Historic Preservation - Technical Procedures

Preservation Tech Notes: Windows 8 Thermal Retrofit Of Historic Wooden Sash Using Interior Piggyback Storm Panels
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Preservation Tech Notes, National Park Service, Pad
Doors And Windows
Wood Windows
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Preservation Tech Notes: Windows 8 Thermal Retrofit Of Historic Wooden Sash Using Interior Piggyback Storm Panels
Last Modified:



Sharon C. Park, AIA
Preservation Assistance Division
National Park Service

This standard includes the bulk of information contained in the
original Preservation Tech Notes developed by the National Park
Service and the Center for Architectural Conservation at Georgia
Tech.  The Preservation Tech Notes are case studies of exemplary
projects designed to provide specific examples of sound
preservation techniques.  To obtain a complete copy of The Window
publications, including figures and illustrations, please contact:

         Historic Preservation Education Foundation
         P.O. Box 77160
         Washington, DC  20013-7160

The Window Handbook, jointly prepared by the National Park Service,
Preservation Assistance Division and the Center for Architectural
Conservation at Georgia Tech, also contains all of the Tech Notes
on Windows and is available for purchase from the Historic
Preservation Education Foundation for $32.00.  The Window Workbook
is available for $49.00.  The two publications together can be
purchased for $72.00.

Oklahoma City, Oklahoma


The Colcord Building, constructed in 1910, is one of the few
remaining historic office buildings in downtown Oklahoma City.
Designed by William A. Wells, the 12 story concrete structure is
elaborately decorated with ornamental terra-cotta panels at the
first, second and twelfth floors.  The bold scale and setbacks of
the wooden windows surrounded by terra cotta are significant design
features of this National Register property.

During a recent renovation of the building, alternatives were
investigated for improving the energy performance of the windows.
After careful evaluation, the decision was made to fit a new storm
panel to the existing sash in a manner that was cost-effective and
also preserved the window's distinctive qualities.


The Colcord Building contained 507 single-glazed, double-hung
window units with each sash containing a single light.  The units
ranged in size from 22" by 66" to 48" by 66", which translated into
approximately 7,700 square feet of glass surface area.  Since
energy costs can be increased by up to 25% as a result of loose-
fitting single-glazed windows, it was financially important for the
owner to upgrade the existing windows or to replace them.

The architect investigated the following approaches:

1.   Repairing the existing windows.

2.   Adding weatherstripping to the existing units.

3.   Adding a second layer of glazing to the existing windows,
    either as a separate storm window or as applied storm panels.

4.   Replacing the existing windows with new double-glazed thermal

The need to seek a cost-effective and yet compatible solution
quickly eliminated two common options.  The first was the use of an
exterior storm window since it would have altered the deep setback
which was a character-defining feature of the building.  The second
alternative was the use of a modern replacement window.  The cost
of replicating 507 double-hung windows out of wood and installing
thermal glazing was beyond the budget of the owner.  A much less
expensive solution, which was investigated, involved the
replacement of all windows with a metal frame, fixed sash and solar
grey  heat absorbing insulating glass.  While the cost estimate of
$300 per unit was within the budget, the architect felt that such
a solution was unacceptable since it would have drastically changed
the building's historic appearance.

The architect then investigated repairing and upgrading the
existing sash.  This work would involve tightening the loose
members of the window sash, adding weatherstripping and installing
an interior storm panel.


While researching new window units, the architect found a
commercially available wooden replacement window that incorporated
a removable storm panel piggybacked onto the single glazed sash.
This second layer of glazing was recessed on the inside of the
primary wooden sash, allowing the double-hung window to operate
without interference from the applied panel.  The architect applied
this concept to the Colcord Building's windows and proposed to
retrofit a recessed storm panel to each existing sash, provided it
would not structurally weaken the sash in the process.

This approach would necessitate routing or cutting out a portion of
the historic wooden sash and would add to the weight of the sash
due to the attached storm panel.  The sash rails and stiles
measured 1 7/8" deep by 2" wide and were in sound condition.  They
were therefore capable of tolerating the required cut 3/8" deep by
1/2" wide.  The issue of the additional weight of the storm panels
was directly related to the choice of materials selected for the
piggyback units.  Since acrylics are purported to be 40% lighter
than glass and 15% more thermally efficient for the same thickness,
the architect investigated their use.  Because of expense and the
tendency of acrylic to bow or discolor due to sun exposure and to
scratch when being cleaned, the architect selected instead glass
panels set in aluminum frames.  By lubricating the existing sash
chains, the sash could accommodate the additional weight of 3 to 8
pounds without affecting operability.

The use of interior storm panels affixed to a sash can create a
potential problem of trapped condensation.  To avoid this
possibility, the architect specified two features.  The first was
a neoprene gasket as an air seal between the wooden sash and the
aluminum frame of the storm panel.  The second was the creation of
ventilation holes in the wooden sash stiles.  The vent holes,
drilled laterally through the stiles, were to provide a minimum of
air circulation should moist air condense between the layers of


The construction manager developed a system of scheduling work
during the tenants' non-working  hours.  The two-man crew
undertaking the repair of the windows could complete six window
units during an evening shift.  To accomplish the work as
efficiently as possible, a temporary workshop with jigs and benches
was set up on the floor where the men were working.  This portable
workshop was relocated as the men progressed throughout the

Each sash was first pulled from the window frame by removing the
parting bead and stop bead from one side of the window jamb.  While
the sash was out of the frame, the frame itself was repaired, old
paint scraped off, and the new spring-bronze weatherstripping
installed.  In addition, the sash chains were lubricated prior to
installation of a plywood panel intended to give temporary closure
to the window while work was underway.

After each sash was taken to the workbench, the glass was carefully
removed and set aside.  The sash was then placed horizontally on
the bench and the inside face of the sash was routed out to create
a 1/2" by 3/8" recessed channel for the storm panel.  Loose paint
and remaining dry glazing putty was scraped from the sash.  At this
point ventilation holes should have been drilled, yet because of a
misunderstanding, the carpenters omitted this feature.

Once the sash was repaired, the original glass was reinstalled and
spring-metal weatherstripping was applied to the underside of the
lower sash rail and the meeting rail.  The routed opening was then
measured and the storm panels were fabricated in the basement by
the storm window subcontractor.  The repaired sash were then rehung
in their original window openings and later the storm panels were


The storm panels were made of single thickness float glass set into
an extruded section of enameled-finish aluminum fitted with an
integral neoprene gasket.  A simple hand crimping tool was used to
affix the metal section around the float glass.

The method for mounting the storm panels posed the final problem
for the architect.  The commercially available storm panels used
concealed retractable clips to hold the panels in place.  These
clips allowed for easy removal of the storm panels for cleaning.
However, because the clips would have added substantially to the
cost of the storm panels and would have required deeper routing of
the historic sash, it was determined that an alternate attachment
method was needed.

As the panels would only be removed for maintenance purposes, the
architect determined that the panels simply could be screwed in
place, using a neoprene gasket set on the back side of the panel as
a seal.  Should the panels ever need to be removed, an electric
screw gun would do the job quickly.


The total cost of repairing the historic wooden windows,
weatherstripping and retrofitting the storm panels was under $100
per window opening.  This was a 66% saving over the original
proposal to use a replacement metal-framed thermal glass unit.
Several factors combined to make the retrofit a cost effective
solution.  The use of traditional tools and standard woodworking
techniques ensured that the work could be easily carried out at the
site by the subcontractor.  Scheduling of the work permitted full
use of offices by tenants during the day.  Furthermore, material
costs were low and the workmen established an efficient method for
repairing and upgrading the existing windows.

The low cost of the retrofit and the added insulating qualities of
the upgraded wooden windows provided the owner with a cost
effective solution.  The architect computed that with the combined
benefits of low initial expense in retrofitting the historic
windows and the decreased fuel bills associated with the improved
thermal performance of the windows, the owner should have a
complete return on his investment in 7 years.


The repair and thermal upgrading of the wooden windows at the
Colcord Building successfully combined historic preservation goals
and cost considerations.  Because the windows were in good
condition, heavily constructed, and of a simple one-over-one
configuration, a recessed interior storm panel was a practical
solution.  The dry Oklahoma climate and the neoprene gasket used
helped to avoid the problems generally associated with condensation
between non-sealed glazing layers.  The vent holes, originally
specified and always recommended, were not necessary in this case.
After four seasons there has been no evidence of condensation.  The
approach used at the Colcord Building is an excellent example of
how historic wooden windows can be economically repaired and
upgraded to meet today's energy needs without replacing historic



-    The Colcord Building
    Oklahoma City, Oklahoma


-    The Colcord Associates
    The Colcord Building
    Oklahoma City, Oklahoma



-    Jack M. Graves, AIA
    Architectural Design Group
    117 Park Avenue
    Oklahoma City, Oklahoma


-    William L. McNatt & Co.
    217 East Sheridan Street
    Oklahoma City, Oklahoma


-    $50,500 for window repair, weatherstripping addition of
    piggybacked interior storm panels and repainting of 507
    windows ($99.60/unit).

                             END OF SECTION
Last Reviewed 2012-02-24