Historic Preservation - Technical Procedures

Preservation Tech Notes: Windows 6 Replacement Wooden Sash And Frames With Insulating Glass And Integral Muntins
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Preservation Tech Notes, National Park Service, Pad
Doors And Windows
Wood Windows
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Preservation Tech Notes: Windows 6 Replacement Wooden Sash And Frames With Insulating Glass And Integral Muntins
Last Modified:



Charles Parrott
Lowell Historic Preservation Commission
U.S. Department of the Interior

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 Lawrence-Wentworth House, originally the home of one of
Lowell's antebellum mill owners, has had numerous alterations and
changes in use since its construction in 1831.  Its original Greek
Revival street facade was altered sometime after the Civil War to
such an extent that it appears more Victorian than Greek Revival.

Beginning at the turn of the century, the single family residence
was converted to a boarding house, a succession of commercial uses,
and finally to offices for a social service organization.  Sometime
during this series of changes, the Victorian double-hung wooden
sash on the first floor were replaced with mill finish aluminum
jalousies.  The Victorian wooden sash, consisting of a two-over-two
(2/2) light configuration, survived on the second floor.

After experiencing several years of sizable increases in energy
costs, coupled with the inherently poor thermal performance of the
jalousie sash on the first floor, the owner, Unitas, Inc., a
service organization to Lowell's Hispanic community, came to the
Lowell Historic Preservation Commission requesting assistance in
replacing these visually obtrusive and thermally inadequate


The Victorian 2/2 sash on the second floor were still in
serviceable condition and were already fitted with storm windows.
Consideration was therefore given to the installation of 2/2
replacement sash and frames on the first floor that would match the
visual qualities of the remaining historic windows and at the same
time incorporate the energy efficiency features of double glazing
and weather stripping.  Another important goal was to reduce cost
without altering the appearance of the windows or affecting their


Studies have shown that when treated with a water repellent coating
and properly fabricated and installed, new wood windows will
provide long service.  Since the exterior wood siding, trim, upper
floor windows and painted masonry would all require periodic
repainting, this maintenance consideration was not a major factor
in the decision to install wooden replacement windows.

A full-scale measured drawing was made of an existing second floor
window as a guide in detailing the replacement window.  This
investigation revealed that the single-glazed 2/2 sash were 1 3/8"
thick, and that the entire width of the box frame was exposed on
the exterior.

In reaching the decision to install wooden windows, the important
technical issue was the treatment of the vertical muntin in both
the upper and lower sash.  The narrow appearance of the 3/4" muntin
in the historic sash presented some problems, since insulating
glass was preferred for the new windows and required a wider muntin
for proper installation.  The alternative of using new single
glazed wooden windows, with a separate interior or exterior storm
unit added, was less desirable in this case since such windows
would be more troublesome to open.

The selected replacement sash were designed to have two individual
lights of insulating glass in each sash with an integral (as
opposed to a "fake" or applied) muntin.  Based on the experience
gained by the Lowell Historic Preservation Commission in previous
projects, the muntin of the new sash was made only 1" wide, closely
matching the appearance of the historic 3/4" wide vertical muntins
remaining in the upper floor windows.  This slight change in muntin
width is hardly noticeable.  The results might have been different
if the old and new sash had existed side by side; if the number of
panes had been greater and the panes themselves been smaller; or if
the historic muntins had been thinner.

The new sash were made 1 3/4" thick, an increase of 3/8" over the
historic sash, in order to allow a sufficiently deep rabbet in
which to set the 7/16" insulating glass and to provide added
support for the double weight of the glass.


Along with full scale working drawings for the new window, written
specifications for both sash and frame fabrication and installation
were prepared.  These documents were sent to several window shops
and installation contractors to obtain separate quotations for
fabrication and installation.

The ten new windows were to be delivered fully primed and
assembled.  Of the ten windows, six were detailed for masonry
openings and four for frame openings.  No more than two windows
were the same size and there were seven different sizes in all.
Only the six principal windows, averaging 21 square feet each, were
of 2/2 configuration.  Replacements for the four smaller jalousie
windows, positioned in less prominent rear or side locations, away
from the front of the building, were designed in 1/1 light
configuration, but were otherwise identical to the larger windows.

Two types of a commercially-available rigid metal weather
stripping, formed from rolled zinc sheets, were installed in
preference to a less permanent vinyl, foamed plastic, or spring-
metal weather stripping.  At the heads, jambs and sills, the
weather stripping consists of a continuous flange over which fit
the grooved rails and stiles.  At the meeting rail, the weather
stripping consists of two interlocking hooks.  The weather
stripping protrudes only a short distance above and below the
meting rails along the jambs and is almost totally concealed when
the windows are shut.  It is extremely durable and is virtually
unaffected by corrosion or chemical decomposition.

The sealed insulating glass units, installed in the fabricator's
shop, were first caulked with a thin bead of non-harding water-
based (containing no oil) sealant.  The sealant was applied at the
corner of the glass unit so as not to touch the butyl compound used
to seal the edge of the insulating glass.  The water-based sealant
serves as an important barrier between the separate butyl-seal on
the insulating glass and the standard oil-based glazing compound as
used in the actual glazing.  The oil-based glazing compound was
chosen in preference to the standard wood molding strips to provide
a cheaper, more flexible and more weather-resistant glazing.  It
also matches the historic glazing treatment.

The historic windows in the Lawrence-Wentworth House were balanced
in standard fashion with sash weights and pulleys.  Since many were
missing on the first floor, less costly spiral tube balances were
specified for the new windows.  The spiral tube balances were
attached at their top to the face of the jamb near the top of the
window.  The longer balance tube for the lower sash, therefore, is
visible above the closed lower sash inside the building, just as
the sash cord is exposed on a weight balanced sash.  The tube
balances, however, are not seen from the exterior and their use
permitted a more energy efficient window frame.  The empty boxes,
which would have held the sash weights, were filled with
insulation; air infiltration was further reduced since there were
no pulley mortises in the frame.

The spiral balances also allowed the use of a less expensive L-
shaped, shop-fabricated frame and the look of the historic box
frame was accomplished with masonry-anchored nailers, steel framing
clips, and flat interior casing stock.

The new wooden frame was thus identical in appearance to the
historic frame on the building.  The width of the historic frame
was reproduced along with the wooden brick molding used to trim the
exterior of the masonry openings.


The ten windows were fabricated to specification, including such
features as wood preservative treatment and sash locks, for $2520
($13.40 per square foot).

The installation work, undertaken in 1983, included preparation of
the window openings; installation of the windows and interior tops;
and the attachment of exterior brick molding and all interior trim,
which had been selected from flat or molded stock.  Priming
unprimed elements and caulking were also included in the
installation work, which totaled $1800 ($9.52 per square foot).

Total cost of the ten windows less finish painting, which was done
as part of the general exterior repainting, was $4320 ($22.92 per
square foot).  Wooden frame half screens mounted on the interior
and set in aluminum tracks were also furnished and installed for a
total of $490 for the ten windows.


The window work on the Lawrence-Wentworth House shows the
practicality of replacing windows on a selective basis.  In
replacing only the first floor windows, significant cost savings
were achieved and the 2/2 Victorian windows on the second floor
were saved.  This project clearly shows that energy conservation
and other cost-reducing measures can be achieved in replacement
windows that reproduce the visual qualities of the historic
windows.  The use of spiral balances and insulating glass, the
increase in the sash thickness,modifications to the box frames, and
the slight widening of the integral wood muntin were accomplished
in a sensitive way in keeping with the Secretary of the Interior's
"Standards for Rehabilitation."  This approach has limitations,
especially when dealing with very thin historic muntins, where to
accommodate the weight of insulating glass and for suitable
glazing, the width of the muntin would have to be increased
substantially.  In many cases, however, involving two and four-
light sash, this application can be adopted without perceptibly
increasing the width of the muntin or diminishing the historic
character of the window.



-    Lawrence-Wentworth House
    48 Lawrence Street
    Lowell, Massachusetts


-    Unitas, Inc.
    48 Lawrence Street
    Lowell, massachusetts

PROJECT DATE:  Early 1983


-    Lowell Historic Preservation Commission
    204 Middle Street
    Lowell, Massachusetts


-    The Window Shop
    250 Chandler Street
    Worcester, Massachusetts


-    Weather Stripping

    Southern Metal Products
    3950 Swinner Road
    Memphis, Tennessee

-    Sash Balances

    Caldwell Manufacturing Company
    64 Commercial Street
    Rochester, New York

-    Sealed Insulating Glass

    Economy Glass Corp.
    315 Columbus Avenue
    Boston, Massachusetts


-    The fabrication cost, including priming, of the 10 widows (7
    different sizes) was $2520 ($13.40 per square foot);
    installation cost was $1800 ($9.52 per square foot); total
    cost was $4320 ($22.29 per square foot).

                             END OF SECTION
Last Reviewed 2012-02-24