Museums, archives, and art storage facilities require special design consideration for a number of

reasons, most notably that the value of the building’s contents often exceeds the value of the building
itself. Even a minor

Museum design tips

The following must be considered when designing museums:

• Design enclosure systems must include air barrier, vapor retarder, and thermal insulation systems that
are sufficient and continuous.

• Design mechanical systems must provide close control over interior temperature and RH levels.

• Avoid interior shading devices at windows. If shading devices are used, provide supplemental heat to
windows to prevent condensation when the shades are drawn.

• Skylight systems should be high performance, with integral condensate gutters and a heat source near
the skylight. Alternatively, a “buffer zone” of dry air between a skylight and laylight may be sufficient to
prevent condensation problems.

shortfall in building performance with respect to heat, air, and moisture control can compromise the
collections and lead to a multi-million dollar problem.

This column diagnoses the three most common moisture challenges with museums, archives, and art
storage facilities and provides design guidance on how to avoid them.

1. Avoid severe swings in temperature and humidity

Museums, archives, and art storage facilities generally have strict requirements for interior temperature
and relative humidity (RH) control. The unofficial museum standard for temperature and RH is 70°F and
50%.
While much attention is given to getting temperature and humidity levels just right, maintaining
relatively constant conditions is generally more important than the levels themselves.

In fact, these set point recommendations are based more on anecdotes than on any rigorous testing or
scientific evaluation. During World War II, conservators from the British Museum found that paintings
hidden in slate quarry caves for protection experienced much slower degradation than when on display
in the museum. The conditions in those caves—approximately 63°F and 58% RH—were loosely rounded
to 60°F and 60% RH, and are considered the optimum museum environment for paintings. To account
for occupant comfort and variability in collections, conservators established the ideal range for
museums and libraries at 60-75°F and 50-65% RH.

In The Museum Environment, first published in 1978, author Garry Thomson stated that “modern day
mechanical systems are capable of maintaining a year-round environment of 70° ±2°F, 50% ±5% RH.”
Although Thomson does not declare that these conditions are ideal for storing artwork or other
artifacts, those numbers and that publication have been considered the “museum standard” for years.

This special report is the second installment of a six-part series from Sean O'Brien on moisture-related
design for specialty buildings.

The series includes:

• Indoor swimming pools/natatoriums

• Museums and archives

• Hospitals

• Ice rinks

• Cold storage facilities

The fact is all materials have different ideal storage conditions. Limiting fluctuations in temperature and
RH helps reduce mechanical stresses in the materials due to thermal- and moisture-related expansion
and contraction. Materials such as wood and paper will expand and contract as their temperature and
moisture content change. Large or sudden swings in those levels can induce stresses in the materials,
leading to premature degradation.

2. Pay careful attention to fenestration and wall systems to avoid condensation

Museum environments typically have an interior dew point temperature of approximately 50°F—much
higher than a typical office building or condominium—and this dew point is constantly maintained
throughout the winter. In non-humidified buildings, the coldest exterior temperatures generally coincide
with the driest interior conditions, reducing condensation risk in most cases. For museums, there is no
corresponding drop in interior dew point at low exterior temperatures, thereby increasing the risk for
condensation.

Condensation will form on interior surfaces that are colder than the ambient dew point temperature.
While this is typically not a concern for walls with continuous insulation, framed walls with insulation
placed between the studs may experience low interior surface temperatures due to thermal bridging
through the framing. In relatively cold climates, such as the Northeast, thermal bridging in museum
walls can drop interior surface temperatures to the point where condensation occurs.

The primary source of condensation, however, is fenestration. Aluminum windows, curtain walls, and
skylights, even when thermally broken, often have interior frame temperatures below 50°F during
colder winter periods (Photo 1, at top). Although insulating glass is generally sufficient to prevent
widespread condensation on the vision areas, condensation may still occur in some cases.

The most common cause of glass condensation is the use of interior shading devices, which can insulate
the framing and glazing from interior heat, while allowing interior moisture to reach those surfaces.
Depending on the shade configuration, surface temperatures may be 10-20°F colder than the fully
exposed condition. This creates a dilemma for museum designers, as shading devices are often required
to control light levels, provide “blackout” capability for special exhibits or functions, and filter out
ultraviolet radiation to protect artifacts from fading and degradation.

In cold climates, designers should consider specifying shading devices integral to insulating glass units
(i.e., “between the panes”) or fritted or coated/laminated glass to provide light control. If interior shades
must be used, a source of heat, such as a fin-tube radiator below the window, should be provided to
warm the space between the window and the shade while the shade is drawn.

Museums often contain design features such as large glazed walls and expansive skylight openings.
Skylights pose a significant condensation risk due to their horizontal orientation, which contributes to
additional heat loss via radiation at night. Skylights are often placed far from heat sources, which results
in localized lower temperatures at the skylights. High-performance glazing and framing, integral
condensate gutters, and local heat sources must be included in the design of museum skylights. A
second option is to use interior laylights below the skylights, which create a captive airspace below the
skylight that can either be heated or dehumidified to reduce condensation risk.

Another common source of condensation in museums is the moisture contained in moving airstreams.
Air leakage across and within exterior walls can carry large amounts of moisture relatively far from the
leakage site, creating the potential for widespread damage in a short amount of time. This is
exacerbated by the typical pressure control scheme in museums, which tends to create positive interior
pressure to prevent exterior contaminants from entering gallery or storage spaces. While positive
pressure keeps contaminants out, it also forces moisture-laden air through any gaps in the envelope.
Since maintaining negative interior pressure is rarely an option for a museum, a well-detailed,
continuous air barrier must be installed to prevent air leakage and condensation. A continuous vapor
retarder is also required for the enclosure, since moisture can diffuse through porous materials in the
walls and roof system.

3. Specify an airtight enclosure to maintain humidity levels

In addition to carrying moisture, moving air, in the form of air leakage through the enclosure, also
carries heat. If air leakage is severe enough, heat and moisture flows can cause variations in the interior
temperature and RH. A daily swing of 10% RH due to cold, dry air infiltration could damage the museum
contents and compromise the museum’s reputation—the ability to maintain interior environmental
conditions within tight tolerances is often a prerequisite to borrow collections from other museums. An
airtight enclosure combined with a well-designed mechanical system is necessary to maintain tight
control over the interior conditions.

GETTING MUSEUM CONVERSIONS RIGHT

In many cases, buildings with historic or artistic significance are converted into museum spaces. Such
buildings are typically older, solid masonry structures with little or no insulation and inefficient
fenestration systems.
Photo 2. Deteriorated interior brick caused by installation of insulation and a vapor retarder

Photo 3. Condensation on existing windows due to leaky interior storm windows

Adding insulation and vapor or air barriers to solid masonry structures can cause significant problems
with moisture accumulation, condensation, and accelerated masonry degradation, since these materials
interfere with the walls’ ability to “breathe” (Photo 2). To complicate matters, windows in these types of
buildings may have historic significance and are required to remain in place. Without modifications,
however, the existing enclosure cannot function effectively under humidified conditions.

Window condensation can often be addressed by installing new interior “storm windows” to provide
thermal resistance and maintain the exterior appearance of the building. In this case, the interstitial
space may also provide room for shading devices. The space must be slightly ventilated with outdoor air
to prevent heat and moisture buildup, and the interior storm window should be airtight to minimize
moisture flow and condensation. Drafty storm windows often result in heavy condensation or frost on
the existing windows (Photo 3).

The simplest solution for walls is to place humidified spaces away from the walls, using those areas for
corridor or other common space. If humidified space must be adjacent to masonry walls, a ventilated
“art hanging wall” can be constructed inboard of the masonry and integrated with the building
mechanical system (Figure 1 and Photo 4, below). This ventilated wall allows artwork to be placed on
exterior walls without the temperature and moisture fluctuations that would otherwise occur. Close
coordination between the architect, enclosure consultant, and mechanical engineer is required for this
system to be successful. Depending on the condition and configuration of the existing walls, insulation
may be allowable in this system without negatively affecting the walls.