Proper Flashing -
Divert. Drain. Dry.
Window Fitters Guide to Window Installation 
17                ©2022 AWDI, LLC Do not reproduce without permission
Regardless of whether a window is installed in a brand new opening or an existing opening, the installation must be inte-
grated into the wall to allow water to be diverted from coming into the cavity and therefore into the interior. Water that
enters the cavity must be allowed to drain; and moisture trapped in the wall or cavity between the window and the fram-
ing, must be allowed to dry to prevent mold, mildew and rot. Simple: Divert, Drain and Dry.
This goal is accomplished through 
proper flashing which not only blocks
the infiltration of water, but also is
lapped in such a way that the water
drains down, and then to the outside.
An integral part of a properly flashed
opening is the sill. Moisture can enter
the cavity from outside and also from
inside. 
When the outside is colder than the
inside, the inside air will carry a lot 
more moisture, and since warm air
moves towards cold air, the warm, 
moist interior air will makes its way 
into the wall where it will condense out
the moisture.  
This moisture, in the wall, when the
opening is properly flashed, will makes
its way down the sides of the new win-
dow towards the sill, where the sill con-
figuration should allow this moisture to
exit.
For the moisture that may be left to 
be prevented from causing problems, 
the cavity must be allowed to dry. 
This is accomplished by not completely
sealing the full perimeter of the new 
window/wall interface allowing the 
space and materials to properly dry.
There are two basic methods of flashing:
Method A where the Weather Resistive
Barrier is applied after the window is
mounted, and Method B where it is
applied before window mounting.
The bottom line is divert the environ-
mental water, drain any that makes its
way inside, and dry whatever is left.
Additional
Flashing
Overlaps
Window
Building
Paper Slips
Under
Flashing
Wraps Over
Jambs
and Sill
SAMPLE MANUAL

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