Historic Preservation - Technical Procedures

Preservation Tech Notes: Windows 10 Temporary Window Vents In Unoccupied Historic Buildings
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Preservation Tech Notes, National Park Service, Pad
Doors And Windows
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Preservation Tech Notes: Windows 10 Temporary Window Vents In Unoccupied Historic Buildings
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Charles E. Fisher
Preservation Assistance Division
National Park Service


Thomas A. Vitanza
Williamsport Preservation Training Center
National Park Service

This standard includes the bulk of information contained in the
original Preservation Tech Notes developed by the National Park
Service.  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.

Monocacy National Battlefield
Frederick County, Maryland


Located on the grounds of the Monocacy National Battlefield, the
Worthington House is a mid-19th century ell-shaped brick farmhouse.
Judging from the modest exterior, it is rather surprising to find
that the building contains noteworthy interior stenciling.  The two
front rooms on either side of the center stair hall and the stair
hall itself all have remarkably intact examples of "trompe l'oeil"
stenciled paneling combined with an egg and dart motif frieze

The National Park Service acquired the 282 acre Worthington
property in 1982, principally to protect this detached portion of
the battlefield from intensive development.  At the time of
acquisition, the farmhouse was vacant and severely deteriorated
with extensive water damage occurring as a result of major leaks
and a predominance of broken and missing windows.  Vines and
saplings were growing up through the building and roof, destroying
the mortar and displacing the bricks.  The one-story porch across
the front had collapsed, causing noticeable dislocation of the
front masonry wall.  In several areas large numbers of the handmade
brick had been scavenged from the exterior, leaving gaping holes in
the bearing walls.

With no immediate use planned for the building, it was necessary to
repair and stabilize the structure or lose it to deterioration.
Work was undertaken using limited funds to make the building
structurally sound, weathertight, and less vulnerable to vandalism.
Rather than using traditional mothballing techniques, which rely
heavily on temporary measures and the introduction of non-historic
elements, the project team utilized high quality but cost-effective
stabilization measures whenever possible to ensure the long-term
preservation of the historic building.  Temporary features, such as
window vents, were designed and installed in such a manner as to be
reversible and to cause little additional loss of historic fabric.


Situated on a very windy knoll, the Worthington House had several
immediate preservation problems.  The interior was waterlogged.
Rain entered through broken and missing windows and through the
deteriorated slate roof.  At the time of acquisition, the structure
had been occupied sporadically for approximately 10 years by
vagrants and had received no upkeep at all.  Rodent and insect
infestation was also contributing to the deterioration of the

Early work focused on the need to make the building as weathertight
as possible, yet allow for adequate ventilation.  Consideration was
given to devising a solution that would incorporate the window work
with a passive ventilation system.  It was recognized that if the
house was tightly sealed with insufficient ventilation, the
building would be particularly susceptible to condensation and
moisture damage.  Another factor to consider was that the building
would remain unheated and unoccupied for an undetermined length of

Neither boarding over the openings nor installing full sash
throughout would provide optimum ventilation on the interior.  This
would be required to deter fungal decay of the wood and to avoid
condensation damage to plaster walls and to their decorative
stencil work.  Hot daytime temperatures followed by cold nights in
the spring and early fall could result in significant condensation
damage to the plaster and stencil work.  Damage would be
particularly acute when nighttime temperatures fell below freezing.
Furthermore, the hot moist air of the long Maryland summer would
create problems, since high humidity can present a favorable
condition for fungal growth.  This is particularly true when the
drying effect of air movement, normally induced in an occupied
building is not present.  The potential for damage in these
circumstances was great.  Once wood absorbs enough moisture from
the hot humid air and if fungal attack begins, the process of wood
decay would enable the fungi to maintain the wood in a wet
condition since fungi reduces wood to water and carbon dioxide.
While such moisture problems could arise throughout the house, the
basement was particularly susceptible to such damage due to
moisture infiltration through the dirt floor, the below grade
location, and seepage through the walls and basement doors.


Since the stabilization plan did not call for the installation of
either a heating or a mechanical ventilation system, the solution
to the air circulation needs was to install window vents.  The
basic "rule of thumb" used by the project staff for determining the
amount of open air needed for good air circulation in this building
is to use 50 percent of the sash units for ventilation.  This
approach has been successfully used by the Williamsport
Preservation Training Center in previous projects.  Depending upon
individual conditions, some adjustment needs to be made based on
the layout of rooms, interior walls, door locations, and number and
location of stair shafts and windows.

Because cross-room ventilation was desirable, the location of the
ventilating louvers was critical.  With proper planning, natural
ventilation could be induced through the "chimney" or "updraft
effect" within the building by which warm air raises and escapes
through higher level vents, to be replaced with cooler air entering
at lower levels.

Good air movement would also tend to equalize interior and exterior
temperatures, thus lessening condensation problems within the brick
walls and on interior painted plaster surfaces.

The window louvers had to be located so as to promote cross-room
ventilation and avoid stagnant air pockets in the rooms.
Furthermore, improvements to the appearance of the exterior of this
long neglected building were desired.  Efforts were taken,
therefore, to locate as many of the louvers as possible on side and
rear elevations, thereby minimizing the visual impact on the front
elevation.  Full double-sash vents could be placed in some side and
rear windows to permit more glass on the front elevation.  Even the
glazing in the reconditioned or replacement windows would help to
facilitate air movement within the building, since the sunlight
passing through the glass would heat inside air and cause it to
rise out through upper floor level vents.  Cooler air entering
through the basement windows would replace the warmer air.

A survey of the building's 31 window openings established that on
the first floor all but one sash were either missing or beyond
repair.  Altogether, only about one-third of the individual sash
units were repairable.  Most of those that were reconditioned
required muntin replacement.  In order to save on the final
production costs involved in repairing or constructing the 52
individual sash units, all sash work was completed in one shop
operation.  The louvered vents were temporarily installed in lieu
of the glazed sash on the bottom half of most window openings as
part of the "mothballing" and stabilization efforts.


Wooden fixed louver vents were custom-made and installed.  The
easily fabricated louvers were sized to fit the lower sash opening
- 34 1/2" wide by 34 1/4" high on the first floor, while those for
the smaller second floor windows were only 25 1/4" high.  Full
units were installed in all single-sash basement windows, since the
window area was much less and the moisture problems more severe.
At the same time, the three attic windows were also replaced with
full louvers to encourage thorough multi-level ventilation.

Custom-built wooden louvers were selected over stock, pre-
fabricated metal vents for the following reasons:  most pre-
fabricated vent systems would require modifications of the historic
jamb in order to get a secure fit; a single style metal unit could
not be found to fit the variety of opening sizes and the depth of
the jamb; costs would be greater than making the custom units; and
most important, it was felt that the thin gauge metal units offered
little or no deterrent to unlawful entry.  The wooden units
presented a more secure system.

The louver frame was designed to fit snugly into the existing sash
tracks and simultaneously to secure the glazed upper sash.  An
added benefit of the 6" stock width is that it provided a fairly
rigid - and thus secure - louver frame.  The louver frame was
constructed of 1" x 6" shelf grade northeastern white pine; the
louver slats were made from 1" x 8" pine.  The spacing of the
louver slats did not exceed 4" in order to provide additional
lateral strength (and security) to the frame.  The relative
closeness of the slats also would make it more difficult to kick
out the grade level units.  The slats were set into the frame at a
45 degree angle by routing a 1/4" deep dado cut into the jamb of
the louver.  The exposed edges of the slats were plumb cut in order
to create a water drip on the exterior.

Prior to assembly, the louver members were primed using an alcohol
base paint in order to get at least one protective coat on all
surfaces.  After assembly, they were given one shop coat of oil
base exterior house paint.  A final coat was applied after
installation.  For aesthetic reasons, the paint color used on the
sash and trim was selected for the final coat on the louvers.

In order to secure the vents in place, common 6d galvanized box
nails were driven through the louver jambs into the sash tracks of
the historic window jambs.  To keep the jamb and stops from being
damaged by the louver installation, temporary blocking was set
between the parting bead and the inner and outer stops.  By
attaching the vents in this location, little damage was done as the
2 nails were driven into the sash track rather than an exposed
portion of the jamb.  Once the building is returned to use, the
lower sash will be installed and the nail holes will be filled with
wood putty.  Since the nails were driven in on the interior of the
building, nearly 3" from the exterior wall, adequate security was
achieved without driving the nails all the way in.  Thus it will be
relatively easy to grab onto the nail heads and back them out when
the vents are eventually removed.

After the louvers were secured in place, 1/16" mesh copper wire
screening was installed on the interior of the louver frame using
a 1/2" square wood frame.  The screening is an integral part of the
louver design.   This seemingly minor detail was necessary to
prevent the recurrence of insect, bird, and rodent infestation.
The 1/16" mesh was specified to keep out the ever-present mud-
dauber wasp, whose hive building instincts have no regard for
historic plaster or paint.

The cost of constructing and installing the louvers in 27 window
openings was around $1,800, including 17 full size louvers, 7
basement and 3 attic units.  This work was undertaken concurrently
with the construction and installation of the reconstructed window
sash and repairs to the frames, sills, jambs, and surrounding
brickwork.  The total cost of the window work was less than $9,000,
involving 31 window openings.


The window louvers installed in the Worthington House have proven
effective over the past two years in providing the necessary
ventilation for the building.  Neither fungal attack nor
condensation damage has recurred, and the interior air lacks even
the typically humid musty odor typically found in many older
buildings.  The louvers provide for good air movement within the
building and a greater equilibrium between interior and exterior
humidity levels and air temperatures, thus helping to protect the
historic plaster and the significant interior finishes.  The
installation of the louver system in conjunction with the other
sash work, and the overall exterior stabilization work has
stimulated an interest in finding a use for the structure.  As a
temporary solution to a complex set of problems, the louver vents
in the Worthington House have resolved a variety of issues.  When
used together with additional weatherproofing measures, this
venting solution can be adopted for use in other buildings being



-    Worthington House
    Monocacy National Battlefield
    Frederick County, Maryland


-    National Park Service
    Antietam National Battlefield
    Sharpsburg, Maryland

PROJECT DATE:  January-June 1983


-    Williamsport Preservation Training Center
    National Park Service
    Williamsport, Maryland

-    Douglas C. Hicks
    Project Supervisor
    Supervisory Exhibit Specialist

-    Thomas A. Vitanza
    Project Leader
    Historical Architect Trainee

-    William Hose
    Exhibit Specialist Trainee

-    Bruce Martin
    Woodworking Specialist


-    Material and labor for construction of the 17 full-size, 7
    basement and 3 attic louvers was approximately $1,800.  The
    material and labor cost for reconstruction of the sash,
    including glazing, painting, sizing and installation was
    around $5,200 (roughly $100 per sash unit), involving 21 pairs
    of double-hung sash and 7 basement and 3 attic windows.  All
    other related work for the 31 openings, including sizing and
    installation of the louvers, repair to window openings
    (repair/replacement of sills and jambs and related masonry
    work), painting, and installation of screening and blocking
    cost between $1,000 and $2,000.  Total window costs for
    complete sash and the louvers as well as installation and
    finish work was between $8,000 and $9,000.

                             END OF SECTION
Last Reviewed 2012-02-24