The third weatherization treatment is to install an additional
layer of glazing to improve the thermal efficiency of the existing
window. The decision to pursue this treatment should proceed from
careful analysis. Each of the most common techniques for adding
a layer of glazing will effect approximately the same energy savings
(approximately double the original insulating value of the windows);
therefore, cost and aesthetic considerations usually determine
the choice of method. Methods of adding a layer of glazing to
improve thermal efficiency include adding a new layer of transparent
material to the window; adding a separate storm window; and replacing
the single layer of glass in the window with thermal glass.
The least expensive of these options is to install a clear material
(usually rigid sheets of acrylic or glass) over the original window.
The choice between acrylic and glass is generally based on cost,
ability of the window to support the material, and longterm maintenance
outlook. If the material is placed over the entire window and
secured to the frame, the sash will be inoperable. If the continued
use of the window is important (for ventilation or for fire exits),
separate panels should be affixed to the sash without obstructing
operability (see fig. 9). Glass or acrylic panels
set in frames can be attached using magnetized gaskets, interlocking
material strips, screws or adhesives. Acrylic panels can be screwed
directly to the metal windows, but the holes in the acrylic panels
should allow for the expansion and contraction of this material.
A compressible gasket between the prime sash and the storm panel
can be very effective in establishing a thermal cavity between
glazing layers. To avoid condensation, 1/8" cuts in a top
corner and diagonally opposite bottom corner of the gasket will
provide a vapor bleed, through which moisture can evaporate. (Such
cuts, however, reduce thermal performance slightly.) If condensation
does occur, however, the panels should be easily removable in
order to wipe away moisture before it causes corrosion.
The second method of adding a layer of glazing is to have independent
storm windows fabricated. (Pivot and austral windows, however,
which project on either side of the window frame when open, cannot
easily be fitted with storm windows and remain operational.) The
storm window should be compatible with the original sash configuration.
For example, in paired casement windows, either specially fabricated
storm casement windows or sliding units in which the vertical
meeting rail of the slider reflects the configuration of the original
window should be installed. The decision to place storm windows
on the inside or outside of the window depends on whether the
historic window opens in or out, and on the visual impact the
addition of storm windows will have on the building. Exterior
storm windows, however, can serve another purpose besides saving
energy: they add a layer of protection against air pollutants
and vandals, although they will partially obscure the prime window.
For highly ornamental windows this protection can determine the
choice of exterior rather then interior storm windows.
The third method of installing an added layer of glazing is to
replace the original single glazing with thermal glass. Except
in rare instances in which the original glass is of special interest
(as with stained or figured glass), the glass can be replaced
if the hinges can tolerate the weight of the additional glass.
The rolled metal sections for steel windows are generally from
1" 1-1/2" thick. Sash of this thickness can normally
tolerate thermal glass, which ranges from 3/8" 5/8".
(Metal glazing beads, readily available, are used to reinforce
the muntins, which hold the glass.) This treatment leaves the
window fully operational while preserving the historic appearance.
It is, however, the most expensive of the treatments discussed
here. (See fig. 6f).