Historic Preservation - Technical Procedures
- Preservation Briefs: 31 Mothballing Historic Buildings
- Procedure code:
- National Park Service, Preservation Assistance Division
- General Requirements
- Special Project Procedures
- Last Modified:
- Preservation Briefs: 31 Mothballing Historic Buildings
- Last Modified:
PRESERVATION BRIEFS 31:
MOTHBALLING HISTORIC BUILDINGS
The link immediately connects to the latest version of National Park Service Preservation Brief 31:
Sharon C. Park, AIA
This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:
Superintendent of Documents
P.O. Box 371954
Pittsburgh, PA 15250-7954
Please call the Publication Order Information Desk at 202/512-1800
or FAX 202/512-2250 to verify price and availability.
When all means of finding a productive use for a historic building
have been exhausted or when funds are not currently available to
put a deteriorating structure into a usable condition, it may be
necessary to close up the building temporarily to protect it from
the weather as well as to secure it from vandalism. This process,
known as mothballing, can be a necessary and effective means of
protecting the building while planning the property's future, or
raising money for a preservation, rehabilitation or restoration
project. If a vacant property has been declared unsafe by building
officials, stabilization and mothballing may be the only way to
protect it from demolition.
This Preservation Brief focuses on the steps needed to "deactivate"
a property for an extended period of time. The project team will
usually consist of an architect, historian, preservation
specialist, sometimes a structural engineer, and a contractor.
Mothballing should not be done without careful planning to ensure
that needed physical repairs are made prior to securing the
building. The steps discussed in this Brief can protect buildings
for periods of up to ten years; long-term success will also depend
on continued, although somewhat limited, monitoring and
maintenance. For all but the simplest projects, hiring a team of
preservation specialists is recommended to assess the specific
needs of the structure and to develop an effective mothballing
A vacant historic building cannot survive indefinitely in a
boarded-up condition, and so even marginal interim uses where there
is regular activity and monitoring , such as a caretaker residence
or non-flammable storage, are generally preferable to mothballing.
In a few limited cases when the vacant building is in good
condition and in a location where it can be watched and checked
regularly, closing and locking the door, setting heat levels at
just above freezing, and securing the windows may provide
sufficient protection for a period of a few years. But if long-
term mothballing is the only remaining option, it must be done
properly. This will require stabilization of the exterior,
properly designed security protection, generally some form of
interior ventilation - either through mechanical or natural air
exchange systems - and continued maintenance and surveillance
Comprehensive mothballing programs are generally expensive and may
cost 10% or more of a modest rehabilitation budget. However, the
money spent on well-planned protective measures will seem small
when amortized over the life of the resource. Regardless of the
location and condition of the property or the funding available,
the following nine steps are involved in properly mothballing a
1. Document the architectural and historical significance of
2. Prepare a condition assessment of the building.
3. Structurally stabilize the building, based on a
professional condition assessment.
4. Exterminate or control pests, including termites and
5. Protect the exterior from moisture penetration.
6. Secure the building and its component features to reduce
vandalism or break-ins.
7. Provide adequate ventilation to the interior.
8. Secure or modify utilities and mechanical systems.
9. Develop and implement a maintenance and monitoring plan
These steps will be discussed in sequence below. Documentation and
stabilization are critical components of the process and should not
be skipped over. Mothballing measures should not result in
permanent damage, and so each treatment should be weighed in terms
of its reversibility and its overall benefit.
Documenting the historical significance and physical condition of
the property will provide information necessary for setting
priorities and allocating funds. The project team should be
cautious when first entering the structure if it has been vacant or
is deteriorated. It may be advisable to shore temporarily areas
appearing to be structurally unsound until the condition of the
structure can be fully assessed. If pigeon or bat droppings,
friable asbestos or other health hazards are present, precautions
must be taken to wear the appropriate safety equipment when first
inspecting the building. Consideration should be given to hiring
a firm specializing in hazardous waste removal if these highly
toxic elements are found in the building.
DOCUMENTING AND RECORDING THE BUILDING:
Documenting a building's history is important because evidence of
its true age and architectural significance may not be readily
evident. The owner should check with the State Historic
Preservation Office or local preservation commission for assistance
in researching the building. If the building has never been
researched for listing in the National Register of Historic Places
or other historic registers, then, at a minimum, the following
should be determined:
- The overall historical significance of the property and dates
- The chronology of alterations or additions and their
approximate dates; and,
- Types of building materials, construction techniques, and any
unusual detailing or regional variations of craftsmanship.
Old photographs can be helpful in identifying early or original
features that might be hidden under modern materials. On a walk-
through, the architect, historian, or preservation specialist
should identify the architecturally significant elements of the
building, both inside and out.
By understanding the history of the resource, significant elements,
even though deteriorated, may be spared the trash pile. For that
reason alone, any materials removed from the building or site as
part of the stabilization effort should be carefully scrutinized
and, if appearing historic, should be photographed, tagged with a
number, inventoried, and safely stored, preferable in the building,
for later retrieval.
A site plan and schematic building floor plans can be used to note
important information for use when the building is eventually
preserved, restored, or rehabilitated. Each room should be given
a number and notations added to the plans regarding the removal of
important features to storage or recording physical treatments
undertaken as part of the stabilization or repair.
Because a mothballing project may extend over a long period of
time, with many different people involved, clear records should be
kept and a building file established. Copies of all important
data, plans, photographs, and lists of consultants or contractors
who have worked on the property should be added to the file as the
job progresses. Recording all actions taken on the building will
be helpful in the future.
The project coordinator should keep the building file updated and
give duplicate copies to the owner. A list of emergency numbers,
including the number of the key holder, should be kept at the
entrance to the building or on a security gate, in a transparent
PREPARING A CONDITION ASSESSMENT OF THE BUILDING:
A condition assessment can provide the owner with an accurate
overview of the current condition of the property. If the building
is deteriorated or if there are significant interior architectural
elements that will need special protection during the mothballing
years, undertaking a condition assessment is highly recommended,
but it need not be exhaustive.
A modified condition assessment, prepared by an architect or
preservation specialist, and in some case a structural engineer,
will help set priorities for repairs necessary to stabilize the
property for both the short and long-term period. It will evaluate
the age and condition of the following major elements foundations;
structural systems; exterior materials; roofs and gutters; exterior
porches and steps; interior finishes; staircases; plumbing,
electrical, mechanical systems; special features such as chimneys;
and site drainage.
To record existing conditions of the building and site, it will be
necessary to clean debris from the building to remove unwanted or
overgrown vegetation to expose foundations. The interior should be
emptied of its furnishing (unless provisions are made for
mothballing these as well), all debris removed, and the interior
broom-swept. Building materials too deteriorated to repair, or
which have become detached, such as moldings, balusters, and
decorative plaster, and which can be used to guide later
preservation work, should be tagged, labeled and saved.
Photographs or a videotape of the exterior and all interior spaces
of the resource will provide an invaluable record of "as is"
conditions. If a videotape is made, oral commentary can be
provided on the significance of each space and architectural
feature. If 35mm photographic prints or slides are made, they
should be numbered, dated, and appropriately identified.
Photographs should be cross-referenced with the room numbers on the
schematic plans. A systematic method for photographing should be
developed; for example, photograph each wall in a room and then
take a corner shot to get floor and ceiling portions in the
picture. Photograph any unusual details as well as examples of
each window and door type.
For historic buildings, the great advantage of a condition
assessment is that architectural features, both on the exterior as
well as the interior, can be rated on a scale of their importance
to the integrity and significance of the building. Those features
of the highest priority should receive preference when repairs or
protection measures are outlined as part of the mothballing
process. Potential problems with protecting these features should
be identified so that appropriate interim solutions can be
selected. For example, if a building has always been heated and if
murals, decorative plaster walls, or examples of patterned
wallpaper are identified as highly significant, then special care
should be taken to regulate the interior climate and to monitor it
adequately during the mothballing years. This might require
retaining electrical service to provide minimal heat in winter, fan
exhaust in summer, and humidity controls for the interior.
Stabilization as part of a mothballing project involves correcting
deficiencies to slow down the deterioration of the building while
it is vacant. Weakened structural members that might fail
altogether in the forthcoming years must be braced or reinforced;
insects and other pests removed and discouraged from returning; and
the building protected from moisture damage both by weatherizing
the exterior envelope and by handling water run-off on the site.
Even if a modified use or caretaker services can eventually be
found for the building, the following steps should be addressed.
STRUCTURALLY STABILIZING THE BUILDING:
While bracing may have been required to make the building
temporarily safe for inspection, the condition assessment may
reveal areas of hidden structural damage. Roofs, foundations,
walls, interior framing, porches and dormers all have structural
components that may need added reinforcement. Structural
stabilization by a qualified contractor should be done under the
direction of a structural engineer or a preservation specialist to
ensure that the added weight of the reinforcement can be sustained
by the building and that the new members do not harm historic
finishes. Any major vertical post added during the stabilization
should be properly supported and, if necessary, taken tot he ground
If the building is in a northern climate, then the roof framing
must be able to hold substantial snow loads. Bracing the roof at
the ridge and mid-points should be considered if sagging is
apparent. Likewise, interior framing around stair openings or
under long ceiling spans should be investigated. Underpinning or
bracing structural piers weakened by poor drainage patterns may be
a good precaution as well. Damage caused by insects, moisture, or
from other causes should be repaired or reinforced and, if
possible, the source of the damage removed. If features such as
porches and dormers are so severely deteriorated that they must be
removed, they should be documented, photographed, and portions
salvaged for storage prior to removal.
If the building is in a southern or humid climate and termites or
other insects are a particular problem, the foundation and floor
framing should be inspected to ensure that there are no major
structural weaknesses. This can usually be done by observation
from the crawl space or basement. For those structures where this
is not possible, it may be advisable to lift selective floor boards
to expose the floor framing. If there is evidence of pest damage,
particularly termites, active colonies should be treated and the
structural members reinforced or replaced, if necessary.
Pests can be numerous and include squirrels, raccoons, bats, mice,
rats, snakes, termites, moths, beetles, ants, bees and wasps,
pigeons, and other birds. Termites, beetles, and carpenter ants
destroy wood. Mice, too, gnaw wood as well as plaster, insulation,
and electrical wires. Pigeon and bat droppings not only damage
wood finishes, but create a serious and sometimes deadly health
If the property is infested with animals or insects, it is
important to get them out and to seal off their access to the
building. If necessary, exterminate and remove any nests or
hatching colonies. Chimney flues may be closed off with exterior
grade plywood caps, properly ventilated, or protected with framed
wire screens. Existing vents, grills, and louvers in attics and
crawl spaces should be screened with bug mesh or heavy duty wire,
depending on the type of pest being controlled. It may be
advantageous to have damp or infected wood treated with
insecticides (as permitted by each state) or preservatives, such as
borate, to slow the rate of deterioration during the time that the
building is not in use.
SECURING THE EXTERIOR ENVELOPE FROM MOISTURE PENETRATION:
It is important to protect the exterior envelope from moisture
penetration before securing the building. Leaks from deteriorated
or damage roofing, from around windows and doors, or through
deteriorated materials, as well as ground moisture from improper
site run-off or rising damp at foundations, can cause long-term
damage to interior finishes and structural systems. Any serious
deficiencies on the exterior, identified in the condition
assessment, should be addressed.
To the greatest extent possible, these weatherization efforts
should not harm historic materials. The project budget may not
allow deteriorated features to be fully repaired or replaced in-kind.
Non-historic or modern materials may be used to cover
historic surfaces temporarily, but these treatments should not
destroy valuable evidence necessary for future preservation work.
Temporary modifications should be as visually compatible as
possible with the historic building.
Roofs are often the most vulnerable elements on the building
exterior and yet in some ways they are the easiest element to
stabilize for the long term, if done correctly. "Quick fix"
solutions, such as tar patches on slate roofs, should be avoided as
they will generally fail within a year or so and may accelerate
damage by trapping moisture. They are difficult to undo later when
more permanent repairs are undertaken. Use of a tarpaulin over a
leaking roof should be thought of only as a very temporary
emergency repair because it is often blown of by the wind in a
If the existing historic roof needs moderate repairs to make it
last an additional ten years, then these repairs should be
undertaken as a first priority. Replacing cracked or missing
shingles and tiles, securing loose flashing, and reanchoring
gutters and downspouts can often be done by a local roofing
contractor. If the roof is in poor condition, but the historic
materials and configuration are important, a new temporary roof,
such as a lightweight aluminum channel system over the existing,
might be considered. If the roofing is so deteriorated that it
must be replaced and a lightweight aluminum system is not
affordable, various inexpensive options might be considered. These
include covering the existing deteriorated roof with galvanized
corrugated metal roofing panels, or 90 lb. rolled roofing, or a
rubberized membrane. These alternatives should leave as much of
the historic sheathing and roofing in place as evidence for later
For masonry repairs, appropriate preservation approaches are
essential. For example, if repointing deteriorated brick chimneys
or walls is necessary to prevent serious moisture penetration while
the building is mothballed, the mortar should match the historic
mortar in composition, color, and tooling. The use of hard
portland cement mortars or vapor-impermeable waterproof coatings
are not appropriate solutions as they can cause extensive damage
and are not reversible treatments.
For wood siding that is deteriorated, repairs necessary to keep out
moisture should be made; repainting is generally warranted. Cracks
around windows and doors can be beneficial in providing ventilation
to the interior and so should only be caulked if needed to keep out
bugs and moisture. For very deteriorated wall surfaces on wooden
frame structures, it may be necessary to sheathe in plywood panels,
but care should be taken to minimize installation damage by
planning the location of the nailing or screw patterns or by
installing panels over a frame of battens. Generally, however, it
is better to repair deteriorated features than to cover them over.
Foundation damage may occur if water does not drain away from the
building. Run-off from gutters and downspouts should be directed
far away from the foundation wall by using long flexible extender
pipes equal in length to twice the depth of the basement or crawl
space. If underground drains are susceptible to clogging, it is
recommended that the downspouts be disconnected from the drain boot
and attached to flexible piping. If gutters and downspouts are in
bad condition, replace them with inexpensive aluminum units.
If there are no significant landscape or exposed archeological
elements around the foundation, consideration should be given to
regrading the site if there is a documented drainage problem. If
building up the grade, use a fiber mesh membrane to separate the
new soil from the old and slope the new soil six to eight feet (200
cm-266 cm) away from the foundation making sure not to cover up the
dampcourse layer or come into contact with skirting boards. To
keep vegetation under control, put down a layer of six mil black
polyethylene sheeting or fiber mesh matting covered with a 2" - 4"
(5-10 cm) of washed gravel. If the building suffers a serious
rising damp problem, it may be advisable to eliminate the plastic
sheeting to avoid trapping ground moisture against foundations.
The actual mothballing effort involves controlling the long-term
deterioration of the building while it is unoccupied as well as
finding methods to protect it from sudden loss by fire or
vandalism. This requires securing the building from unwanted
entry, providing adequate ventilation to the interior, and shutting
down or modifying existing utilities. Once the building is
de-activated or secured, the long-term success will depend on periodic
maintenance and surveillance monitoring.
SECURING THE BUILDING FROM VANDALS, BREAK-INS, AND NATURAL
Securing the building from sudden loss is a critical aspect of
mothballing. Because historic buildings are irreplaceable, it is
vital that vulnerable entry points are sealed. If the building is
located where fire and security service is available then it is
highly recommended that some form of monitoring or alarm devices be
To protect decorative features, such as mantels, lighting fixtures,
copper downspouts, iron roof cresting, or stained glass windows
from theft or vandalism, it may be advisable to temporarily remove
them to a more secure location if they cannot be adequately
protected within the structure.
Mothballed buildings are usually boarded up, particularly on the
first floor and basement, to protect fragile glass windows from
breaking and to reinforce entry points. Infill materials for
closing door and window openings include plywood, corrugated
panels, metal grates, chain fencing, metal grills, and cinder or
cement blocks. The method of installation should not result in the
destruction of the opening and all associated sash, doors, and
frames should be protected or stored for future reuse.
Generally exterior doors are reinforced and provided with strong
locks, but if weak historic doors would be damaged or disfigured by
adding reinforcement or new locks, they may be removed temporarily
and replaced with secure modern doors. Alternatively, security
gates in a new metal frame can be installed within existing door
openings, much like a storm door, leaving the historic door in
place. If plywood panels are installed over door openings, they
should be screwed in place, as opposed to nailed, to avoid crowbar
damage each time the panel is removed. This also reduces poinding
vibrations from hammers and eliminates new nail holes each time the
panel is replaced.
For windows, the most common security feature is the closure of the
openings; this may be achieved with wooden or pre-formed panels or,
as needed, with metal sheets or concrete blocks. Plywood panels,
properly installed to protect wooden frames and properly
ventilated, are the preferred treatment from a preservation
There are a number of ways to set insert plywood panels into window
openings to avoid damage to frame and sash. One common method is
to bring the upper and lower sash of a double hung unit to the mid-point
of the opening and then to install pre-cut plywood panels
using long carriage bolts anchored into horizontal wooden bracing,
or strong backs, on the inside face of the window. Another means
is to build new wooden blocking frames set into deeply recessed
openings, for example in an industrial mill or warehouse, and then
to affix the plywood panel to the blocking frame. If sash must be
removed prior to installing panels, they should be labeled and
stored safely within the building.
Plywood panels are usually 1/2" - 3/4" (1.25-1.875 cm) thick and
made of exterior grade stock, such as CDX, or marine grade plywood.
They should be painted to protect them from delamination and to
provide a neater appearance. These panels may be painted to
resemble operable windows or treated decoratively. With extra
attention to detail, the plywood panels can be trimmed out with
muntin strips to give a shadow line simulating multi-lite windows.
This level of detail is a good indication that the building is
protected and valued by the owner and the community.
If the building has shutters, simply close the shutters and secure
them from the interior. If the building had shutters historically,
but they are missing, it may be appropriate to install new
shutters, even in a modern material, and secure them in the closed
position. Louvered shutters will help with interior ventilation if
the sash are propped open behind the shutters.
There is some benefit from keeping windows unboarded if security is
not a problem. The building will appear to be occupied, and the
natural air leakage around the windows will assist in ventilating
the interior. The presence of natural light will also help when
periodic inspections are made. Rigid polycarbonate clear storm
glazing panels may be placed on the window exterior to protect
against glass breakage. Because the sun's ultraviolet rays can
cause fading of floor finishes and wall surfaces, filtering pull
shades or inexpensive curtains may be options for reducing this
type of deterioration for significant interiors. Some acrylic
sheeting comes with built-in ultraviolet filters.
Securing the building from catastrophic destruction from fire,
lightning, or arson will require additional security devices.
Lightning rods properly grounded should be a first consideration if
the building is in an area susceptible to lightning storms. A high
security fence should also be installed if the property cannot be
monitored closely. These interventions do not require a power
source for operation. Since many buildings will not maintain
electrical power, there are some devices available using battery
packs, such as intrusion alarms, security lighting, and smoke
detectors which through audible horn alarms can alert nearby
neighbors. These battery packs must be replaced every three months
to two years, depending on type and usage. In combination with a
cellular phone, they can also provide some level of direct
communication with police and fire departments.
If at all possible, new temporary electric service should be
provided to the building. Generally a telephone line is needed as
well. A hard wired security system for intrusion and a combination
rate-of-rise and smoke detector can send an immediate signal for
help directly to the fire department and security service.
Depending on whether or not heat will be maintained in the
building, the security system should be designed accordingly. Some
systems cannot work below 32 F (0 C). Exterior lighting set on a
timer, photo electric sensor, or a motion/infra-red detection
device provides additional security.
PROVIDING ADEQUATE VENTILATION TO THE INTERIOR:
Once the exterior has been made weathertight and secure, it is
essential to provide adequate air exchange throughout the building.
Without adequate air exchange, humidity may rise to unsafe levels,
and mold, rot, and insect infestation are likely to thrive. The
needs of each historic resource must be individually evaluated
because there are so many variables that affect the performance of
each interior space once the building has been secured. A
mechanical engineer or a specialist in interior climates should be
consulted, particularly for buildings with intact and significant
interiors. In some circumstances, providing heat during the
winter, even at minimal 45 F (7 C), and utilizing forced-fan
ventilation in summer will be recommended and will require
retaining electrical service. For masonry buildings it is often
helpful to keep the interior temperature above the spring dew point
to avoid damaging condensation. In most buildings it is the need
for summer ventilation that outweighs the winter requirements.
Many old buildings are inherently leaky due to loose-fitting
windows and floorboards and the lack of insulation. The level of
air exchange needed for each building, however, will vary according
to geographic location, the building's construction, and its
general size and configuration.
There are four critical climate zones when looking at the type and
amount of interior ventilation needed for a closed up building:
hot and dry (Southwestern states); cold and damp (Pacific northwest
and northeastern states); temperate and humid (Mid-Atlantic states,
coastal areas); and hot and humid (southern states and tropics).
Once closed up, a building interior will still be affected by the
temperature and humidity of the exterior. Without proper
ventilation, moisture from condensation may occur and cause damage
by wetting plaster, peeling paint, staining woodwork, warping
floors, and in some cases even causing freeze thaw damage to
plaster. If moist conditions persist in a property, structural
damage can result from rot or returning insects attracted to moist
conditions. Poorly mothballed masonry buildings, particularly in
damp and humid zones have been so damaged on the interior with just
one year of unventilated closure that none of the interior finishes
were salvageable when the buildings were rehabilitated.
The absolute minimum air exchange for most mothballed buildings
consists of one to four air exchanges every hour; one or two air
exchanges per hour in winter and often twice that amount in summer.
Even this minimal exchange may foster mold and mildew in damp
climates, and so monitoring the property during the stabilization
period and after the building has been secured will provide useful
information on the effectiveness of the ventilation solution.
There is no exact science for how much ventilation should be
provided for each building. There are, however, some general rules
of thumb. Buildings, such as adobe structures, located in hot and
arid climates may need no additional ventilation if they have been
well weatherized and no moisture is penetrating the interior. Also
frame buildings with natural cracks and fissures for air
infiltration may have a natural air exchange rate of three or four
per hour, and so in arid as well as temperate climates may need no
additional ventilation once secured. The most difficult buildings
to adequately ventilate without resorting to extensive louvering
and/or mechanical exhaust fan systems are masonry buildings in
humid climates. Even with basement and attic vent grills, a
masonry building may not have more than one air exchange an hour.
This is generally unacceptable for summer conditions. For these
buildings, almost every window opening will need to be fitted out
with some type of passive, louvered ventilation.
Depending on the size, plan configuration, and ceiling heights of
a building, it is often necessary to have louvered openings
equivalent to 5%-10% of the square footage of each floor. For
example, in a humid climate, a typical 20' x 30' (6.1m x 9.1m)
brick residence with 600 sq. ft. (55.5sq.m) of floor space and a
typical number of windows, may need 30-60 sq. ft. (2.75sq.m -
5.5sq.m) of louvered openings per floor. With each window
measuring 3' x 5' (.9m x 1.5m) or 15 sq. ft. (1.3sq.m), the
equivalent of two to four windows per floor may need full window
Small pre-formed louvers set into a plywood panel or small slit-type
registers at the base of inset panels generally cannot provide
enough ventilation in most moist climates to offset condensation,
but this approach is certainly better than no louvers at all.
Louvers should be located to give cross ventilation, interior doors
should be fixed ajar at least 4" (10cm) to allow air to circulate,
and hatches to the attic should be left open.