Combining 
Two or More 
Windows
Window Fitters Guide to Window Installation 
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A mulled unit is the combining of 2 or 3 (or more)
units into one opening. Replacement windows can 
be mulled side by side (a picture window flanked by
double hung on each side or two double hungs), or
“stacked” on top of each other (a transom over a 
double hung or a picture window over an awning
window). The most common mulled units are side 
by side.
Window units can be joined together, and then
installed as a combo unit. These units can be joined
side by side.
Structural mulling enables the units to be joined
side by siade or stacked. The mulls are anchored
then to the opening frame.
Basically, windows can be placed together in 
multiple units into any opening.In fact, the same
installation procedures (blindstop installation or
other techniques) will still apply. Mulled units, in
essence, become a single unit that will install and
mount similarly to single units. The mull systems
effectively seal the joint.
How to Join with "H" Mulls
If you desire to mull two or more units, order 
windows to include an “H” mullion for each joint 
and a continuous head expander, and sill angle 
long enough to fit the entire width of mulled unit.
SAMPLE MANUAL

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