Historic Preservation - Technical Procedures

Preservation Tech Notes: Windows 11 Installing Insulating Glass In Existing Wooden Sash Incorporating The Historic Glass
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Preservation Tech Notes, National Park Service, Pad
Doors And Windows
Wood Windows
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Preservation Tech Notes: Windows 11 Installing Insulating Glass In Existing Wooden Sash Incorporating The Historic Glass
Last Modified:



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.  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.

Chicago, Illinois


The Delaware Building was constructed shortly after the Chicago
Fire of 1871 using pre-cast concrete panels on the outside.
Originally six stories in height, the Italianate-style office
building received an additional two floors around 1890 that were
faced with sheet metal to resemble the appearance of the facade
below.  The lower two floors were originally dressed with
ornamental cast iron arches.  Later removed, they have been
recently restored.  Individually listed on the National Register of
Historic Places, the building has an impressive interior atrium
that survives, providing light to offices on the upper floors.

The windows are set in a deep reveal, paired in an alternating
pattern of wide and narrow bays.  The wooden sash are double hung
and most have arched tops.  The window openings are richly detailed
with keystones flanking columns and heavy surrounds, and are
separated in pairs on most floors by various pilaster treatments.
As part of the rehabilitation of the building in 1982, the existing
wooden windows were repaired, weatherstripped and retrofitted with
an additional sheet of glazing using a technique that permitted
creation of sealed insulating units in each sash.


The 102 wooden windows on floors 3 through 8 were over 100 years
old yet still in good condition, partly because their deep setback
from the face of the building provided some protection from the
weather.  There was considerable air leakage, however, due to lack
of weatherstripping, cracked putty seals around the glass, and
shrinkage and cracks in the caulk around the outside frame.  Since
tenants would be responsible for their heating and cooling bills
upon completion of the rehabilitation work, some form of double
glazing was considered desirable.

The windows had both square and arched tops and the size varied
considerably, making the cost of a good quality aluminum
replacement unit quite high.  The 2 1/2" thick windows varied in
height from 6 1/2' to 9', depending on the floor level, and the
windows on the two-story addition differed in width from those
below; overall the windows ranged in width from 3 to 4 feet.


Since the sash were in reasonably good condition and contained only
single lights, the owner decided to retain them and install
insulating glass.  Due to the varying sizes and the arched tops, it
was discovered to be more economical to created sealed insulating
glass utilizing the historic glazing than to remove the existing
glass and install all new insulating glass units.

The 2 1/4" thick sash were in good condition and could handle the
additional weight of insulating glass.  Because of the height of
the windows and the fact that the building could be climatically
controlled by individual tenants year round without opening the
windows, the top sash were to be fixed shut.  The additional weight
of the glass meant that the lower sash would need to be rebalanced
or somehow modified for ease of operation.

The technique used for double glazing the windows, incorporating
the existing glass, was developed in Europe and in use there since
the mid 1970s.  This technique uses only one additional sheet of
glass, placed off the inside edges of the rails and stiles, to
create a sealed insulating unit.  Since the metal spacer placed
between the glass sheets is set slightly off the edges of the rails
and stiles, the retrofitted insulating unit reduces the historic
glass exposure by about 1/2" along all sides.  This tends to limit
the applicability of this technique for double glazing to upper
floor windows with large lights since at such distances the
reduction in exposed glass area is not readily visible.  At the
third floor level and above, the 1/2" encroachment along the edges
of the glass on the large sash at the Delaware Building was
determined not readily visible from the street.  The architect felt
that the additional expense to avoid any sight encroachment was not
justified for an alternate approach entailing routing out the 2
1/4" thick sash on the inside putty rabbet to a greater depth in
order to accept an all new sealed insulating unit.  Such an
alternative, however, might have been appropriate had the interior
moldings along the inside rail and stile been more decorative since
the added single pane would obscure such detailing.  Fortunately in
the case of the Delaware Building, the molding was quite plain.


Modification of the window sash was accomplished off site.  Each
window was tagged, working one floor at a time, prior to shipment
to a shop facility nearby.  Once in the shop,necessary sash repairs
were made, loose paint scraped off, and the glass on the inside
surface carefully cleaned.  Then a standard hollow rectangular
aluminum spacer was sized to fit the opening, filled with a
desiccant to absorb any residual moisture that would be entrapped
in between the glass sheets, and the corners of the spacer then
were edged sealed  Both sides of the spacer that faced the glass
were coated with a strip of polyisobutylene.  The polyisobutylene
strip on the room side also had two parallel copper wires running
the entire length, terminating through one corner.  With the sash
lying flat, the spacer was set snugly up against the inner surface
of the existing glass, then the sheet of new glass was placed upon
it.  Small setting blocks were placed under the bottom edge of the
glass.  A 3/16" tolerance was left between the bottom of the spacer
and new glass and the edges of the rails and stiles.  This space,
which later is filled with a silicone seal, helps to compensate for
any surface irregularities on the inside of the rails and stiles.

The primary seal for the insulating unit was then created by
applying an electrical charge for about 5 to 10 minutes through the
two copper wires.  The wire-resistant heating melted the butyl
compound on both sides of the spacer since the aluminum spacer
easily conducted the heat generated by the wires to the
polyisobutylene strip on the opposite face.  As a secondary seal to
retard moisture collection between the glass and help secure the
new glass in the sash, a silicone compound was applied in the space
between the wood frame, the old glass and the edge of the new glass
and the metal spacer.

To complete the glazing work, a flat piece of 5/8" wide painted
aluminum trim was adhered with silicone along the inside perimeter
of the light at trim to cover the spacer.  A similar trim piece,
normally applied to the outside, was judged not to be necessary in
this instance.


Due to the street noise, it was assumed that with climate control
available to each tenant mechanically the year round that the lower
sash would rarely be opened.  Rather than rebalance the lower sash
weights, inexpensive "jiffy" springs were added.  These springs
permit  the sash to be opened for outside access for maintenance
work and occasional use by the tenants.  While the bottom sash were
weatherstripped, the upper sash were fixed closed through
attachment of two small wooden blocks screwed into the jamb at the
bottom corners of the sash.  The upper sash were then caulked
around the top and sides to minimize air infiltration.


The cost of creating sealed insulating units installed was $17,403
for the 102 windows, averaging $170.62 per window in 1982.
Additional expense to scrape, paint, caulk, weatherstrip and
enhance the operability of the lower sash averaged $47.63 per
window, bringing total cost for the windows to $22,262 (average
$218.25 per window), less than half the cost of good quality
aluminum replacement windows.  While the creation of sealed
insulating units in place was labor intensive, the cost of doing so
was lower than the cost of installing all new sealed units into the
existing frame.  The frames did not have to be rabbeted further to
receive the additional thickness of the unit glass, as in the case
of all new sealed units; furthermore, only a single sheet of glass
per window had to be cut with a curved top.


The incorporation of the existing glass into a sealed insulating
unit was accomplished in the Delaware Building without any damage
to the historic sash and without any loss of historic material.
The retrofit work, which is easily reversible, has performed well
since its completion in 1982.

There are a number of companies in the United States which market
similar systems.  The particular system used in the Delaware
Building was tested in the late 1970s by the Danish Institute of
Technology, an independent government testing institution, and has
been marketed in the U.S. since the early 1980s.  The work carries
a 5 year manufacturer's guarantee, typical of the insulating glass

Two construction details should be carefully examined when
evaluating available systems.  Particular care needs to be taken
with the corner keys (which hold the metal spacers together), since
they need to be coated individually with a sealant in order to
maintain the continuous primary insulating seal around the
retrofitted unit.  Also the butyl compounds used in primary seals
can vary in their resistance to the effect of ultraviolet solar
radiation.  It is recommended, therefore, that flat exterior trim
be applied around the edges of the rails and stiles - even where
not needed visually - in order to reduce the possibility of the
degradation of the seal as a result of sunlight.

Attractive cost savings over all new sealed units can be realized
with this retrofit system, particularly when dealing with arched
top sash or windows that vary in size.  There are, though, certain
obvious limitations.  Where the interior moldings around the inside
edges of the rails and stiles are decorative, the manner in which
the additional sheet of glass is installed would obscure such
detailing.  With multipane sash and typical muntin sizes, the
encroachment onto the historic glass area would normally be
excessive even where the window structurally might accept the
additional glass weight.  Yet certainly this form of retrofitting
insulating glass into large single-light windows on upper floors of
structures the size of the Delaware Building easily can be achieved
in many cases permitting the preservation of both the material and
appearance of the historic windows while improving their energy



-    Delaware Building
    36 West Randolph
    Chicago, Illinois


-    Delaware Building Partnership
    Chicago, Illinois



-    Fred Matthias, AIA
    Bernheim, Kahn, and Lozano
    Chicago, Illinois


-    Wilbert R. Hasbrouck, FAIA
    Historic Resources
    Chicago, Illinois


-    Gail Wallace
    Signa Systems, Inc.
    307 North Michigan Avenue
    Chicago, Illinois


-    The total window cost for floors 3 through 8 (102 windows) was
    $22,262, with the per window cost being $170.62 for the
    insulating glass work and $42.63 for painting, caulking and

                             END OF SECTION
Last Reviewed 2012-02-24