Preservation Tech Notes: Windows 2 Installing Insulating Glass In Existing Steel Windows

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Preservation Tech Notes, National Park Service, Pad
Doors And Windows
Steel Windows
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 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 Foundati= on
         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 Mergenthaler Linotype Building is a six-story printing loft
building located in the South Loop Printing House National Register
District in Chicago.  The masonry building was constructed in 1883
and altered in 1917 with the addition of steel multipane hopper
windows on the side and rear elevations.  Recently renovated, the
building now has residential condominiums in the upper stories and
commercial space on the first floor.  

The historic steel windows on the otherwise plain secondary
elevations are a significant architectural feature, clearly visible
from the street.  The 15 large 20-light windows on the side of the
building measure 6' wide by 7' high with a center-pivoting hopper,
while the 9 smaller 12-light windows are the same height but only
3'9" wide.  On the rear elevation, there are an additional 38 steel
windows of 15 lights, measuring 3' by 7'.  The individual lights
are 14" by 20", a size common to pre-World War II steel sash.

During the renovation work on the building, the windows were
repaired at the site and insulating glass was used in place of the
existing single panes to improve the energy performance of the
windows and the marketability of the condominium units.


The architect for the project, Kenneth Schroeder, AIA, with the
Chicago firm of Kenneth Schroeder and Associates, was confronted
with a series of problems in dealing with the steel windows.  As is
commonplace, lack of routine maintenance had led to rusting and
mild corrosion of portions of the windows.  Some of the sections,
especially the hopper units, were bent, bowed or racked and a
number of the =AC" wire glass panes were broken.

In converting the building to residential use, ways of improving
the thermal comfort of the occupants were explored and cost-
effective means were studied for reducing energy consumption.
Furthermore, a design had to be selected for additional windows
that needed to be created in order to satisfy air and light
requirements for the new use.  Four possible approaches to the
windows were examined:

    -    Repair and repaint the existing windows;

    -    Repair existing windows and install insulating glass;

    -    Replace existing windows with custom-made, double-glazed
         steel sash; or

    -    Remove existing units and install new aluminum double-
         hung thermal sash.

The following were specific considerations in assessing the four
window alternatives:

1.   Any window alteration would have to be compatible with
    the historic character and configuration of the existing
    steel windows.

2.   Work on the windows beyond repair and repainting -
    including the possibility of all new replacement units -
    would have to be cost-effective in terms of increasing
    the marketability and value of the condominiums.

3.   The existing wire glass would have to be replaced with
    clear glazing since the wire glass was considered
    inappropriate for the new residential use.

4.   Any work on the windows would have to be accomplished
    within a tight time schedule.

5.   Addition of screening was desirable but not critical
    because the downtown area was not particularly insect-

Of the four possible window treatments, the two replacement options
- custom-made steel sash of a matching design, and bronze anodized
aluminum double-hung sash - were too expensive for the project; the
latter was less expensive but still cost on the average between
$400-450 per window.  Furthermore, the use of double-hung sash in
the large openings would have changed the historic character of the

For the required new window openings, a manufacturer of steel
windows was located who offered a stock unit similar to the
original design but with noticeable differences in the horizontal
proportions and overall dimensions.  These stock units were
significantly less expensive than the custom steel windows and were
selected for use in new openings since they were compatible with
the heavy commercial style and character of the historic windows.


The decision to repair the existing windows was based largely on
the costs involved.  Reglazing the windows using 3/8" insulating<= br> glass ran approximately $12 per double pane, while reconditioning
the frames and hardware and repainting added another $50-75 to the
average cost of each window.  The average cost of $250 per window
for this work was significantly lower than any of the proposed
replacement units, and most of the repair cost was a result of the

Only about half of the glazing cost was directly attributable to
the use of insulating glass since from the outset, the residential
use of the building tended to dictate from a marketing perspective
that the existing wire glass be replaced.  Thus it was decided that
in lieu of only repairing and reglazing with clear =AC" glass, which would have been the cheapest of the four identified window options,
sufficient benefits could be derived by using double glazing to
improve the window's energy performance.

In deciding what type of glazing to install, the depth of the T-
shape muntin bar was a constraining factor.  Since at least the
1930s, companies have been manufacturing double-glazed steel
windows.  In such cases however, the depth of the muntin bars was
usually greater than the 1=AC" found on the windows in the
Mergenthaler Linotype Building.  By using 1/8" glass instead of the
=AC" wire glass, it was possible to use insulating glass in the
existing windows, though an optimum distance between the double
panes in terms of energy performance could not be obtained.  

Thus it was decided that the cost of double glazing alone was
justified considering the advantages derived from increased energy
performance and reduced likelihood of condensation forming on the
windows despite the new residential use.  The double glazing also
created no significant weight problem since =AC" wire glass was being<= br> removed.


After the metal glazing angles and glass were removed, the old
frames were scraped off and existing paint layers and rust were
removed using sand under air pressure.  After cleaning, sections
from several old windows that had been removed were welded into the
remaining windows to replace missing or deteriorated metal pieces.
At the same time, racked metal sections, especially the center-
pivoting hopper units, were realigned by applying heat and
pressure.  All the metal was then painted with an anti-corrosive


The original wire glass had been held in place by glazing angles
bolted to the muntin bars.  In removing the glass it was decided,
due to deterioration of the glazing angles and the labor-
intensiveness of reinstallation, that new spring glazing clips and
steel sash glazing compound would be used instead.

The insulating glass had been delivered to the site, vacuum sealed
and with a continuous glazing edge.  After the bed putty was
applied to the sash, the insulating glass was installed with
glazing clips and face putty.

New replacement handles and chains were readily found for hopper
units where such pieces were broken, deteriorated or missing.
After the windows were painted and caulked, there still was air
infiltration around the hopper units, so adhesive-backed foam was
added as weather stripping around the hopper unit.


There were no real unforseen problems with the window work and the
job was finished on time and within budget.  Screens were not added
to the windows although it would be possible do to so if an
individual resident felt there was a need.  Condensation on the
windows has not been a problem due to the use of insulating glass;
reduced air infiltration after repair, caulking and weather
stripping; the presence of a good heat source near the windows; and
the southern exposure of the large windows.

On the whole, the reconditioned windows are performing well and are
providing the desired improvements in personal comfort.  There was
no real problem in marketing the condominiums and residents have
capitalized on the interior design opportunities provided by the
large windows and the loft space.

The Mergenthaler Linotype Building rehabilitation has shown that
the repair and upgrading of historic steel windows can improve
energy performance in a cost-effective manner and that the light on
the interior space from the large thin frame steel sash can be an
amenity in residential conversions of loft buildings.  Similar
rehabilitation projects have shown that there are a variety of ways
- both traditional and innovative - to preserve the character and
design of historic steel windows and to have them function



-    Mergenthaler Linotype Building
    531 South Plymouth
    Chicago, Illinois


-    Harper Realty
    134 South LaSalle Street
    Chicago, Illinois



-    Kenneth Schroeder, AIA
    Kenneth Schroeder and Associates
    714 South Dearborn
    Chicago, Illinois


-    The average cost for repairing each window, installing
    insulating glass, reconditioning the hardware and finishing<= br>     was $250.

                             END OF SECTION
Last Reviewed 2012-02-24