Window Fitters Guide to Window Installation 
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Insulating the Frame and
Sash Pockets
There are 3 major performance problems associated with
old windows and doors. Convection, Conduction and
Radiance are usually well handled by replacing the old
operating window or door with a modern, well manufac-
tured new window with high performance glazing.
The real culprit, in most poorly performing old windows, is
air infiltration. It is well documented, that a crack, 1/16”,
around the perimeter of the old sash or the old window
frame will cause drafts, and the feeling of “cold” as if there
were a hole in the wall the size of a brick. 10 windows or
more, and the hole is like 10 bricks, and that can make a
cold winter day feel unbearably cold and drafty.
Too often, replacing old
drafty windows with
new, high efficiency
replacement windows,
doesn’t change much
in the way of drafts
and comfort. In fact,
the air-tight replace-
ment window magni-
fies the air leaks
around the new 
window. 
Uninsulated sash
pockets and window
frame cavities are the
number one complaint.
The new window may
be well insulated but
the hidden space
between the old 
window frame and 
the rough opening
needs to be insulated 
if the replacement is 
to succeed.
Empty Cavity
Between Old
Frame and
Rough Opening
SAMPLE MANUAL

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