|Aluminum storm, painted.
The traditional method of double glazing is the use of exterior
storm windows, which achieves a U-value for the window comparable
to insulting glass. The typical exterior storm window greatly
reduces air infiltration, lowers the maintenance cost of the
historic window, and extends its useful life. Unfortunately,
exterior storms can affect the visual appearance of the historic
windows, although less so where single-lite historic sash are
involved. While it was common at the turn of the century to
match the divided lite pattern in the primary sash with that
of the exterior storm, today's single-lite storm panels tend
to alter the reflective qualities and shadow lines cast by the
primary window, and also obscure features such as muntins.
Several steps can be taken to lessen the visual impact of storm
windows. The simplest is to have the storm factory window painted
to match the color of the primary window and trim. The second
step is to specify a half-screen to be mounted on the inside,
since it is the screening material in the typical storm/screen
combination that most dramatically affects the appearance of
a historic window. By mounting a half-screen on the inside (a
typical feature earlier in the century), the sight lines of
the storm unit are simplified by the reduction from a typical
three-track to a two-track frame. Also commercially available
are custom single-track, two-panel units with a simple subframe
set within the jamb. Some single- track systems are designed
so that the panels can be removed from the inside for cleaning
and, for summer use, screen panels inserted for ventilation.
This single-track design, can significantly reduce the storm
unit's impact on the window's historic appearance. In either
case, the storms will also result in considerable sound reduction,
which is important to buildings exposed to high street noise.
Fisher, Chuck. Rehabilitating
Windows in Historic Buildings: An Overview (GSA 08500-03),
Preservation Technical Procedures, U.S.
General Services Administration
Exterior-mounted storm windows must have "weep holes"
at the bottom of the frame to allow any moisture that collects
between the primary window and the storm window to drain out.
Even though these drainage holes subtract from energy savings,
not having them will eventually cause the primary window frame
to rot, and possibly make them impossible to operate.
Efficiency and Renewable Energy, U.S. Department of Energy