Preservation Tech Notes: Windows 13 Aluminum Replacement Windows With Sealed Insulating Glass And Trapezoidal Muntin Grids

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Preservation Tech Notes, National Park Service, Pad
Doors And Windows
Aluminum Windows
Last Modified:



Charles Parrott
Lowell Historic Preservation Commission

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.

Lowell, Massachusetts


The three connected buildings of the former Suffolk Manufacturing
Company in Lowell, known as Mills Nos. 5, 6, and 8 were built in
1862 (No. 5) and 1880 (Nos. 6 and 8), at a time when the city of
Lowell was one of the predominant centers of the textile industry
in the United States.  Located within the Locks and Canals National
Register Historic District, this impressive complex of mill
buildings housed, in more recent years, a synthetic textile weaving
operation under the ownership of the Wannalancit Textile Company.
In 1981 the buildings were acquired for development as the
Wannalancit Office and Technology Center, a project involving
286,000 square feet of rental space.

The long unbroken pattern of some 900 small-paned windows dominates
the facades of these buildings and is clearly their most
significant architectural element.  At the time of the conversion
to an office complex, the original double-hung windows, arranged
with 12-over-12 lights, had survived in Mills Nos. 6 and 8, but
virtually all the sash and some of the frames had been removed from
Mill No. 5.


Appropriate window treatments quickly emerged as the major
preservation design problem in this $8.5 million rehabilitation.
From the outset it was established that the appearance of the
historic windows would have to be retained on all elevations; this
meant duplicating such features as the reveal, trim detailing,
double-hung configuration, and particularly the multi-pane
appearance created by the muntin pattern.

The first step was to evaluate the condition of the original
windows that had survived in Mills No. 6 and 8 to establish whether
the historic windows were repairable.  Based on an indepth
inspection, the windows were judged beyond reasonable repair due to
their deteriorated condition.  Once the decision to replace the
historic sash had been reached, consideration was given to
replacing them with matching custom-made wooden windows.  The
developers obtained a quotation of $875 per window for fabrication
and installation of 900 matching wooden replacement units, with
integral muntins in a 12-light configuration and the addition of an
interior storm panel.  Due to cost, the developers decided to
consider other alternatives.

The decision was reached to evaluate the cost and appearance of a
non-wooden, double glazed, prefinished, single-hung window with
applied muntin grids on the exterior rather than integral muntins.
The objective was to determine whether a non-wooden commercially
available window could loosely match the configuration and
appearance of the historic windows.  To achieve this result,
several window manufacturers were invited to install field mock-
ups.  Six manufacturers responded - four aluminum windows were
installed as part of the selection process, along with one vinyl
and one aluminum-cladded wooden unit.

The criteria used in evaluating the field mock-ups included
performance and cost, but focused primarily upon appearance due to
the significant contribution of the windows to the historic
character of the building.  In considering the various proposed
window replacements, criteria established for the window work

1.   Retaining the historic reveal of the window - the location of
    the sash relative to the outer wall surface.

2.   Matching the double-hung window style and having sufficient
    depth between the glass planes of the upper and lower sash to
    create an appropriate shadow where the upper sash overlaps the
    lower sash.

3.   Matching as closely as possible the proportions and the width
    of the sash members, such as the stiles and meeting rails.

4.   Duplicating the glass pattern of 12 lights in each sash.

5.   Having an applied muntin grid that was not only permanent, but
    that closely approximated the historic shape and width and
    also had sufficient depth to create good shadow lines.

6.   Having appropriate paint color.

7.   Retaining the appearance of the historic frame and brick
    molding details.

8.   Incorporating energy conservation features.

One of the aluminum window mock-ups did contain real muntin bars,
dividing each sash into 12 individual, double-glazed lights.  The
width of the muntins, however, was far too wide.  With the
insulating plate, there were technical limitations that prevented
the manufacturer from making any major modifications, especially in
the size of the muntins.


One of the aluminum windows did stand out as the most promising
match of the historic units, and with further modifications
eventually was selected as the replacement window for the project.
The replacement unit was one of the manufacturer's standard single-
hung sash models modified to accept an exterior muntin grid on each
sash, creating the appearance of 12 divided lights.  The window
assembly also incorporated a custom-extruded and curved aluminum
pan over the existing wood frame and trim.


The historic wooden frames were retained and used as a structural
subframe on which to fasten the new aluminum trim and frame
members.  A custom extruded aluminum pan was used that fitted
snugly around the exposed face of the original frame and maintained
the face width and depth of the casing at both jamb and head.  The
original wooden brick molding (which covers the joint between the
masonry and the window frame) was fitted with a custom-extruded
aluminum pan.

At the head of the wooden frame, the aluminum pan was constructed
in two pieces, both custom extrusions.  The flat face of the lower
piece was cut on a curve to follow the segmental curve of the
masonry opening.  The upper piece, which replaced the original
brick molding, was custom-bent to the segmental curve and blind
attached to the lower piece of aluminum.  An additional aluminum
pan was fitted tightly to the original wooden window sill.

When the aluminum window frame was inserted, it included its own
sill in the place where the bottom rail abutted the sill of the
original window.  This second sill unfortunately added an element
not present in the original window.


A new type of applied muntin grid was developed to give the
appearance of a 12 light division in each sash.  To avoid the
flatness of most applied metal muntin grids, the aluminum sections
were extruded in a trapezoidal shape to resemble more closely the
historic shape of the rabbeted wooden muntin and beveled putty
seal.  The 3/4" wide muntin bar has an exterior depth of 1/2",
dimensions nearly identical to those of the original wooden
windows.  Even though the muntin bar does not extend through to the
inside, the field mock-up showed that the shadow lines were
sufficiently strong to create from the exterior the overall effect
of a 12-light sash.

The manufacturer's stock sash accommodated the muntin grid with
only minor modification.  The stock sash contained a single light
of 7/8" thick insulating glass (two 1/8" glass panes separated by
a 5/8" air space) fastened in a 1-1/4" thick sash.  As modified for
the muntin grid, the insulating glass was narrowed to 1/2" (two
l/8" glass panes separated by a 1/4" air space) to provide the
depth for the grid to be contained within the sash.  The beveled
edge of the muntins was continued around the glass edge of the
rails and stiles as well, in order to duplicate the angled putty
seal line of the original sash.  The grid was securely set into the
sash frame as the sash was being assembled.

Visually, the exposed face of the aluminum window is wider than the
wooden sash because the face of the new frame and sash are
practically in the same plane and read as one.  As a result, the
face width of the new upper sash stiles (which visually includes
the vertical face of the new aluminum frame) is wider (2-1/2" vs.
1-1/2") than the face width of the historic sash.  At the lower
sash, the new frame stands out as an additional member not present
in the historic windows.

An increase in size also occurs at the meeting rail.  The resulting
encroachment on the historic glazed area is noticeable, though
fortunately the large window opening lessens the visual impact of
the heavier meeting rail.  Redesign of the meeting rail to achieve
a narrower face dimension was not attempted for this project
because any change in these extrusions would have necessitated
retesting of the window with the new meeting rail to determine its
compliance with the standard Architectural Aluminum Manufacturer's
Association (AAMA) specification for this window - a costly and
time-consuming process.


Windows of Lowell's textile mills were usually panted light colors
- often white - in the 19th century.  Through paint research, it
was established that an off-white was the original color of the
windows at the Suffolk Mills; later in the 20th century they had
been painted dark green.  In an effort to recreate the light value
of the original color without necessarily duplicating the exact
hue, a cream color was selected.  The thermosetting acrylic enamel
paint was factory-applied to the extrusions before window assembly.
Due to the large number of windows involved, the cost of the custom
color was negligible and did not affect the construction schedule.


The historic window openings were prepared for the new aluminum
windows by removing interior sash stops, the parting beads, sash
pulleys and exterior brick molding at the head.  For each window,
the head, jamb and sill panning was then applied using the
extrusions custom-made for this project.

The new window units were delivered to the site preassembled.  The
operable bottom sash was temporarily removed from the new aluminum
window unit, and the unit (frame and fixed upper sash) was set into
the old sash opening and screwed into the pulley stiles of the old
window frame.  Finally, the operable bottom sash was installed and
all exterior joints caulked.  Later, flat wood trim was placed
around the interior joints and complete the enclosure of the
original frame.


The replacement of the windows at the Wannalancit Office and
Technology Center was a pioneering effort in using stock aluminum
windows specifically modified for large historic industrial
buildings with single-hung sash and multiple divided lights.  The
results achieved in adapting these existing stock units are
noteworthy, including the high cost savings achieved by adapting
existing units rather than developing and producing completely new
aluminum windows.  Both the developers and the preservation review
groups involved felt that the performance criteria and visual
considerations were satisfactorily met in this case, especially
considering the then-prevailing state-of-the-art.  Subsequent
modifications in similar window systems over the past several years
have achieved even a closer match of the visual characteristics of
historic windows.

Insulation and infiltration values of the new aluminum windows were
considered acceptable by the owner; the modifications made to the
stock units only minimally reduced the energy efficiency.  The
insulation U-value of the window with 1/2" insulating glass
measures .62 while the 7/8" insulating glass achievable in the same
window without the muntin grid would have been .54.  The
infiltration rating for this DH-A2.5 H.P. specification window with
a fixed upper sash and 7/8" insulating glass without the grid is
.08 cfm per foot of crack - well below the .50 maximum allowable
set by AAMA.  Presumably the 1/2" insulating glass with grid in the
same window had little effect on the air infiltration rating,
although this assembly was not tested.  Besides the energy savings
that will be realized, the maintenance cost of the windows over the
first 20 years is anticipated to be less than wooden windows,
principally because of the factory finish.

The windows likely will require more frequent cleaning than normal.
Atmospheric dirt deposited on the exterior glass surface has tended
to wash behind the muntin, depositing a slightly heavier than
normal dirt film just below the muntin as the dirt washes down
under normal weathering.  Fortunately both the fixed upper and the
operable lower sash are removable from the inside for cleaning.

For purposes of preserving the historic appearance of the building,
matching the visual details of the historic wooden windows at the
Suffolk Mills was of foremost importance.  It was felt that the
aluminum window maintained the overall appearance of the original
window.  This was achieved by retaining the historic frame width,
suing custom aluminum brick moldings (including a curved head
molding), setting the two sash in offset glazing planes as in the
traditional double hung window, incorporating an exterior grid on
each sash simulating the trapezoidal muntins of an authentic wooden
window, and using appropriate paint color.

Development of these window features shows that the aluminum window
industry can make improvements in window lines to meet the needs of
the historic retrofit market and that such changes can be made
without altering the operating characteristics inherent in these
windows.  Other aluminum window designs, no doubt, can also be
successfully modified to achieve similar results, although some
custom work, such as special extrusion for pannings and grids,
painting finish, and fitting will often be necessary.

The philosophical issue of using what is essentially an artificial
recreation of a true divided light muntin grid was considered
during the development of this project.  The primary concern in
this project was visual - not physical - separation of the panes.
Even if cost were not a concern, the state-of-the-art development
of insulating glass muntin widths narrow enough for divided lights
with muntins under 1" wide had not been achieved in an aluminum
window.  The fabrication of a deep trapezoidal exterior muntin grid
in aluminum - one that was permanently and solidly attached - was
something of a milestone itself.  If not the first instance of its
use in this way, it was one of the earliest.  The initial response
to the request for the exterior grid was the supply of a double
ovolo molding extrusion by the manufacturer.  Through discussions
with representatives of the Lowell Historic Preservation Commission
and the National Park Service, the developer and the window
manufacturer were persuaded that the trapezoidal shape should
instead be used, as it was the historically correct shape for the
exterior of the muntin and one far more appropriate in the
replacement unit.

The window solution at Suffolk Mills was an important improvement
over past efforts in similar rehabilitation of large scale historic
mill complexes where aluminum windows had been installed as
replacement units.  While in many cases the historic character of
specific buildings would preclude the use of such a retrofit
solution, it has applicability to many large-scale buildings where
the existing windows are beyond repair and where replacement with
wooden windows, even though upgraded in thermal performance, is not
a viable alternative.  The experience of this project also suggests
that opportunities exist for still other refinements of aluminum
window design and detailing.  Foremost, it illustrates the need for
advance planning and the willingness of the developer, the window
manufacturer and preservation groups to work together, as they did
in this case, to improve the quality of replacement windows
installed in historic buildings.



-    Wannalancit Office and Technology Center
    Mills Nos. 5, 6 and 8 of the former
    Suffolk Manufacturing Company
    650 Suffolk Street
    Lowell, Massachusetts


-    Doborth and Fryer, Inc.
    650 Suffolk Street
    Lowell, Massachusetts

PROJECT DATE:  Spring 1983


-    Perry Dean Rogers, Inc.
    177 Milk Street
    Boston, Massachusetts


-    North American Manufacturing, Inc.
    551 Concord Street
    Holliston, Massachusetts


-    Atlantic Window Co.
    15 Carr Road
    Saugus, Massachusetts


-    National Park Service
    Lowell Historic Preservation Commission
    Massachusetts Historical Commission


-    The total fabrication and installation cost was $415 each for
    906 windows on Mills Nos. 5, 6 and 8. The supply cost was $240
    per window; the remaining $175 included removal of the old
    window and the installation and trimming of the new window.
    The per square foot cost was $12.20 on windows 4' wide by 8
    1/2' tall.  An additional 250 windows will eventually be
    installed in Mill No. 10.

                             END OF SECTION
Last Reviewed 2012-02-24