This paper was presented at 1993 AIC Meeting in Denver. Mr. Lull's
presentation at the 1993 annual meeting also discussed renovations to the
Pinkney House at the Kern County Museum in Bakersfield, California. A paper
on that project may be found in the Objects Specialty Group Postprints 1991,
Volume One, pages 102-107.
Recent Planning Experience in Balancing Collection and Building Preservation
Needs: Improvements to the Mercer Museum
William P. Lull
The improvement of the collection environment in the Mercer Museum in
Doylestown, Pennsylvania, presents several unique challenges. These include
an historic solid concrete building; no present heating, cooling or humidity
control; an environmentally sensitive collection; little hope of installing
a ducted air system; and problems with condensation, high light levels and
water leaks. The Museum developed a multi-disciplinary approach under an
IMS grant to address these problems. Working independently, the original
precept of the consultants at the time of the grant application was to
develop a scheme of humidity-controlled heating to reduce high humidity
conditions. However, when the consultants met for the on-site meeting after
inspection of the building a different approach was developed that would be
less intrusive to the building fabric and more consistent with other
treatments. The solutions identified included interior window treatments
for light and condensation control, improvements to the exterior building
details, and improvements to the operable windows and ventilation.
Contingencies were developed for modest humidity-controlled heating and
increased ventilation in the event that the other treatments proved
inadequate to control the high humidity and high temperature problems.
The main building at the Mercer Museum is an important part of the Museum
collection. It no only holds the bulk of Mr. Mercer's original collection
but is itself an artifact of early poured-in-place concrete construction.
It distinguishes itself as a building originally intended to be a Museum and
built in the form of a castle, is an important landmark for the area. The
building has problems in the collection environment it provides for the
collection. While there are some inherent benefits to the building as an
unconditioned space it has some environmental problems, primarily high
humidity, high temperatures and water leaks. To address this complex
problem the Museum, under the supervision of Mr. Cory Amsler, Curator of
Collections, assembled a multi-disciplinary team under an IMS grant. The
collection needs were represented by Kory Berrett, Berrett Conservation
Studio, the building needs were represented by Dale Frens, Frens and Frens
Architects, and the environmental control planning was developed by William
2. COLLECTIONS ENVIRONMENT OBSERVATIONS AND EVALUATION
This section describes the existing conditions of the collection environment
to provide a context for the past environment of the collection, the
environmental evaluation, and the need for the improvements.
ARCHITECTURAL/GENERAL. The main building at the Mercer Museum is built of
poured-in-place concrete. It has an exposed concrete roof with no roof
membrane. The windows are mostly fixed sash consisting of cast concrete
frames emulating double-hung windows. A few windows are operable wooden
frames opened for ventilation.
COLLECTIONS DISPLAY AND STORAGE. The collections are stored and displayed
in two basic methods. Many of the large pieces are open to the general
space where air may freely circulate and where visitors may view the objects
from many sides. Most other parts of the collection on display are shown in
closed display rooms located around the perimeter of the building. These
rooms usually have an exterior exposure, usually with a window, and an
interior wall with a window for visitors to view the objects. Some of these
rooms are hardly larger than a display case while others are complete rooms.
Most storage of the collection is in similar closed rooms away from visitor
HUMIDITY TOLERANT ENVELOPE. The building is single-glazed and the walls and
roof apparently have no cavities or other heterogeneous aspects to their
construction; they are simply solid concrete. Since the building is not
heated and humidified the classic condensation conditions of a warmed
humidified interior exposed to a cold building envelope would not be
expected to occur; however, the high thermal mass of the concrete and ready
translation of outside conditions to inside apparently allows conditions to
occur that lead to condensation on the glass and window frames.
HVAC SYSTEM INFRASTRUCTURE. The building has no HVAC distribution
infrastructure. The only air handling system in the building is the smoke
exhaust system in the south tower which is used for summer ventilation.
This is controlled on a subjective basis when the weather is hot by turning
on the fan using a timer. There are no other methods for cooling,
heating/reheat, humidification, dehumidification, filtration or other
tempering of the building environment. The museum complex has hot water
heating boilers but these serve other buildings.
The following problems directly relate to the effect of the environment on
the collection. They are generally based on the previous collection survey
reports by conservators and the on-site discussions of the project team.
CONDENSATION STAINING. One of the major problems is condensation on the
interior of the concrete frame windows. The Museum staff notes that this
"usually occurs when a cool night follows a warm day." It was observed on
the sixth floor west windows at the time of the September site visit and is
generally characterized by the staff as occurring on the east and west sides
of the building. Staining from condensation runoff is apparent at most
windows which may be eroding the concrete. In very cold weather the Museum
staff reports the condensation can form ice.
COLLECTION SPOTTING. Collection objects kept near the windows show surface
spotting and staining which is apparently due to condensation at the
PARTICULATE CONTAMINATION. As noted by previous project conservators there
is a clear level of particulate contamination a portion of which is
suspected as coming from the exposed interior concrete surfaces. The
particulate observed in the collection areas at the was generally
lightweight with some soot. The lightweight matter was suspected as coming
from the collection or the museum visitors.
Analysis of samples was performed to determine how much of the particulate
is from the concrete structure itself. The 23 December 1992 analysis by Dr.
George Segan Wheeler concluded "the particulates derive primarily from the
cement which is probably degrading by infiltration of water from the
exterior." That report indicates that most of the particles were in the
size of 4 to 80 microns with 20 to 30 microns average, with the fibers also
noted as "probably from the cement."
HUMIDITY CONDITIONS. Mr. Berrett identified a problem with occasional high
humidity conditions in winter and problems with occasional humidity
excursions of over 20% RH in all seasons. The open windows are suspected as
allowing outside weather conditions to be rapidly transmitted into the
BROKEN/LEAKY OPERABLE SASHES. Several of the operable windows are broken or
their sashes do not seal well. This allows in the infiltration of moist air
and some water when it rains since these are casement windows.
RAIN LEAKS. There are several isolated places noted by the staff where rain
leaks into the building. This not only poses a long-term threat to the
building but increases the internal high humidity problems.
TEMPERATURE CONDITIONS. Mr. Berrett noted that summer high temperatures
were most problematic on the fifth and sixth floors in the areas with poor
ventilation. The ventilation is currently limited to the operable windows.
LIGHT EXPOSURE. Mr. Berrett identified light exposure problems from the
4. TEAM APPROACH
Working independently, the original precept of the consultants at the time
of the grant application was to develop a scheme of humidity-controlled
heating to reduce high humidity conditions, using a temporary mock-up of the
heating scheme as a test. However, when the consultants met for the on-site
meeting after inspection of the building a different approach was developed.
Mr. Berrett's priorities for improving the environment were listed along
with Mr. Frens' planned treatments for preservation of the building and Mr.
Lull's possible environmental control treatments. Priority was placed on:
a) isolating the interior environment from exterior humidity extremes, b)
controlling condensation, c) ventilating for high temperatures at the upper
floors, d) controlling particulates, and e) reducing daylit light levels.
Mr. Frens was able to offer several passive treatments he would otherwise
suggest for preservation of the building that could lead to improvements for
the interior environment, including treatment of the building envelope to
address water leaks and rebuilding of the operable windows used for
ventilation. Mr. Lull helped the team identify the limitations of the
existing structure for supporting not only a conventional HVAC system but
even a humidity-controlled heating system to reach all the spaces; virtually
any ducted air system would be physically, historically and aesthetically
intrusive and yet would be required to provide air to condition the closed
display rooms, called "glazed alcoves" by Mr. Mercer, which are key to the
museum display program. Window condensation was addressed through planned
secondary interior glazing to prevent the interior moisture from reaching
the cold panes of glass. This also provided the opportunity for using
tinted glazing to reduce light levels and glare for the collection on
display. Improved ventilation would be provided to address the heat gain at
the upper floors.
Since the performance of many of these improvements could not be readily
quantified backup plans were developed for dealing with high temperatures
and high humidity. If temperatures were still too high an increased
ventilation scheme was developed. If humidity levels were still too high in
cool weather humidity-controlled heating can be provided through the use of
the original tunnel under the main floor which many speculate was intended
by Mr. Mercer for steam heating pipes .
The important aspects of the plan were the use of treatments that were
otherwise indicated for preservation of the building, and the selection of
passive techniques which would not require any permanent intrusion or
modification to the building. No major energy costs would be incurred
allowing the improvements to have a minimum impact on the museum's operating
budget. The contingency of using central humidity-controlled heating, while
having a possible significant energy cost, was confined to a tunnel which
would again cause no changes to the building's historic fabric and any leaks
from the new system could not reach collection areas.
DISQUALIFIED TREATMENTS. The following possible treatments to address the
humidity and particulate problems were identified but disqualified by the
project team. In many cases, as indicted, (*) this was due to a requirement
for installation of a ducted air system.
1. DEHUMIDIFICATION.* High humidity could be reduced in summer through the
use of a central dehumidification system. Mr. Lull pointed out that
local dehumidifiers in rooms might require additional electric power
distribution within the building, and that responsible use of them would
require provision of a condensate pumping and removal system. He
pointed out that they would also increase and decentralize the fire risk
in the building and would have a significant energy cost.
2. FILTRATION.* The smaller air borne particulates could be filtered out
of the air through the use of a central filtration system. Mr. Lull
indicated that this would have had a significant increase in annual
3. AIR-CONDITIONING.* High temperatures could be addressed through
conventional cooling which might also serve some or all of the
dehumidification functions. Mr. Lull indicated that this would have had
a dramatic increase in annual energy costs.
4. CLEANING OF CONCRETE. The interior exposed surfaces of concrete could
be cleaned to remove any particulates present on the surface. Mr. Lull
suggested a HEPA-rated vacuum be used to assure all particulates were
caught. The preponderance of the project team felt that the dust
analysis indicated this was not necessary as a formal project and could
be addressed as a general staff activity with normal vacuums.
5. MAIN ROOF MEMBRANE. If the patching and/or sealants used in the repair
of leaks proves ineffective in making the main roof weather tight then a
roof membrane might be needed, although this would not be historically
accurate. This might cause a problem with condensation on the underside
of the new membrane depending on outside temperatures, inside
temperature records and conditions for balanced vapor flow to avoid
condensation. Mr. Frens indicated that the roof problems related
primarily to edge and boundary conditions and not to water penetration
of the roof itself so a membrane would be of little benefit.
* This option generally requires a ducted air system or piping system. With
the preponderance of the collection located in closed or semi-closed rooms a
ducted air system would require duct penetrations through the poured
concrete walls and floors and an exposed duct system in the building.
Options based on all-water piped systems were similarly discarded since they
still required penetrations of the concrete and brought with them a great
risk from leaky and frozen pipes, and would pose an unmanageable maintenance
burden from the many small terminal devices to be maintained. Each of these
could also be expected to have significant energy costs, not only for the
fans to move the air but the operating costs for heating, dehumidification
5. CONSERVATION ENVIRONMENT IMPROVEMENTS
The following improvement plan was developed to address as many of the
problems as possible. They may have a significant cost and should generally
be designed by an architect and/or engineer. They are the improvements that
were developed with the project consultants to strike a balance between
improved environmental conditions, minimum impact on the structure of the
building and reasonable owning and operating cost implications for the
1. CLEANING IMPROVEMENT. To deal with the particulate problem the
preponderance of the project team felt that the use of conventional
vacuum cleaning would be appropriate and that no special filtration
would be required since no particulates were found at the level that
would require a HEPA vacuum. For convenience in cleaning the collection
the vacuum should be the type that attaches to the waist with a belt.
2. REMOVE GLASS PANELS AND UNDERCUT DOORS. To promote air flow in the
various closed exhibit rooms the project team agreed with Mr. Berrett's
suggestion that one or more panels of glass be removed and that the
doors be undercut. This should be carefully done so that security of
the contents is preserved and might involve the use of heavy security
screens where necessary.
3. ACRYLIC PANELS FOR WINDOW CONDENSATION. The fixed-sash windows with
condensation problems should be considered for interior-applied
secondary glazing with acrylic panels. To be effective the panels need
to seal against the interior air reaching the original exterior window
glass. Since condensation apparently forms on the thin window frame
mullions and muntins the panel should preferably span the window,
applied to the thicker part of the wall. As reported by Mr. Berrett, to
reduce light level and glare the panels should be tinted. (See later
discussion of acrylic panel tests.)
4. REPAIR ENVELOPE LEAKS. The water leaks at various areas should be
repaired. This should include patching window problems, miscellaneous
wall conditions, a roof membrane for the balcony, and other conditions
determined by the restoration architect.
5. RENOVATED OPERABLE SASHES. The current operable sashes are often
broken, inoperative or ineffective in keeping rain out. The restoration
architect should consider redetailing the windows for more effective
seals and replacement of casement and sliding sash windows with awning
windows for better protection from rain ingress. All non-fixed sashes
should be made operable for current or future ventilation requirements.
(See next item.)
6. BETTER VENTILATION OF SUMMER HEAT GAIN AND HUMIDITY. The ventilation
system should be improved in its ability to address high temperature and
high humidity conditions. This would involve improving the ventilation
air discharge, thermostatic/humidistatic control of the ventilation, and
motorized damper operation of the renovated window sashes.
CONTINGENCY IMPROVEMENTS. These improvements are contingency improvements
in the event that the main improvements prove ineffective in meeting project
goals. These have not been identified as part of the main improvements
because they have a significant capital or operating cost, or are more
intrusive to the building fabric.
1. CENTRAL HUMIDITY-CONTROLLED HEATING. If the main improvements do not
provide sufficient control over high humidity conditions when the
temperature is low outside then humidity-controlled heating might be
used. While a ducted air distribution system might be too intrusive on
the historic structure, the building might be centrally heated with
steam, glycol/water or hot water piping located in the service tunnel
located below the floor in the main level. This area could be easily
served by piping running to a heat source to the north or south, and
would present little risk to the collection from leaks since it is below
collection levels. To enhance the heating effect the existing floor
grille might need to be opened up and additional grilles might need to
be added. Improvements to the tunnel might be necessary to improve
cleanliness, maintenance access and discourage pest ingress. The
control of such a system would need to avoid rapid temperature changes
and might need to seek a backward-averaged humidity level, rather than
be particularly responsive to acute conditions. This option would have
significant energy use implications.
2. IMPROVED CENTRAL VENTILATION. If the renovated windows and automatic
controls still prove inadequate for removal of summer heat (when outside
conditions are favorable) then further improvements should be considered
in the following order:
a. ADD EXHAUST FAN TO ALTERNATE TOWER. The other tower could be
equipped with a ventilation system similar to the existing system
except better suited for continuous duty. It should make full use
of the available window areas for exhaust louvers and should use a
fan capable of greater air flows at lower noise levels.
b. REPLACE EXISTING FAN SYSTEM. The existing fan system could be replaced
with a fan capable of greater air flows at lower noise levels.
This option would have significant energy use implications.
3. LOCAL FANS IN CLOSED AREAS. If the other improvements prove inadequate
to address condensation or mold problems in the closed exhibit alcoves
then fans might be used in them to increase air flow. With the fans
would come some additional energy use, additional maintenance, and added
risk of fire.
4. LOCAL HUMIDITY-CONTROLLED HEATING. Similar to the central system, this
would be the use of electric heaters in in the closed exhibit alcoves
with humidistats used for control in addition to thermostats. With the
heaters would come significant additional energy use, additional
maintenance, added risk of fire, and possibly the need to install
additional electric service to the rooms where it is used.
5. RECIRCULATED AIR. Mr. Berrett felt strongly that a central recirculated
air system be considered for destratification and improved air turnover
in the collection spaces for more even conditions and to suppress mold
growth. The application of this may ultimately be limited by the
available shaft space for vertical ducts inside the building. Creating
additional vertical shaft space is possible and options might include an
added exterior chase, an interior shaft created by penetrating a
vertical series of galleries, or a new exterior tower that might house
the shaft and new HVAC equipment. This option would have a significant
energy use implication, particularly if high velocity/high pressure air
distribution was required. A major advantage would be that such a
system might offer the opportunities for filtration, and for heating,
cooling and dehumidification at significant additional energy use.
6. INTERIOR CONCRETE SEALANT. A concrete sealant might be applied that
could help reduce the generation of particulates while also reducing the
infiltration of water vapor into the building from the concrete. Mr.
Frens indicated that it would be hard to find an effective sealant that:
a) would not change the appearance of the concrete, b) could be easily
reversed, c) that had a proven track record of performance, and d) that
would not need to be reapplied after a period of time, such as 10 years,
to renew its efficacy. The preponderance of the project team felt that
treatment of the concrete was not necessary to control the particulate
problem, which was attributed primarily to housekeeping, and was
otherwise an option of last resort.
ACRYLIC PANELS. A test of the possible treatment of the windows with
acrylic panels was performed by Mr. Frens. The panels were effective at
reducing light and UV levels while having very little impact to the
appearance of the building from the outside. The reduction of light
transmission was generally not noticed from the inside. There was a problem
with condensation within the cavity created by the application of the panel.
Although different panel types were tried including an application of a
panel with holes, each was plagued by moisture formation within the cavity
between the glazing surfaces. Two additional treatment option tests were
identified to prevent both surface and cavity condensation.
a. SEALED CONCRETE. The whole-window treatment might be tried again but
this time a strippable sealant, as suggested by Mr. Frens, might be used
on the exposed concrete to inhibit moisture migration into the cavity
between the glazing elements.
b. SINGLE-PANE PANELS. To reduce the exposure of concrete in the cavity
between the glazing elements the acrylic panels could be applied to each
window pane instead of the whole window. To eliminate the exposure of
concrete the acrylic panels could be applied with a glazing tape that
would at once seal the concrete at the edge of the window pane and hold
the acrylic panel in place. This treatment test should include summer
conditions to be sure there will be no "popping-off" effect, expansion
damage or other problems due to dimensional change in the acrylic panel
as it warms and cools. To help avoid this the new acrylic panel should
be under-sized sufficiently to allow for expansion.
This approach has the advantage of having a minimum impact on the
appearance of the window and can provide the same tinting or UV
protection as the large panels. It also avoids reworking some parts the
window frame edge condition that Mr. Frens indicated would be required
to support the whole-window treatment. The single-pane treatment has
the disadvantage of not providing as large a sealed air space so the
thermal benefit for controlling condensation may be limited, and they
may have a higher cost due to greater labor in fitting and installation.
This treatment will also not protect the window muntins and mullions
The results of these tests should identify the final details for
condensation control at the windows.
William P. Lull is a graduate of the Building Technology program at MIT, a
principal and senior conservation environment consultant at Garrison/Lull
Inc., and is Adjunct Associate Professor of Building Technology at New York
University. He has formerly worked as a designer and project manager for
architects, engineers and government agencies. Mr. Lull has been an invited
lecturer for many groups and author for several publications. He has
consulted on collection environments in many museums, libraries, archives
and historic structures in the US and throughout the world.