5 Weather Barriers Critical
to Successful Installation
3. Air Barrier: The air
barrier function (resisting 
the flow of air in either 
direction) is provided by the
drywall, seal to the sub-sill,
seal between the sub-sill and
the window frame, the seal
between the window frame
and the glazing, and the 
glazing, itself.
4. Thermal Barrier:
The thermal barrier consists 
of the chambered frame, the
insulating glazing package,
and the insulated wall 
assembly. The interfacing of
the thermal barrier can be
achieved filling cavity voids
between window and wall
with foam insulation and any
sealing of interior and exterior
creating a dead-air space.
5. Vapor Barrier:
The vapor barrier (resisting
vapor diffusion) is provided 
by materials of low vapor 
permeability located near the
interior of the wall and win-
dow assemblies and include 
a polyethylene sheet or 
insulation liner, window
frame, builders tape between
jamb, sill, header and wall,
and the interior glazing. 
1. Water Shedding
Barrier: The water barrier is
provided by the glazing, the
glazing tape between the glaz-
ing and window frame, the
exterior surface of the window
frame, the sealant between
the window frame and the sill
drip flashing, the sill drip
flashing, the drip cap, and the
exterior surface of the wall
cladding.
2. Moisture Barrier: 
The exterior moisture barrier
function is provided by the
exterior glazing, the seal
between the glazing and the
window frame, the seal
between the window frame
and the sub-sill membrane,
the sub-sill membrane, and
the exterior building paper
(house wrap).
5
3
4
1
2
These barriers are meant to
control the environmental 
conditions and manage their
impact on the interior comfort. 
The ideal installation is
achieved when each of 
the 5 barriers in the wall, 
is integrated with the corre-
sponding barrier in the 
window.
• Water – Exterior water
such as rain, snow, etc.
• Moisture – semi liquid
“water” trapped behind the
exterior siding caused by 
natural evaporation or 
condensation within wall
components
• Air – Movement of air by
wind or pressure differential
• Thermal – Material Heat
transfer
• Vapor – Migration of 
warm moist air to drier 
cooler air (producing 
damaging condensation) 
1. WATER BARRIER
    FLASHING/
     DRIP CAP
1. STUCCO/
 WATER 
   BARRIER
2. MOISTURE 
BARRIER
3. AIR
         BARRIER
4. THERMAL
    BARRIER
5. VAPOR 
    BARRIER
1. SIDING/
WATER 
  BARRIER
There are 5 critical weather barriers built into every wall
separating the exterior environment from the interior. 
There are similar barriers designed into the components
of every window and door.
Window Fitters Guide to Window Installation 
8                ©2022 AWDI, LLC Do not reproduce without permission
SAMPLE MANUAL

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