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A Systematic Approach to Evaluating Historic Wood Windows for Repair and Restoration

March 16, 2014

A Systematic Approach to Evaluating Historic
Wood Windows for Repair and Restoration
Steven R. Marshall, RRC, CDT, LEED AP;
Olga Hathaway; and Catherine Matathia, PE, LEED AP
Gale Associates, Inc.
163 Libbey Parkway, Weymouth, MA 02189
Phone: 781-335-6465 • Fax: 781-335-6467 • E-mail: ken@gainc.com
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Abstract
Historic windows are a key component of the fabric and character of a building. In many
cases, the windows are neglected and left to deteriorate. The visible deterioration of historic
wood and steel window systems, along with energy concerns, make these windows prime
candidates for replacement with modern metal- or plastic-based systems. This intermediate
presentation will provide architects and engineers with an overview of the materials and
construction methods of these historic window systems, with a primary focus on evaluation
and design for their restoration. A methodical evaluation, condition assessment rating
system, and repair techniques, including energy-saving weatherization techniques, will be
discussed.
Speakers
Steven R. Marshall, RRC, CDT, LEED AP – Gale Associates, Inc.
Steven Mars hall is a senior project manager for his firm. He specializes in site
investigations, design, administration, and coordination of roof, wall, and window projects.
Marshall has extensive experience in evaluation, repair, or replacement of roof, window,
curtain wall, storefront, door, and masonry wall projects. He has been responsible for the
management of over 30 historic window projects.
Olga A. Hathaway – Gale Associates, Inc.
Olga Hat haw y is a senior project engineer for her firm. She holds a master of science
in building conservation from Rensselaer Polytechnic Institute and a BSCE from
Northeastern University. Hathaway specializes in historic restoration of the exterior building
envelope. Her projects incorporate many National Register and landmark buildings. She has
also authored several technical articles, including “Restoring Historic Terra Cotta,” which
was featured in a national publication.
Nonpresenting Author
Catherine A. Matathia, PE, LEED AP – Gale Associates, Inc.
Cat herine Matat hia is a project engineer for her firm. She graduated from Tufts
University with a BSCE. She specializes in structural engineering and building envelope
evaluations and fieldwork for building projects, including investigations, analysis, design,
coordination, specifications, and construction administration. She has been responsible for
the field evaluation and assessment of historic wood and steel windows at over 30 historic
buildings at a National Historic Site. This includes field drawings of window details and
hands-on assessments of each window component.
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Renovation and preservation of historic
structures create many imposing obstacles
for a design professional. Maintaining a
structure’s historic fabric and character
is critical to success, with window restoration
playing a major role. Consultants
evaluating window assemblies have to fully
understand repair techniques used by professional
window restoration conservators.
In addition to familiarity with historic window
constructions and styles of different
periods, the consultant must comprehend
the types and origins of wood decay, selective
wood repair methods, sash stabilization
techniques, glazing, and hardware restoration
or replacement to properly and effectively
evaluate and design the rehabilitation
of historic windows. This knowledge helps
to restore the windows in a manner that
maintains the maximum amount of historic
significance of the window assembly and
helps protect against unnecessary and/or
damaging repairs that might be performed.
COMPILE ARCHITECTURAL AND
HISTORICAL INFORMATION
Confirm purpose and history of windows
In order to provide and execute an
appropriate repair and/or restoration plan,
the original intent and purpose of the
windows in the structure
should be considered.
For example, long runs of
fixed, continuous, clerestory
windows in a warehouse
are presumably to
provide adequate lighting
rather than ventilation
to the interior spaces.
Alternatively, casement
windows in a residential
building circa 1920 function
as an aesthetic element
and can provide a
relatively large amount
of fresh air and sunlight
to the interior. Additional
window factors, including
provisions for views of the outside and their
contribution to the overall historic significance
of the building façade, should also be
considered.
Establishing the building’s period of significance
is critically important. The period
of significance is often identified by an
historical architect or preservationist during
a study of the building and is then
incorporated into an historic nomination
form. It relates to when the building was
considered important, based on an historical
event, association with a significant
person, distinct characteristics of design
or construction, or its potential to yield
important information. Depending on the
treatment selected, all of the building’s
remaining historical features could be preserved
equally, even if they are of different
vintages, or the entire structure could be
restored to one time period that has been
identified to be its period of significance.
For example, a building that was originally
constructed in the 18th century could have
had multiple additions and changes in use,
leading it to incorporate wood windows from
the 18th, 19th, and 20th centuries, all of
which may be historic. Unless one period of
the building is identified to be of specific
significance, the appropriate approach
may be to restore each of the windows to its
specific time period.
Additional research should be performed
to determine if some or all of the windows
have been replaced and just appear to be
original. In some instances, when buildings
undergo major renovations during their history,
the original windows could have been
completely replaced. If original drawings
are not available, care should be taken to
measure and compare muntin, brick mold,
sash rail, and frame profiles for each window
throughout the building. Windows that
look original may not be when compared
to the other windows in the building that
may have been salvaged during the major
renovation. Typically, the slight changes in
the profiles of the sash rails and muntins
can differentiate one series of windows from
another. It is not uncommon to salvage the
existing window frames and only replace
the sashes.
Compile Historic Documentation
Gather any available drawings, details,
building elevations, and historic photographs.
These will assist in determining the
original configuration and location of the
A Systematic Approach to Evaluating Historic
Wood Windows for Repair and Restoration
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Photos 1 and 2 – An historic
photo of the Boston Fire
Station, Boston, MA (left),
and an historic post card
of the Beverly City Hall,
Beverly, MA (right), helped
identify the appropriate
window configurations for
these buildings.
windows and if any significant changes or
alterations have been performed. Sources
for this information include building owners,
libraries, local historical societies, preservation
groups, etc. Useful resources and
historical data include the following:
• Original building plans, façade elevations,
window schedules, and window
and wall details—any information
that indicates how the windows
were designed or constructed
• Previous reports or photographs that
document the windows or building
façade
• Knowledge of or records of local
practices or what was normally
installed by builders at that time
and in that region
• Books on traditional building details
or historic building codes
• Architectural guide books to identify
style of building, its vintage, and
typical associated window styles
See Photos 1 and 2.
ESTABLISH DOCUMENTATION
SYSTEM
After compiling the available historical
data and researching the building’s service
history, it is necessary to perform a cursory
review of the existing conditions of the
façade and windows. This preliminary and
limited review will allow for a more accurate
window assessment plan to be established.
Performing a cursory review of the windows
to determine general characteristics including,
but not limited to, material type, configuration,
operability characteristics, and
hardware, will help establish and streamline
the documentation system and improve
efficiency when performing more rigorous
field evaluations.
In addition to the initial review, a unique
numbering system for each window or
rough opening should be established. The
intent of the numbering system is to simplify
the documentation process and provide
accurate data collection for each window.
It also serves as a method to link the various
data forms (i.e., photos, window elevation,
reports, checklists and building elevations)
together so they can all be accessed
through this numbering system. Typically,
this alphanumeric window identification
should include the single letter representing
the referenced building’s exterior elevation
(i.e., N, S, E, W), floor number, and alphabetical
window designation starting from
left to right. For example, the window identification
number “N3A” would indicate the
first window from the left, on the third floor
of the building’s north elevation. If multiple
buildings were involved, the building name
or number would precede the identification
number. This unique numbering system
should be established prior to performing a
detailed window survey to reduce potential
confusion when the field notes and information
are compiled in the office at a later
time.
When using the building’s original architectural
drawings, we strongly recommend
comparing them to the building’s existing
façade prior to developing the numbering
system. It is not uncommon for windows
to have been enclosed or altered since the
building’s original construction. Once the
actual number of windows is known, then
the numbering system can be developed,
and modifications can be noted on copies of
the elevation drawings for reference in the
final report.
Prior to initiating field evaluations, a
window designation and condition/defect
checklist for each common window type
(i.e., wood) should be generated. This list
may include, but not be limited to, the following:
• Building name/number
• Window material type
• Window designation number
• Window operability type
• Window glazing type
• Window components and characteristics
• H ardware types
• Defect codes related to materials,
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Figure 1 – Window defect checklist.
including wood, glazing putty, paint,
etc. Defect codes are typically a
numbered rating system, such as a
zero to four (0-4), where 0 is “good
condition” or “no repairs required,”
and 4 is “major decay and substantial
replacement parts” or the “entire
window is required.” If the window is
blocked or concealed in some way,
then it should be noted that the unit
is not visible (NV) and could not be
evaluated.
• Window elevation sketch to show
appearance, dimensions, and location
of defects
An example of a window defect checklist
for a wood window is referenced in Figure 1.
ACCESS METHODS TO EVALUATE
WINDOW S
One of the most important aspects of
performing window assessments is the field
evaluation. After compiling the existing
architectural and historical data and establishing
a documentation system, field evaluations
are performed. Often, the exteriors
of windows are not easily accessible due
to adjacent buildings, site limitations, or
height restrictions. Due to historic window
complexities and potentially unique characteristics,
it is critical to have “hand reach”
access from the interior and exterior of each
window to establish accurate restoration
methods. There are several access methods
that may be utilized to reach difficult areas:
• Aerial Lifts — Aerial lifts, which
generally range in height from 30
to 150 feet, provide access and an
observation platform with the ability
to articulate to precise locations on
the building façade. Note that flat
and accessible grounds adjacent to
the building are required for the use
of an aerial lift.
• Swing Staging — Swing staging
offers a suitable platform for observation
and testing but is more suitable
for straight vertical drops with
a flat building geometry. Roof access
is required to set up and move
the swing staging, which can have
high cost implications and extensive
down time.
• Ground Observation — Ground
observation using high-powered binoculars
is useful to spot potential
problem areas or simply to verify
or acquire quantities of components.
High-powered binoculars and
vantage points—such as adjacent
buildings or roof levels—will help to
improve the field data collected.
• Rope Access — Rope access allows
for close-up observation of an elevation
when other access (i.e., aerial
lifts and swing staging) is too restrictive.
Rope access must be performed
by a qualified, properly trained person.
It is also necessary to provide
safety tie-offs and anchor points,
which can be limited on historic
facilities.
HISTORIC WOO D WINDOW
CON STRUCTION
Typically, historic wood windows were
manufactured using old-growth wood. Oldgrowth
wood, commonly used until approximately
60 years ago, is significantly more
durable than the new-growth wood generally
used today. Furthermore, the joints
of historic windows are typically mortise
and tenon, which is more durable than
the common mitered and glued joint used
today. The material
quality of the windows
and the ability
of these windows
to be disassembled
are critical factors
to achieve a fully
restored window,
which will result in a
reduction of air leakage
through the window
sashes. Air leakage
is the primary
cause of thermal loss
through an existing
wood window and a
common complaint
of building occupants
with historic
wood windows.
In general, wood
windows consist of a
sash and frame. For
the sake of simplicity,
the typical components
of a wood
double-hung window
will be described
below.
The sash (also
known as the operable
portion of the window) is typically constructed
of side stiles and rails at the top
and bottom. The stiles (right/left) and rails
(top/bottom) frame and secure the glass,
and are typically joined with a mortise and
tenon joint with an added fastener in the
form of a wood peg or wire nail. The sash
stiles and rails are then rabbeted on the
exterior face in order to receive glass and
glazing putty. A divided-lite sash is constructed
in a similar manner with the addition
of bars and muntins. The rails would
be mortised in order to receive the bars, and
the muntins are then installed between the
bars. A double-hung sash requires meeting
rails at the top of the lower sash and at
the bottom of the upper sash. The meeting
rails typically have a beveled surface that
fits together when the window is closed.
For ease of operability, double-hung windows
incorporate a parting bead, which is
a pressure-fit piece of wood that sits within
the frame groove in order to separate the
upper and lower sash when opening and
closing the window.
The window sash is set into the frame
and held in place with the parting bead and
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Figure 2 – Anatomy of a double-hung window.
sash stops. The head and jamb sections of
a window frame are typically constructed
of a sash channel, blind stop, and casings.
Within this pocket, created by these components,
are the sash weights and chords
or chains. The bottom rail typically sits over
the sill and adjacent to the interior stool.
Depending on the existing wall construction
of the building, additional decorative
trim, molding, and casings may be present.
Figure 2, taken from The Old House Journal,
presents the typical “anatomy” of a doublehung
window.
Additionally, wood windows come in
various configurations and operability classifications
including, but not limited to,
fixed, awning, casement, pivots, and project
in/out windows. These typical types are
represented in Figure 3.
IN-DEPTH FIELD ASSESSMENT
In order to establish proper restoration
techniques, identification and accurate
documentation of defective components are
required. As previously noted, the exterior
and interior conditions of each window are
documented through a specific evaluation
process. Certain defective characteristics
are more suitably determined from either
the interior or exterior of the window. For
example, operability should be tested from
the interior. Ropes and pulleys of wood
windows are reviewed from the interior
once exposed in the window pocket. Glazing
putty conditions are more easily observed
and assessed from the exterior.
A typical assessment plan would include
the following: Every accessible window of
the building will be evaluated at arm’s
length on the exterior and interior. Aerial
lifts, swing staging, or rope access will be
utilized to evaluate windows above the
ground level on the exterior. Study team
members will complete checklists for each
window by sketching the window elevation,
measuring the rough opening, sketching
significant details, noting all window
characteristics and deficiencies, and taking
photographic documentation. To maximize
the data catalogued, it is recommended that
each checklist, sketch, and other notes be
completed for the exterior and the interior
of every window. Photographic documentation
will include an overall photo of each
window from the exterior and interior, along
with any notable defects or characteristics,
such as brick molds, muntin profiles, wood
joinery, sash profiles, and hardware.
Document and detect both interior and
exterior components for the following:
• Wood components
• H ardware
• Paint
• Glazing
• Sealants
• Operability
Documentation on field checklist and
window elevation should contain the following:
• Defect numerical rating system
• Defect symbols for repairs (i.e.,
rotted, checked, or cracked wood;
peeled paint; failed sealant; wood
gouge; cracked glass)
In the above-mentioned spreadsheet,
note that each window component has a
level of defect code pertaining to each item.
For example, the wood of each parting
bead may be in satisfactory condition, with
no splitting or rotting. However, the parting
bead paint may be missing or failed.
Therefore, an appropriate restoration technique
for this particular component would
likely be to salvage the existing parting
bead, which will be prepared, primed, and
painted for reinstallation.
Glazing putty and paint deterioration
can be accurately represented as a percentage
or visually shown on the window elevation
sketch. Based on the cost of repairs
and economies of scale, it is considered
reasonable to assume that a 30% or greater
failure of a component may warrant complete
removal and restoration. For example,
glazing putty that is approximately 50%
deteriorated is significant enough to warrant
removal and replacement. This will
keep all glazing putties at similar installation
periods and help reduce the potential
for yearly failures as older putties left in
place start to deteriorate. Replacing all
provides a more consistent and predictable
maintenance schedule.
IDENTIFICATION OF HAZARDOU S
MATERIALS
It is important to note that historic
windows identified for restoration may
have received previous repairs or undergone
repainting campaigns prior to 1978, when
the use of lead paint became prohibited.
Therefore, prior to removing the existing
paint, the material must be sampled and
tested for lead. Often, a full restoration
of the existing windows begins with the
abatement of the existing lead paint, which
requires stripping the windows down to
bare wood. Window sealants and putties
could also incorporate hazardous materials,
such as asbestos or polychlorinated biphenyls
(PCBs), which must also be removed
prior to any restorations. It is not necessary
to test for the presence of hazardous materials
to complete an evaluation. If the primary
requirement of the evaluation is to just
note the current condition of the windows,
then it may not be necessary for hazardous
material testing at that time. If budgeting
is part of the report, then testing should
be performed, since it can significantly
increase the cost of the restoration, especially
if PCBs are present. PCBs have been
known to leach into porous substrates,
including wood and masonry. Since testing
for these materials will add cost to the initial
evaluation, they should be discussed prior
to arriving on site. It is recommended that
persons trained in sampling and testing be
responsible for collecting the materials.
PLANN ING FOR REPAIRS
Once the condition assessment of the
windows is complete, the designer is in a
position to consider the scope of repairs/
rehabilitation. Depending on the goal and
budget of the project, the scale of repairs
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Figure 3 – Typical window types.
Note that these diagrams are to show
operability types only and do not
reflect the typical multilight glazings
and decorative muntins of historic
wood windows.
may vary from stabilization of the windows
to reduce further deterioration, to wholesale
restoration of the sashes and frames,
to thermal upgrades such as the addition
of weatherstripping or storm windows. The
levels of repairs for wood windows are best
classified by “Technical Preservation Brief
9: The Repair of Historic Wooden Windows,”
published by the National Park Service
(NPS), and are as follows:
• Routine maintenance
• Stabilization
• Splices and parts replacement
• Energy efficiency improvements
through weatherstrippings, interior/
exterior storms, and/or double glazing
REPAIR METHODS
While many of the wood windows surveyed
could appear to be in failed condition
to an untrained eye, the majority of the
deterioration may be occurring on the exterior
surfaces and limited to failed sealants,
paint coatings, and putties.
The proper repair of wood windows
is an involved and intricate process and
requires a skilled tradesman. Improper
materials and/or repairs can have a detrimental
effect on historic window renovations,
which can reduce, instead of extend,
their service life. Complete restoration of the
windows includes the removal of the existing
sashes and repair of the existing frames
in place. At the sashes, weatherstripping
can be replaced or added, defective muntins
replaced, damaged wood repaired and
repainted, and putties and damaged glazing
units replaced. At the frames, parting beads
and balances can be replaced, wood components
repaired, weatherstripping added,
all components repainted, and perimeter
sealants replaced. If thermal upgrades are
desired, options—including the addition of
insulated glazing or a storm window—can
be considered. All of these techniques are
further described below.
One of the most elementary and critical
steps for the stabilization of the existing
windows is the maintenance of the paint
coatings. Failed paint coatings can result
in damage of the wood components, resulting
in extensive repairs in order to return
the systems to proper operation. Proper
repainting of operable units involves removing
the existing sashes and painting the
frames and sashes separately. Painting the
sashes while they are in place can result
in a buildup of paint at the frame-to-sash
interface, which may cause the sashes to
be stuck in place and the windows to be
inoperable. In addition, painting would not
be able to extend within the sash channel,
leaving portions of the wood sash untreated.
Proper painting should also consist of
scraping off loose or chipped paint to limit
the buildup of paint layers. For components
that are removable—such as existing
sashes—one means of removing paint
is via infrared heating. For components
that remain in place, such as the frame,
chemical strippers are often utilized. Both
methods are typically used in combination
with hand scraping and sanding. Note that
all paint removal methods need to be done
in accordance with proper lead-abatement
procedures if lead-based materials were
identified.
Another key component of window stabilization
is the replacement of the existing
window putties. As previously mentioned,
the putties may be a hazardous material
and need to be tested prior to the work. If
the putties are found to contain asbestos
or lead (from being painted over with leadbased
paint), abatement of this material is
required. The replacement of putties both
weatherizes the windows and allows for
access to the glazing units to replace any
cracked glass pieces. Perimeter sealants
should be replaced in a similar method and
may also contain lead, asbestos, or PCBs.
In addition to paint and putty replacement,
many historic windows may also
require additional wood repairs. There are
two main types of wood repairs: epoxy and
dutchman.
Epoxy repairs consist of using an epoxy
resin or paste to fill in cracks, gouges, and
rot (after removing the existing rotted wood)
to stabilize the wood. Epoxy repairs are
often required at sash corners where the
joinery has failed. Epoxy repairs require
significant preparation of the existing wood
and may need several applications of epoxy
to completely fill the damaged area. Epoxy
requires time to set, and the repairs need to
be sculpted and shaved to match the profile
of the surrounding wood finishes.
Dutchman repairs involve repairing
existing areas of rotted or missing wood
with new wood. Ideally, the new wood
should match the species of the original
wood. If the species of the original wood
cannot be matched, the new wood should
be mahogany, cedar, or oak to be as similar
as possible to the density of the original
old-growth wood. With a dutchman repair,
the area of rotted wood is cut out, and the
replacement wood (dutchman) is cut to
match the opening. The dutchman is set
into the opening with a resin. Dutchman
repairs are most suitable for large-area
repairs. See Photos 3 and 4.
Other wood window stabilization repairs
include replacing broken or missing hardware,
sash lifts (if present), balances, and—
in isolated instances—weights. The main
factor of these repairs is locating appropriate
materials. For example, the original
windows are likely to use brass, bronze,
and steel for hardware. Balances are typically
supported or connected by rope or
metal chain and use metal tape pulleys.
Most historic weights are brass, but steel is
not unusual. Sometimes these replacement
materials are difficult to locate and require
searching in specialty restoration sites or
salvage yards.
Once the window has been stabilized,
its performance can be further improved
with the addition of weatherstripping to the
existing frame and/or sash components.
Weatherstripping can be metal, pile type,
or neoprene. Sheet metal weatherstripping,
which is composed of copper or brass, can
sometimes be found in the original window
assembly, nailed to the sash along the base
and at the meeting rail (two joints through
which air leakage can occur). If in good
condition, this weatherstripping may be salvageable.
If not, it can be replaced in kind,
or alternative forms of weather stripping
can be considered. Other commonly used
weatherstripping techniques include the
addition of metal weatherstripping to the
jamb tracks and integral pile weatherstripping
set into the sash stops (at the interface
of the stop and sash).
While the previously described repairs
both stabilize and weatherize the existing
wood windows, there are several options for
further improvements to the window’s thermal
performance. The first option, which
may be considered the most common and
the least invasive since it does not alter the
existing window fabric, is the addition of a
storm window. A storm window provides a
second seal, which acts like a secondary
glazing. The dead space between the window
and storm increases thermal resistance
for the assembly and reduces air leakage.
Furthermore, a low-emissivity (low-e)
glass storm window will provide additional
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thermal performance to the existing singleglazed
unit, making it comparable to a
double-glazed replacement system. Storm
windows can have a metal or wood frame
(aluminum is most common) and can be
installed on the exterior or interior of the
existing window unit. Properly installed
storm windows are continuously sealed
around the perimeter, with weeps left at
the bottom to allow air and condensation
to escape to limit the potential of condensation
between the window and storm. See
Figure 4.
Another means of thermal improvement
of the sash is replacing the existing
single glazing with insulated glazing panes.
This option is partially dependent on the
configuration of the existing sash, whether
the current wood is sufficiently thick and
durable to accommodate the much thicker
and heavier insulated glazing unit in lieu
of the original single glazing. For operable
sashes, the result of the added glass weight
will also likely require the replacement of
the existing sash pocket weights to accommodate
the heavier sashes.
If the addition of insulated glazing to
a window is feasible, there are still several
factors that should be considered from both
an aesthetic and performance standpoint.
From an aesthetic and historic perspective,
it is important to note that the insulated
glazing will impact the existing muntin profiles
and change shadow lines. The other
main consideration is the amount of thermal
improvement that can be achieved and
if alternate means, such as storm windows,
may provide a more feasible option.
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Photo 3 – Epoxy repair of
checked wood with resin.
Photo 4 –Dutchman repair
of wood sash corner.
CON CLUSION
The restoration of historic wood windows,
in lieu of replacement, is both the
historically appropriate and sustainable
option when repairs or replacement of
existing window systems are considered.
Too often, the choice to replace existing
wood windows is made by those who do not
have the appropriate qualifications to both
review the conditions of the existing windows
and understand that these windows
were constructed of quality materials in a
manner that facilitates their repair. In many
instances, the apparent conditions of the
existing wood windows make them an easy
target for replacement. However, the actual
defects may only be surface-deep and easily
repaired by the appropriate tradesman to
restore the condition of the wood components,
or even thermally improved with the
addition of weatherstripping, storm windows,
and/or insulated glazing. The repairs
and upgrades result in a window unit that,
with minimal maintenance,1 would extend
the service life of these windows by up to
30-50 years, which would likely exceed the
life of new metal or vinyl replacements.
Footnotes
1. Typical maintenance for wood windows
after a wholesale restoration
would include painting every five to
ten years and replacement of failed
sealants and glazing putties.
2 9 t h RC I I n t e r n a t i o n a l C o n v e n t i o n a n d T r a d e S h ow • Ma rc h 2 0 – 2 5 , 2 0 1 4 Ma rs h a l l e t a l • 1 3 1
Figure 4 – Wood window restoration details: stabilization repairs at the sash and frame and addition of weatherstripping
and exterior storm.