By Michael C. Henry

In the past half century, expectations of thermal comfort in North America have been shaped by the increased availability of climate control technology and equipment and by the comparatively low cost of operating these systems. As conservation professionals, we have come to expect that climate control technology can alleviate the potential damage to museum collections from extremes and fluctuations in temperature and relative humidity. Both expectation levels—comfort and conservation—have resulted in sophisticated, energy-intensive climate management systems for old and new buildings.

In the latter part of the same period, the climate science community arrived at the overwhelming consensus that global consumption of fossil fuels significantly contributes to higher atmospheric temperatures, changes in climate patterns and precipitation, and rising sea levels.

Against this backdrop, as stewards of cultural heritage, we should review our current approaches to environmental control and revisit traditional building design and use as part of our environmental management strategies for collections. This may give us solutions that promote not only the conservation of our material culture but also the conservation of our global environment.

The Interior View and How We Got There

After World War II, the increased availability of environmental systems, especially air-conditioning, for human comfort and industrial applications provided the museum community with the technology to control the interior conditions of collection spaces. Coincident with the availability of hardware was the accessibility of fuel and electric power to operate these systems, generally at favorable costs, especially in the case of electric power.

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In 1969, when Reyner Banham published his seminal book The Architecture of the Well-Tempered Environment, air-conditioning was an expensive option on American cars, and central air-conditioning had become available in new postwar housing. Nearly thirty years later, historian Gail Cooper noted in Air-conditioning America: Engineers and the Controlled Environment, 1900–1960, "Largely as a consequence of modern design and construction imperatives, then, air-conditioning moved quite rapidly from a luxury to a necessity in the building industry." When the National Building Museum presented the 1999 exhibition Stay Cool! Air Conditioning America, 90 percent of newly constructed American homes featured central air-conditioning, and two-thirds of existing homes had central air-conditioning, while one-third had room or window air conditioners. Also in 1999, penetration of factory-installed air-conditioning in the American automobile and light truck market approached 100 percent.

Across the United States, the availability of year-round interior climate control in buildings and vehicles has profoundly changed public and personal expectations of environmental comfort and the individual's relationship with the natural environment. Systems not only automatically intercede in controlling the interior environment; in addition, building design evolved to eliminate many of the traditional features for individual control, such as operable windows and shading devices.

In the span of one generation, most people in the United States have come to expect that personal environmental comfort will be maintained by heating and air-conditioning systems. In the course of this change, we have become disconnected from the seasonal shifts of climate, its nuances, and its daily manifestation as weather. Our observations of weather and climate are largely secondhand, reported by media meteorologists or Internet weather services, with an emphasis on the catastrophic extremes that strain our mechanical systems and energy supply infrastructure, upsetting our artificially maintained comfort.

The Building as a System

It is important to recall that many older buildings predating the development of four-season climate management systems typically have some inherent capability to moderate external influences on interior conditions. In these older structures, the building itself was the system for ventilation and human comfort. The design and construction of these buildings relied on certain materials, an overall form, and horizontal and vertical communication between interior spaces. A key component of the interior conditioning of older buildings was occupant operation of building features—such as windows, doors, and shutters or shading devices—which moderated the influence of the exterior on the interior while capitalizing on favorable external aspects, such as breezes, for ventilation and comfort.

By contrast, the majority of buildings from the late twentieth century rely on centralized mechanical systems to moderate the effects of the exterior climate on the interior conditions. In these buildings, should the mechanical systems fail to operate or receive the necessary electrical power, the combination of building materials, building form, and spatial arrangement may actually exacerbate the adverse effects of the outside environment on interior conditions.

Older buildings that have been retrofitted with contemporary mechanical systems are likely to have had modifications so that they perform more like modern, tightly sealed buildings. James Pitot's early nineteenth-century house on Bayou Saint John in New Orleans, currently the subject of a Getty-funded Conservation Planning Grant, is a typical example of the impact of central systems. A traditional two-story Creole cottage, the house has numerous features to moderate the effects of the hostile New Orleans climate. Deep galleries protect the interior spaces from sun and driving rain. The second-floor galleries are intended to provide protected exterior living spaces, the importance of which is evidenced by the presence of architectural trim such as baseboards and, in some instances, chair rails. Interior spaces are configured for cross ventilation through multiple doors and windows that open onto the protected galleries. The house incorporated seasonal operating features, no longer extant, such as curtains and shades hung above the gallery railings to provide privacy and to exclude insects when the galleries were transformed into living spaces in the hot summer months. The original loose-fit slate roof resisted wind uplift from tropical storms, and the heated mass of the roof created a nighttime thermosiphon, exhausting room air into the attic through the second-floor ceilings constructed from gap-spaced painted boards, cooling the rooms below.

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With the introduction of central air-conditioning into the Pitot House in the late twentieth century, the building underwent a variety of changes. The ventilating ceiling was closed off with attic insulation, and the roof was replaced with tight-fitting composition shingles and roofing felts. The attic is no longer a solar-powered passive ventilator, and the doors and windows to the galleries must be kept closed to stabilize the conditioned interiors. Ephemeral and fugitive methods of managing the climate of the interior of the gallery, such as the gallery curtains seen in an 1830 sketch, have long since disappeared.

These losses are not unique to the Pitot House. They are examples of losses of climate-specific operative features at many older buildings that have been retrofitted with centralized heating and air-conditioning systems. These changes illustrate the subtle transformation that takes place when the decision is made to mechanically control the interior climate for occupant comfort or collections conservation or when it is necessary to secure or seal the structure against pollutants, pests, or unwanted entry.

Contrast the losses of historic environmental management features at the Pitot House with the National Historic Landmark Gibson House (1859) in Boston, which has not been air-conditioned and retains its original three-story-high ventilation and light shaft. The shaft, a functionally sophisticated and architecturally refined feature, distributed heated air to upper floors in winter and exhausted hot air from all floors in summer, while distributing much-needed natural light to windowless interior spaces and the stair hall. Building occupants operated the interior window sashes according to need, as indicated by the thermometer placed by one such window.

The impacts of centralized systems are compounded in older buildings considered historic by virtue of their architectural, historical, or cultural significance. In historic buildings, the interior environmental management must also address the preservation issues posed by the building itself. The dual mandate to preserve historic building fabric and prevent deterioration or damage to the collections sets the stage for potentially competing or conflicting objectives.

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In responding to the tension between buildings and collections, the 1991 New Orleans Charter, adopted by the American Institute for Conservation of Historic and Artistic Works and the Association for Preservation Technology International, endorses balancing the need to involve collections professionals as well as building professionals in decision making. Nonetheless, the presumed necessity of a retrofitted centralized heating and air-conditioning system for comfort and conservation will ultimately drive many of the decisions to alter the building envelope and eliminate the operability of original climate management features. These issues were recognized in the engineering, architectural, and conservation communities, and as a result, professional guidelines were developed to resolve the tension between environmental control and building type (American Society of Heating, Refrigerating, and Air-Conditioning Engineers Handbook, chap. 21).

As various monitoring devices and methods were developed, environmental monitoring became an increasingly important component of managing the environment in museums. With the advent of solid-state digital data loggers for measuring and recording environmental conditions, monitoring became increasingly economical. These devices were quickly embraced by museum professionals. The digital data collected could be readily analyzed and presented with personal computer software, thus alerting collections stewards to the variations in temperature and relative humidity in their museum, library, or archives spaces.

In some instances, the data presentation and digital display could imply a level of measurement precision that exceeded the performance specifications of the logging device and the variability of the conditions being measured. One outcome was the expectation that the capability to precisely measure interior conditions implied a capability to control, with the same degree of precision, the machinery and systems that maintained the interior conditions. This expectation tended to be codified in environmental specifications for collections conservation.

Engineers, architects, and mechanical contractors, being problem solvers by training and inclination, were responsive to the challenge of close control for collections environments. However, the resultant systems came at a premium in installation and operating costs. Furthermore, the complexity and lack of transparency of the control systems served to distance museum personnel from the very systems intended to protect the collections.

Increasingly, as building occupants, we are less adaptive to exterior conditions, choosing to rely on systems to provide near-uniform conditions regardless of place, activity, or time. From a conservation standpoint, this view is reinforced by our professional focus: a museum professional will be object or collection oriented, while an engineer will be oriented to the design of a control system for a well-defined and contained environment.

In the long term, this tightly focused, interior-centric point of view will prove unsustainable without some accommodation of larger factors, including the building, the exterior environment, and the global climate.

The Global View and Climate Change

In February 2007 the Intergovernmental Panel on Climate Change (IPCC) issued its fourth assessment on the future of global climate, Climate Change 2007: The Physical Science Basis. The IPCC, an international network of leading climate scientists, concluded that the link between human activity and increased global warming is "unequivocal." The report states that of the human activities that contribute to global warming, the largest influence is the generation of carbon compound emissions from fossil fuel combustion for transportation, for the generation of electricity for uses such as lighting and cooling, and for heating.

The IPCC report identifies several future climate trends in the twenty-first century, all of which are directly related to human activity:

  • warmer and fewer cold days and nights over most land areas,
  • warmer and more frequent hot days and nights over most land areas,
  • more frequent warm spells or heat waves over most land areas,
  • more frequent heavy rainfalls over most land areas,
  • increased drought in areas affected,
  • increased intense tropical cyclone activity,
  • increased incidence of extreme high sea level.

The report also notes a future increase in atmospheric moisture vapor; depending on air temperature, this change may result in increased relative humidity.

As stewards of cultural heritage, we cannot afford to look at these trends as merely a problem for environmental scientists, industry, or government. Climate change and global warming are of great importance to cultural heritage stewards in two respects: because of their impact on cultural heritage and because of the ways in which mitigating this impact contributes to global warming.

First, consider the potential impact of climate change on conservation of cultural heritage, particularly cultural landscapes and fixed property, such as buildings. In 2005 the Centre for Sustainable Heritage (CSH) at University College London released its milestone study Climate Change and the Historic Environment. Based on 2002 projections for trends in climate change in the United Kingdom, CSH evaluated the possible consequences of those projected trends on UK cultural heritage resources. The implications are sobering.

Rising sea levels are a real concern. Less obvious climatic factors threaten as well. Changes in the extrema range, intensity, and frequency of climate variables such as temperature, atmospheric moisture, wind, and rainfall will lead to acceleration of existing deterioration mechanisms or to the initiation of new mechanisms. Buildings, the first line of defense for the collections, may lack the capacity to resist higher wind loads. The rainwater systems of buildings and sites may be undersized for more intense but less frequent rainfalls, leading to excess surface water or even flooding. Changes or variations in soil moisture can change soil volume, leading to stresses and cracking in foundations. Some of the problems projected by the CSH study are already being experienced in the United Kingdom and Europe. While the study focuses on the United Kingdom, the study provides a sense of the type and scale of effects that might be experienced by cultural heritage resources elsewhere.

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Rising sea levels are a real concern. Less obvious climatic factors threaten as well. Changes in the extrema range, intensity, and frequency of climate variables such as temperature, atmospheric moisture, wind, and rainfall will lead to acceleration of existing deterioration mechanisms or to the initiation of new mechanisms. Buildings, the first line of defense for the collections, may lack the capacity to resist higher wind loads. The rainwater systems of buildings and sites may be undersized for more intense but less frequent rainfalls, leading to excess surface water or even flooding. Changes or variations in soil moisture can change soil volume, leading to stresses and cracking in foundations. Some of the problems projected by the csh study are already being experienced in the United Kingdom and Europe. While the study focuses on the United Kingdom, the study provides a sense of the type and scale of effects that might be experienced by cultural heritage resources elsewhere.

The costs of mitigating the risks or repairing the resultant damage from these new climate factors will be great. In the case of catastrophic climatic events, there will be cultural heritage losses that cannot be restored, as in the recent devastation of New Orleans and the U.S. Gulf Coast. In such circumstances, given the larger societal priorities, cultural heritage needs are not likely to be adequately funded.

In addition to the direct effects of climate change on cultural heritage, we must be aware of how our actions in cultural heritage conservation contribute to the generation of the carbon compounds that lead to global warming. For example, in the United States, it is estimated that air-conditioning accounts for up to 20 percent of our electrical power use, 71 percent of which is generated by burning coal, petroleum, or natural gas. The energy cost of close artificial control of interior environments is higher than for more relaxed control, especially with respect to relative humidity. Therefore, tight performance targets for artificial interior environments for collections of all types, significance, and value add to the electrical power needs and fossil fuel consumption for buildings and sites. Unless our systems are powered by carbon-neutral energy sources, such as wind or photovoltaic power systems, we are contributing to the primary factor in global warming. As Pogo, the cartoon strip philosopher, commented on the state of the environment in 1971, "we have met the enemy and he is us."

Measures for protecting cultural heritage must not contribute to the exacerbation of the very climatic effects that can threaten its longevity. Protective activities could set up a positive feedback loop that intensifies, rather than attenuates, the conservation problem and its costs. As global warming increases the extrema and range of exterior conditions such as temperature and relative humidity, we cannot respond by tightening control of the interior environment with higher capacity mechanical systems that consume more energy and emit more carbon compounds.

Sustainability: Integrating the Viewpoints

Our stewardship responsibilities to future generations are not limited to the protection of material evidence of our significant objects, buildings, or landscapes. Our unwritten intergenerational compact requires that we transmit this cultural legacy within an environmental, economic, and social context that allows for viable stewardship in the future. This principle is a fundamental tenet of sustainability.

A sustainable approach to cultural heritage is an overarching philosophy that should permeate our thoughts and actions. Environmental management, one aspect of the implementation of this philosophy, is singularly important because of its consequences for cultural heritage conservation, energy consumption, and capital and operating costs.

Revisiting our environmental management strategies for preventive conservation in the light of a sustainability mandate is critical. Environmental management is a large component of the energy consumption and carbon emissions at our institutions and sites. We can reduce the potentially adverse impact of our environmental management strategies if we:

  • redefine our performance criteria for conservation environments by taking into account the robust qualities and vulnerabilities of the collections when compared to the exterior environmental threats specific to the location;
  • reduce carbon emissions (and operating costs) without necessarily reinvesting in new heating and air-conditioning systems, by implementing broader criteria for interior environmental control;
  • account for, and fully credit, the passive and operable features of the building that can moderate the environment and afford protection for the contents and collections, and rely on these features rather than on mechanical systems to the extent practical;
  • improve or enhance the inherent environmental performance qualities of the building envelope;
  • evaluate new or alternative environmental management technologies as part of systems replacements in the near future; consider the feasibility of carbon-neutral power generation, such as wind- or solar-generated electricity, for specific  energy needs;
  • plan for new buildings that moderate the exterior environment without excessive energy consumption.

As we undertake these new approaches to environmental management, it is important that we inform and educate the public as to the need for our action and how we are addressing that need.

In striking a balance between collections stewardship and environmental responsibility, we will undoubtedly face competing needs that challenge our past assumptions and practice. However, it is likely that we will also discover new opportunities to enrich our interpretation of both collections and historic buildings.

Michael C. Henry, Principal, Watson and Henry Associates, is an engineer and architect who has worked extensively in the field of historic preservation and on environmental issues of historic buildings housing collections. He teaches in the Graduate Program in Historic Preservation at the University of Pennsylvania and was 2005–06 Fulbright Distinguished Scholar at the Centre for Sustainable Heritage, University College London.