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Historic Preservation - Technical Procedures
Preservation Tech Notes: Windows 9 Interior Storm Windows: Magnetic Seal
Preservation Tech Notes, National Park Service, Pad
Doors And Windows
Preservation Tech Notes: Windows 9 Interior Storm Windows: Magnetic Seal
PRESERVATION TECH NOTES: WINDOWS Number 9
INTERIOR STORM WINDOWS: MAGNETIC SEAL
Charles E. Fisher
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.
The Williamsburg National Historic Landmark District consists of
approximately 500 buildings that have been restored or
reconstructed to their 18th century appearance. A vast majority of
the buildings are owned by the Colonial Williamsburg Foundation and
many are open to the general public as part of an education
Twenty-three of the non-exhibition buildings, most of which are
used for rental housing, were selected for retrofitting storm
windows as part of Colonial Williamsburg's ongoing energy
conservation program. A simple commercially available interior
storm window system was chosen and certain modifications were made
for aesthetic and functional reasons.
***REHABILITATION DESIGN PROBLEM***
In improving the energy performance of windows in historic
buildings, installing a system of double glazing is usually one of
the first considerations in areas with long heating seasons.
Installation options may include adding a traditional exterior or
an interior storm window, routing out the frame of the historic
sash to insert a double-glazed thermal unit, or under certain
conditions, replacing the deteriorated historic sash with a new
double-glazed one that preserves the historic character of the
In assessing these options, it is understandable that the initial
cost of installation will be a major consideration, but it should
not be the controlling factor. In working with historic buildings,
emphasis should be placed, not only on maintaining the historic
appearance, but also on preserving as much of the historic building
material as possible. In addition, there are often special
considerations based on the use of the historic building, as in the
case of those owned and operated by The Colonial Williamsburg
When Colonial Williamsburg began an accelerated program of energy
conservation for their many historic and reconstructed buildings,
they surveyed those buildings not open to the public to ascertain
the possibility of improving thermal performance and reducing the
air infiltration of the windows. Since nearly 40% of the windows
in the historic buildings are all or in part original, the decision
was made to examine the possible use of storm windows. To minimize
the visual and physical alterations to the windows, especially as
viewed from the outside, an interior storm window system was
considered the best approach. Since half-screens had previously
been installed on the inside of most of the buildings and these
were still in good condition, new screen units were not required.
The following design criteria were thus established for the
interior storm window systems:
1. The storm window had to be largely invisible to the general
public as viewed from the outside.
2. No mechanical fasteners could be used that would damage the
3. The storm window needed to be removable or self-storing to
allow natural ventilation during the summer months.
4. Installation had to be achieved with minimum inconvenience to
5. Air infiltration needed to be appreciably reduced both for
energy reasons and for improved personal comfort of the
6. The storm units needed to accommodate the irregularities of
the historic windows and also the varied assortment of
draperies, blinds and window trim.
7. As viewed from the interior, the storm windows had to be as
unobtrusive as possible.
8. A reasonable payback period for the windows through improved
energy performance was required.
***REHABILITATION DESIGN SOLUTION***
The interior storm window system selected by Colonial Williamsburg
consisted of a clear acrylic sheet in a polymer frame that
contained a flexible magnet on the back side. When the storm
window was set against an adhesive-backed steel strip pre-attached
to the historic interior window trim, a magnetic seal was formed.
The advantages to this design solution were the ease of
installation without any damage to the original woodwork or sash
and the small visual impact it had on the exterior appearance of
Two potential drawbacks of this and other types of interior storm
windows were recognized from the beginning. First, the addition of
the interior storm window could result at times in a slight shadow
effect created by the wooden muntins of the original sash
reflecting off the storm glazing. The other potential problem was
that condensation could be trapped between the original sash and
the storm unit. In the past, condensation had occurred during
winter months, particularly on the north facades where high air
infiltration due to prevailing winds was common and also in rooms
with high humidity, such as bathrooms and kitchens. With the new
storm windows in place, it was assumed that there would be fewer
condensation problems. If moisture runoff from condensation did
occasionally collect on the sills of individual windows, it could
be wiped off. Based on the results from the test buildings,
however, condensation has not been a problem to date. Building
conditions, use, and climatic differences might lead to different
results, however, on other projects.
***SINGLE-UNIT STORM WINDOW***
Two different storm window designs, single unit and split unit,
were utilized in order to accommodate the varying window sizes and
operability requirements. The least expensive design was the
single-unit storm window covering the entire window opening and
used in buildings with year-round climatic controls in which
operable windows were unnecessary.
There are size limitations for the use of the single unit. For
example, had the windows been as large as 4 by 8 feet or
particularly long and narrow, a split unit would have been required
to avoid potential problems from thermal expansion. Other
allowances, for expansion and contraction would also have been
necessary had any of the storm units been set within the jamb
rather than surface-mounted. In the case of the buildings at
Colonial Williamsburg, the window sizes were such that any thermal
expansion, according to the contractor, would only result in the
storm window slightly "creeping" along the steel strips with the
magnetic seal not breaking.
Most of the historic windows at Colonial Williamsburg had been
trimmed with a stool so that a magnetic seal was obtained only on
three sides of the polymer frame. A small metal angle frame or
wood stop could have been added to the stool to permit a magnetic
seal on the fourth side but such alterations were avoided.
A slightly different polymer frame configuration was used for the
bottom of the storm window in cases where a window stool existed.
The sill framing was attached to the acrylic sheet like the typical
polymer frame, with the exception that the bottom was extruded with
a 1/4" U-channel that had a latex gasket glued in place to form a
***SPLIT-UNIT STORM WINDOW***
The storm window design most commonly used at Colonial Williamsburg
consisted of a separate upper and lower part with an
interconnecting meeting bar. This style was used in windows where
the original double-hung sash needed to remain operable. A split-
unit interior storm window has an advantage over a single unit in
that it did not need to be removed and stored in the summer months.
By attaching the lower unit to the upper unit via a magnetic seal,
the storm window could be left in place, thereby avoiding problems
of storage and reducing the chances of the acrylic sheets being
scratched or warped.
The design of the split units met two important considerations:
(1) the meeting bar of the interconnecting units could be located
so as to have minimal visual impact from the exterior, and (2) a
sound connection between the split units was created. To
accomplish the latter, the standard 1/4" U-channel with a latex
gasket used on the stool framing was, in turn, used on the upper
horizontal edge of the lower window unit. On the lower horizontal
edge of the upper window unit, a slightly larger U-channel was
incorporated into the bottom of the polymer frame to fit snugly
over the upper facing U-channel and gasket used on the top of the
lower unit. This feature thus served to diminish air infiltration
and also provided additional reinforcing along the midsection of
the complete storm unit.
The placement of this meeting bar primarily depended on its visual
impact on the outside. Where the historic upper and lower sash
were the same size, the meeting bar of the interior storm was lined
up as closely as possible behind the meeting rail of the historic
sash. In a number of windows, however, the pane configuration made
it harder to disguise the appearance of the interior storm meeting
bar from the outside. These windows had 6 panes over 9, the bottom
sash being considerably larger. In such cases, it was specified
that the bottom of the top storm unit would stop at the first
horizontal muntin below the meeting rail on the historic sash.
***SELF-STORING FEATURE OF THE SPLIT-UNIT WINDOW***
To allow for the storage of the lower unit, an adhesive-backed
steel strip was attached to the face of the vertical polymer frame
on the upper unit. Thus the two vertical framing members in the
upper unit had a magnet on one side and a steel strip on the other.
The addition of the metal strip allowed the lower unit to be
magnetically attached to the steel on the upper unit when in a
To keep the upper unit from creeping downward due to the additional
weight of the piggybacked lower storm, a 3/8" shelf with a 3/16"
upward turn was formed in the two wood-adhering vertical steel
strips at a point aligning with the bottom of the upper unit.
A similar feature was added at the bottom of both single and split
units when no stool existed in the historic window. To deter
possible creeping and, as a safety measure, the bottom of the
vertical steel strip was turned 90 degrees outward to act as a
small shelf for the storm window. A small plastic tip was attached
to the sharp edge of this shelf to protect draperies.
***FABRICATION AND INSTALLATION***
For the convenience of the contractor and the residents of each
building, it was decided not to fabricate the windows at the job
site. Working on one building at a time, the contractor measured
each window, cut the acrylic sheets, and attached the polymer
frames at the shop. Where the historic windows were skewed - some
having lost their rectangularity due to settling - the storm units
were cut oversize at the shop to allow for custom fitting at the
site. The protective masking on the acrylic was left in place
until final installation in order to reduce the chances of
scratching. It was necessary, however, to roll the masking back
from the edges when attaching the polymer frames.
Prior to shipment to the site, the adhesive-backed steel strips,
which were to be mounted to the wooden window trim, were cut and
temporarily affixed to the bar magnet set in the polymer frame; the
pull-off tape was left on. Affixing the adhesive steel strip to
the bar magnet greatly facilitated proper alignment onto the
existing wooden window during the final installation.
After removing the blinds and draperies, the prefabricated storm
window was held up to the historic window and checked for any
necessary adjustments. For windows that were skewed, the polymer
frames and the acrylic glazing were easily cut onsite for a correct
fit. When ready for installation, the masking tape was removed
from the adhesive side of the steel strip and, with the steel strip
still attached to the polymer frame (via the magnet), the window
was aligned and pressed firmly against the wooden trim. In this
way, the steel strip adhered to the wooden trim while maintaining
a correct alignment with the magnet in the polymer frame. In some
cases, due to irregularities in the surface of the wooden trim, it
was necessary to build up behind the steel strips using double
faced tape to accommodate gaps as much as 1/8". The storm window
was then checked for ease of removal and the protective masking
removed from the acrylic. Two workers were able to install about
25 windows in an average working day.
The unobtrusive nature of these storm windows is an advantageous
feature, along with the fact that no physical damage or alteration
to the original window is required.
Building tenants have noticed a major increase in personal comfort
during the winter months, especially while sitting near the
windows, and have experienced a substantial reduction in outside
noise. Specific energy savings have not been calculated, but
tenants report significant energy savings during the first winter.
It is anticipated that the acrylic glazing will be scratched over
the long term, especially since some of the tenants have elected to
store the lower storm units elsewhere during the summer rather than
piggyback them. Colonial Williamsburg gives its tenants specific
cleaning instructions including information on available products
to remove minor marks.
There are available for the split units separate screens which can
be installed in the bottom half and removed during winter months.
These were not used at Colonial Williamsburg because of interior
half screens already existing within the jamb.
These single and split-unit storm windows have been successfully
used in both commercial and residential rehabilitation projects in
other states. By minimizing physical damage and changes in the
visual qualities of the historic window, this interior storm window
has provided two features desirable for other historic buildings.
- Building: 23 Buildings
Owned by the Colonial Williamsburg Foundation
PROJECT DATE: 1982-1983
- Tom Taylor Jr., Chief, Architectural Conservator
Bill Black, Project Architect
Paul Hurley, Superintendent, Building Maintenance Department
The Colonial Williamsburg Foundation
Post Office Box C
- Sam Massey, Jr., Window Consultant and Contractor
Post Office Box 210
- Magnetrim Window - Energy Options of Virginia, Inc.
8439 Glazebrook Avenue
- Magnetite Window - Viking Energy System Co.
275 Circuit Street
- Manufacture and installation of 461 interior storm windows
(7,410 square feet) on 23 buildings - $34,124 ($5.28 square
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