
installation with caulk, capping and trim as 
necessary.  Now you can remove any provided straps.
To mull vertically (stacked units), remove any 
operating sash.  Using the “H” mullion, cut it 1/8”
less than opening width. Place the head expander if
provided) on the top window, and mount the sill
angle (if necessary) “below” the lower window.
Carefully install mulled unit into opening.  Re-install
and align any removed sash. As before, remove any
provided straps, and complete the installation with
caulk, capping, and trim as necessary.
How to Join with "U" Mulls
It is possible to join and stack multiple windows 
to accommodate various architectural, design, 
and structural situations. When stacking windows,
or where you want the windows joined prior to 
installation, snap-in "U" Mulls work better than 
"H" mulls because they are structurally joined to
both sides of the windows. 
Common to welded (fusion corner joints) vinyl win-
dows and especially in awning and casement types,
the frame - inside and outside on all perimeters -
contains a groove that will accept a specially 
extruded "U" shaped mull that snaps in to "lock" 
the units together. If the windows you are installing
have this groove, then it is preferable to use the "U"
mull. It is important to note, however, that the weight
of vinyl replacement windows with 3/4" insulated
glass can be excessive and the mulls may not be
strong enough to hold the weight if a multiple unit is
to be lifted into an opening already mulled. The rule
of two people per window also applies to multiple
units. Therefore, be sure you have enough help when
lifting mulled units into place.
When stacking windows or mulling more than two
units together, in most instances, it is easier and
more efficient to mull the units prior to installation.  
However, when the combination unit will be unwieldy
or extremely heavy, partial mulling can be done
before installation, and the remaining units attached
after they are placed in the opening.
The available, U-shaped,  “snap” mulls are applied on
the inside and the  outside of the multiple window
units.  Place the two windows together and press the
legs of the mull down into the slots in the window
frame.  As shown in the diagram, the tabs on the
mull legs snap into the slots holding the two windows
firmly together.  It is important that the mulls be
accurately pre-trimmed to the proper length prior to
use because they are difficult to remove.  Since the
mulls protrude from the face of the window frame,
they should be trimmed to allow for any moulding or
trim to be applied after installation.  This will provide
a neater, more easily weather-sealed job. 
Head Expander
"H" Mullion
Sill Support
Snap-in
Mull
Snap-in Mull
Lower Window
Upper Window
Window Fitters Guide to Window Installation 
78              ©2022 AWDI, LLC Do not reproduce without permission
SAMPLE MANUAL

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