1. Begin by removing the sashes and
any foam insulation, jamb adjusters
by backing out the screw for the
adjuster.
2. Cut a piece of plywood, exterior
grade preferably, to fasten to one win-
dow jamb. Take care to align to the
top and bottom. Secure with three
#8x1/2” Philips Head Pan Screws.
Repeat this step on the other window.
3. Apply a bead of caulk sealant
along the interior edges of the “H”
Mullion legs.
4. Place the ”H” Mullion on to the
jamb of the first windows and align
the Mull with the top and bottom of
the window.
5. Nest the second window into the
“H” Mullion and squeeze the windows
together making the frames seated
completely into the Mull. Use a putty
knife to help slip the Mull over the
two windows.
6. Stand the assembly upright and
check the alignment. If necessary use
a rubber mallet to align.
7. Starting with the upper installation
holes, screw the windows into the “H”
Mullion using #8 x 1-5/8” Phips Pan
Head Screws.
8. Repeat by anchoring through the
lower installation hole. Do this with
both windows, angling both screws so
as not to interfere with each other.
9. Applying a Bedding Sealant, seal
the open cavities completely on both
ends of the “H” Mullion so there are
no voids where water can penetrate.
10. Place two metal straps along the
top of the Mulled Unit and use
#8x1/2” Philips Pan Head Screws to
secure the straps. Repeat on bottom
of the Mulled Unit.
11. Re-install the upper and lower
sash of each window. If a Head
Expander and Sill Angle are needed,
they can span both windows.
Structural “H” Mulls
Window Fitters Guide to Window Installation 
79              ©2022 AWDI, LLC Do not reproduce without permission
SAMPLE MANUAL

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