Historic Preservation - Technical Procedures
Preservation Tech Notes: Windows 16 Repairing And Upgrading Multi-Light Wooden Mill Windows
Preservation Tech Notes, National Park Service, Pad
Doors And Windows
Preservation Tech Notes: Windows 16 Repairing And Upgrading Multi-Light Wooden Mill Windows
PRESERVATION TECH NOTES: WINDOWS Number 16
REPAIRING AND UPGRADING MULTI-LIGHT WOODEN MILL WINDOWS
Christopher W. Closs
Closs Planners, Inc.
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.
Dover, New Hampshire
Sawyer Woolen Mills, built between 1864 and 1892 and operated by
the Sawyer family, produced uniforms for the Union Navy and later,
high quality worsted cloth and cashmeres. In 1899 Sawyer Mills
went bankrupt and was absorbed as one of the eight original mills
that formed the American Woolen Company. This national textile
giant dominated the domestic woolen industry for a half a century
and, in 1954, became part of Textron Corporation. The complex
consists of 22 interconnected structures, comprising a quarter of
a million square feet of space.
Eleven hundred multi-light double-hung wooden windows provided
maximum natural light and ventilation for the buildings, while
strongly defining the architectural appearance, function and scale
of this nineteenth century workplace.
Architectural planning for the rehabilitation of Sawyer Mills for
use as apartments began in mid-1983. The developer recognized
early in the planning process the importance of the design
characteristics of the original wooden windows and their critical
role in preserving the historic character of the mills.
Remarkably, nearly all of the original window sash remained in
1983, having survived more than a century in the harsh New England
climate and changing corporate ownership. After undertaking a
survey of existing window conditions and exploring alternative
windows in wood, vinyl, and aluminum, the developer was convinced
that repair was both feasible and economically realistic. With so
many windows involved, thorough planning of the window repair work
well in advance of construction was crucial.
Cost, technical capability and window performance were the key
considerations in developing the repair approach. The window
survey enabled the general contractor to estimate the number of new
window sash needed and to form an overall view of the repair work
required for sash, frames and sills. To facilitate this aspect of
planning, a window and millwork consultant was retained to analyze
survey data and to develop shop drawings and specifications for
The general contractor evaluated staff capability to direct and
execute such a large task and concluded the job could be done
effectively. Fortunately, the general contractor owned a complete,
mobile millwork shop managed by a master craftsperson and staffed
with several highly-skilled tradespeople.
The performance that could be anticipated from the repaired windows
was of paramount concern to the architect and owner. Two hundred
twenty-two apartments were planned, each with central heating and
air conditioning. Easily operable primary window sash combined
with storm window units were essential in order for the complex to
be operated economically. To meet the New Hampshire Energy Code,
respectable U and R values would have to be achieved from both
windows and exterior walls, which were uninsulated.
The window survey concluded that 800 of 1100 windows were
considered repairable. Eight different configurations of sash were
found, including several sizes of each configuration.
Following a close evaluation of typical deterioration problems in
a random selection of sash, repair criteria were established. In
cases where more than one frame member or where more than two
joints were deteriorated beyond repair, the sash was discarded.
Salvaged components were used for repair on other units. Muntins
were retained if at least half of the grid remained serviceable.
A significant number of window frames and especially sills were
found to be in unserviceable condition and required replacement.
Some frames had bowed, impairing operability, while many sills were
so deeply checked or split that an adequate water-shedding surface
could not be recreated. Panning the sills with aluminum was
rejected as an alternative to replacement because of the difficulty
in achieving a proper flashing detail without further damage to
frames and brickmolding.
Several types of interior storm windows were evaluated, including
a new style with vinyl frames and Lexan glazing. The type selected
was a standard triple track, aluminum, one-light-over-one unit,
with special narrow frame profile and a half screen. A key design
constraint imposed on the supplier was the requirement that the
interior perimeter of the storm window could not visually encroach
from behind the glass area of the primary sash, which would create
an unobtrusive appearance from outside.
The considerable tasks of removing glazing and paint, repairing
sash frames, sanding, priming, re-glazing and painting eight
hundred windows required organizing the work flow and labor force
in a logical sequence. The decision was made to undertake the work
on the site and to establish a mobile millwork shop adjacent to the
window repair shop so that the former could continuously supply the
latter. The shops had to be relocated only once during
construction. A key factor making the on-site window repair
approach feasible was that the millwork shop's variable workload
from window repairs was supplemented by specialty orders from
outside contracts, eliminating costly down-time.
The repair crews consisted of three groups: a millwork shop foreman
and assistant; a team of four window repairers (who were trained at
the beginning of the project); and a two-person window removal and
re-installation crew. Except for the shop foreman, these groups
were rotated routinely to avoid monotony and enhance safety and to
build skills within the crews, so that reserves were available in
the event of illness or injury. The window consultant and millwork
foreman provided skills training for the repair and installation
crews and ensured quality control of the repair work.
High quality materials were used in repairing sash and milling new
sills and window frames. Canadian eastern white pine, grades #1
and #2, were used exclusively. Canadian #1 white pine was the most
cost-effective material that could be procured knot-free and that
has proven to wear well in window construction.
Considerable money was saved by re-using original materials
wherever possible. All glass from the old sash was carefully
removed and stacked for re-use; approximately 60 percent of the
original glass was reused in the repaired sash. Cleaning the glass
was labor intensive and not entirely successful, since some
surfaces remained a little cloudy as a result of etching over the
years. An effort was made not to mix new glass with the old in
repaired sash in order to minimize differential reflectivity
outside. Existing sash cord pulleys were removed, cleaned and
lubricated, and reinstalled with new sash cord in each window
opening. Cast iron counter weights were found in their pockets for
the most part, and were also re-used.
***SASH REPAIR PROCEDURE***
Original sash were removed from their openings in groups of 20 to
30 units by the installation crew, who carefully checked that both
frames and sash were marked with the corresponding window survey
number. Once delivered to the repair shop, individual sash were
de-glazed, the glass stacked by size, and residual dirt, putty and
paint removed from the frames at the first work station. If simple
repairs only were needed (e.g., muntin replacement, filling holes
or gluing a cracked muntin), they were done and the window sent on
for sanding and priming. If more substantial repair was needed,
the sash was sent to a work station where milled bars and rails, a
whole muntin assembly, or mortise and tenon parts could be fitted.
Wherever possible, whole muntins were saved from otherwise
deteriorated sash and used for spare parts during repairs,
resulting in additional savings in milling costs. The repair shop
had 4 full-time employees.
The most typical problem encountered was loose or failed mortise
and tenon joints; this was remedied by drilling out the old pegs
with a slightly oversize bit and then drawing the sash frame
together tightly with pipe clamps. Fluted, hardwood cabinet
dowels, liberally coated with waterproof glue, were then driven in
to secure the joints. To allow for natural movement, the mortise
and tenon joints were not glued. In some instances, deteriorated
tenons were cut off and bored out, and new tenons installed, using
glue to secure the tenons in their seats but not inside the joints.
This worked well providing the receiving mortises were sound. If
muntins required selective replacement, this was done before
clamping. It was critically important to "true" each sash square
before re-pinning the corner joints.
Common tools used through this stage of the operation included an
electric de-glazing iron, propane torch with both narrow orifice
and flame spreader for putty removal, wire brushes and several
types of paint scrapers with varying profiles. Standard 1/4" or
3/8" (chuck size) hand-held electric drills, were used for joint
It was not necessary to remove all the paint from the wood sash
frames, but only enough to sand smooth and create a fresh bondable
surface to which paint could be successfully applied. In practice,
roughly 50-60 percent of the paint was removed.
Once the sash for a complete window were made structurally sound,
frames were hand-sanded and fully primed with a shellac-based
sealer. Sash were then reglazed in conventional manner and stacked
to await final finish with two coats of exterior-grade, oil-based
paint. The wood edges of the sash were left unpainted to avoid
interfering with hand-planing during fitting in final installation.
***ONSITE MILLWORK SHOP***
All repair and milling of replacement frames, sills, and
brickmolds, and components for such special features as the wooden
belltower finial and interior louvered office blinds, was performed
on-site. The mobile millwork shop was located in a room adjacent
to the repair shop and occupied an area 35' x 45'. The basic
equipment of the millwork shop included: 18" bandsaw, 10"
tablesaw, 36" lathe, a jointer, 12" planer, a molder/shaper
machine, several routers, and a floor-model drill press with
Profile gauges were used to create molding machine knives ground
specially to match the historic brick-mold that trimmed the window
openings. Templates were made of the arc of each type of segmental
arch window head, so that reproduction of deteriorated features
would be precise. Because the white pine stock available was of
insufficient dimension to replicate the width of the original arch
head, a bandsaw was used to cut segments which were laminated in
three pieces to form replacement arched window frame heads.
Replacement window sills were laminated similarly.
To maintain production and minimize waste, the millwork shop
continuously supplied the repair and re-installation operations
with common components such as muntins, bars and rails, frames,
sills and brickmold. Approximately one-third of the frames and
brickmolding required replacement. Where complete new frames were
required, these were produced and assembled by the millwork shop,
ready for priming.
***REINSTALLATION AND STORM WINDOWS***
New matching wood sash, manufactured in Springfield, Massachusetts,
were required to fill three hundred openings where the originals
were missing or beyond repair. Deliveries of the new sash were
scheduled to match the installation capacity of the project crews
with the output of the millwork shop. The new sash were delivered
pre-primed. A second re-installation crew was trained and put in
service during the peak production period in early 1985.
The re-installation crew was responsible for repairing or
installing new frames, mounting brickmolds and installing repaired
and new sash. Two "gun carriages" were constructed to facilitate
frame installation and to ensure safety for the crews. Each
carriage served as a cantilevered work platform (if fitted with
dolly wheels they would have resembled a naval gun carriage). They
permitted the installation crew to work safely on the exterior of
the window opening without staging (much of this work had to be
done over the river). The carriages had stops on the interior,
were counter balanced, and were moved from opening to opening as
frame repairs and brickmold work proceeded.
Re-installation of the sash was done in batches of 20 to 30 pairs
of sash, which although indexed to the original openings, often
required planing of the edges of stiles to achieve smooth
operability. For this a handheld, 3 1/2" power planer was
The installation of the aluminum interior storms occurred after the
primary windows were in place. Installation of the storm windows
was monitored carefully to ensure that aluminum frames were set in
a continuous bead of silicone caulk to provide a tight weather
seal. This was made easier by the absence of decorative casing on
all of the windows; storms were simply screwed fast to the flat,
three-inch weight pocket covers. A new beveled 1" wooden strip was
applied around the outside perimeter of the weight pocket covers
and the heads, and caulked forming an uninterrupted seal with the
brick masonry wall. Weight pockets were not insulated since the
counterweights remained operable.
To reduce the chances of moisture being entrapped between the storm
unit and the primary window, the repaired wooden sash were fitted
somewhat loosely, thus allowing for adequate venting. In practice,
this approach worked well. The first units installed were checked
during the winter of 1984 and exhibited no excess moisture or frost
In an energy-conscious era, this project shows that the repair of
historic wooden windows in mill buildings can be cost-effective and
energy-efficient with proper planning. The following measures need
to be considered in the design solution:
1. Proper and detailed survey evaluation is made of the existing
2. Repair procedures are designed and integrated into the overall
schedule nd work flow of the rehabilitation project.
3. Skilled and semi-skilled personnel are available, and
provisions are made for any necessary training.
4. Modern methods for upgrading the energy performance of
existing windows are integrated into the design.
The rehabilitation of the original wooden windows at Sawyer Mills
was successful in meeting historic preservation standards,
aesthetic considerations and window performance objectives.
Seventy-three percent of the original windows were preserved. The
actual costs only slightly exceeded the original budget allowance
of $440 per opening, excluding the new storm windows. The thermal
performance of the windows was upgraded and ease of operability
restored for the new residential use; both were accomplished with
little difficulty and at minimum cost. Moreover, the introduction
of interior storm windows was an entirely reversible solution that
caused no change or alteration to historic material and was
expected to reduce maintenance cleaning costs.
- Sawyer Mills
Dover, New Hampshire
- Sawyer - Bellamy Mill Associates
Dover, New Hampshire
PROJECT DURATION: 1983-1985
- Keyes Associates
Providence, Rhode Island
- Paul Mirski, AIA
Enfield, New Hampshire
- Christopher W. Closs, MNRP
Closs Planners, Inc.
Concord, New Hampshire
- Arthur L. Pepperman II
Heritage Preservation, Inc.
Laconia, New Hampshire
- Bonnie Brae Construction
East Waterboro, Maine
- The rehabilitation of Sawyer Mill cost approximately
$12,000,000. The window work cost about $616,000, or an
average of $555 per window including the $90 per window cost
for the fabrication and installation of the interior storm
- 800 Windows (Sash Repair)
Sash Repair $300 per window
Sill and frame work
(repair & replacement) $100
New wooden brick molding $20
Painting (included in overall
TOTAL $470 per window
- 300 Windows (Sash Replacement)
Sash Replacement $250 per window
All other costs the same
as the repair work
TOTAL $420 per window
Miscellaneous Window Cost: $15,000
- 1100 Interior Storms
Fabrication and Installation $90 per window.
Grade #1 and #2 eastern white pine (Canadian) DAP Glazing
END OF SECTION