With single-light wooden sash, it may be possible to retrofit
insulating glass within the existing sash frame. The sash frame
needs to be in relatively good condition and of sufficient size
to handle the additional glass weight. Mechanical routing of
the glazing rabbet is usually required to accommodate the additional
thickness of the insulating unit. In addition, the sash weights
probably will need to be augmented if operable windows are desired.
It is always important to establish beforehand whether there
is sufficient room in the weight pockets to allow for the additional
For the same single-lighte sash, another alternative is to
"piggyback" an aluminum-frame storm panel onto the
interior portion of the sash. This procedure requires that the
inside edge of the rails and stiles on the room side be rabbeted
to allow insertion of the storm panel.
As with retrofitting insulating glass, the approach to double
glazing has an advantage over separate storms in ease of operation
and lower maintenance. In piggy-backing the storm panel onto
the sash, care must be taken to get as good a seal as possible
where the metal frame abuts the wood sash frame.
A good weatherstripping system should be used on the back side
of the metal storm frame, and the metal frame should fit snugly.
These measures are necessary to reduce the likelihood of condensation
between the two pieces of glass. Weep holes should be provided,
preferable on the stiles rather than the rails. Removable clips
or set screws to secure the storm panel to the wood sash are
recommended; these permit occasional cleaning of the glass and
allow access for maintenance work. Local glass shops can provide
the necessary materials.
The use of retrofitted insulating glass or piggyback storm
panels is generally limited to single-light sash, although it
may be possible on some two-light sash to achieve similar results,
provided the muntin is wide enough and strong enough.
One additional use of insulating glass with existing wood sash
is worth mentioning. For buildings in downtown urban areas or
along busy highways, there is an increased desire to reduce
noise from the outside. While sound reduction can be achieved
through good weather-stripping, the addition of a storm window
with insulating glass in combination with the existing window
can yield superior sound attenuation over a triple-glazed replacement
Fisher, Chuck. Rehabilitating
Windows in Historic Buildings: An Overview (GSA 08500-03),
Preservation Technical Procedures, U.S.
General Services Administration