How to design a museum building that protects the collection without destroying itself

Copyright Creative Commons licence - free to use, author must be acknowledged, no changes allowed.

Mechanical air conditioning has become so standard a component in modern public and commercial buildings that its unglamorous components are now becoming design icons. Sometimes these components are sneaked into the facade with understated wit, as in Venturi's extension to the National Gallery London (Figure 1) or, with bravado, as dominant features, as in Richard Rogers Lloyds building (Figure 2).

?Figure 1: The back entrance of the 1991 extension to the National Gallery, London.
Architects: Venturi, Rauch and Scott Brown.

Figure 2: The Lloyds building in London,1984.
Architect Richard Rogers.

Air conditioning, defined as the combination of heating, cooling, humidification and de-humidification, uses a lot of energy, compared with just warming a building (Figure 3). It also uses a lot of the building. Humidity can only be controlled by piping air around the building. Up to a quarter of the volume of the building can be consumed by this pipework and its associated machinery. Air conditioning lasts about twenty years, after which it is either worn out or inefficient compared with current technology. Air conditioning in a museum presents further particular disadvantages. Because air is being moved about, air conditioning is noisy, particularly when it is fitted into old buildings, so that the pipes have to be narrow and tortuous. Air conditioning requires continuous oversight, being capable of filling an exhibition room with steam within minutes if a control fails. The burden of maintaining a technical skill far from the core expertise of the museum staff strains the budget and generates competition for funding between the core jobs, research and exhibition curating, and the mundane necessity of fuelling and staffing the climate control.

?Figure 3: Air conditioning is major drain on the resources of a museum. It takes up space, makes a noise, uses more fuel than heating and requires trained technicians to maintain it. And it doesn’t last nearly as long as the exhibits.

The extreme separation between the career paths of the scholar of art and the microclimate technician has not been satisfactorily bridged by the museum conservator. Conservators are mostly trained on top of a non-scientific basic education. Even those well versed in basic science are hampered by the surprising absence from the syllabus, even at university level, of the science of microclimate. Architects suffer the same limitations in their education. The conscientious curator, faced with specifying for the architect the technical requirements for a new building, or a major rebuilding, will consult her conservator, who in turn will turn to a national or international standard to define a climate, relying on the authority of a prestigious institution rather than on an informed critical analysis of the nature of the building, the artefact collection, and the local climate.

Standards for museum and archive climate are unusually hard to formulate because the specification has to optimise the durability of all materials ever used by man. There are some specialist stores, photographic archives for example, that have standards based on research on specific materials. The general purpose standards, however, are timid documents, assuming that conditions comfortable for mankind cannot be too bad for the products of human ingenuity that were made to share the human environment. They furthermore assume that the more constant the better. Knowing of the existence of air conditioning capable of maintaining a constant interior climate regardless of the outside weather, the standards committee specifies a constancy that can only easily be obtained by air conditioning. It is a safe choice for which they will not be criticised. The blame for failing to comply with the standards falls on the design engineer, who protects his reputation by conservative design with ample reserve capacity, whose running costs are borne by the institution.

It is undeniable that perfect climatic stability can only prolong the survival of artefacts. But a climate that is controlled by a sensor discretely placed in an air conditioned room cannot prevent the insult of direct sunlight warming a glazed picture 40 degrees over the temperature so carefully controlled by the sensor mounted out of the sun's glare. Even restrained exhibition lighting subjects exhibits to a 6% variation in relative humidity, according to whether the light is on or off. In practice, the stability promised by electronically controlled air conditioning is very seldom realised in the microclimate around individual objects.

To say that the promise is not always achieved is not the same as to accuse air conditioning of causing harm. Yet it does cause harm, even when it functions correctly. The congenial winter temperature we expect as visitors, combined with the moderate relative humidity thought necessary for the art, combine to stress the building, often itself a valued item of solid history. This assertion is best illustrated by a case history, which describes how the renovation of an old building to conserve its ancient appearance while conforming to modern standards of collection care, human comfort and fire resistance, proved incompatible with a faithful restoration of a historic building. 

The Arts and Industries Building in Washington DC (Figure 4) was designed as a museum and opened in 1881. The spires provided ventilation by the stack effect: warm air rises through the spire, being replaced by cooler air rushing in through the door. Much later, air conditioning was installed.

?Figure 4: The Arts and Industries Building in Washington D.C. Architect Adolf Cluss.
Opened 1881. The exhibition area is all on one floor. It was originally ventilated by the stack effect: warm air rose through the spires, drawing fresh air in through the doors. The roof was replaced in 1975 but has now deteriorated so much that the building is now (2006) closed. According to the US National Trust for Historic Preservation this is one of the 11 most endangered historic structures in the United States.

In preparation for the centenary of the building, it was restored within by clearing out the functionaries who had carved out offices from the immense single exhibition floor of the building. It was restored without, including a new roof, designed to imitate the old roof in appearance while satisfying modern standards for thermal insulation and fire resistance.

The new roof became famous shortly after the re-opening of the building in 1976. It rained inside during fine sunny spring weather (Figure 5). A year long investigation proved that the rain originated as condensation from the humidified indoor air. The process is described in figures 6 - 12.

?Figure 5: Shortly after re-opening to the public in 1976, eye witnesses reported rain from clouds forming within the central rotunda of the Arts and Industries Building.

?Figure 6: The rain proved on analysis to be a solution of fireproofing salts
originating in the plywood panels supporting the roof. *Koktaite is a hydrated calcium ammonium sulphate.

Figure 7: The stack effect is the architectural expression of the hot air balloon. In winter, warm humidified air rises within the high single room of the building, finding its way out through imperfections in the roof construction. Cool air enters through the doors.

Figure 8: The roof is made from many prefabricated boxes. Top and bottom are of plywood. The insulation is fibreglass. There is polyester sheet laid on the lower plywood to act as an air barrier. The outer cover is lead coated copper strips with crimped joints. There are abundant cracks through this construction for warm air to penetrate. During the winter, water condenses on the lower surface of the upper plywood, aided by the hygroscopic fireproofing salts.

Figure 9: In early summer, the sun warms the dark grey lead roof surface to about 80°C. Water vapour distills down to the relatively cool surface of the polyester foil where it condenses and eventually runs down to escape through cracks in the ceiling panels.

Figure 10: This diagram shows quantitatively the annual cycle of winter
condensation followed by summer distillation. The top line shows the direction of air movement. There is a steady upward movement during the winter which quite suddenly reverses with the onset of summer. The second line shows the relative humidity (RH) just below the upper plywood panel. It increases steadily during the winter, never reaching 100%, because the fireproofing salts absorb water at a lower RH and thus buffer the RH to about 80%.The roof gains about 30 tons of water during the winter and early spring. This water is evaporated rapidly in summer, so that by August the roof is dry again.

Figure 11: A detailed examination of a week of data from sensors disposed through the thickness of the roof shows that condensation only occurs during brief periods of intense solar heating, shown by the black areas marking the overlap of temperature and dew point. The stability of the roof is finely balanced. Summer rain within can probably be prevented by reducing the winter temperature and relative humidity in the building.

Figure 12: Here is a summary diagram of the winter charging of the roof
followed by summer discharge of water into the interior of the building. The roof is now, after 30 years of this stressful cycle, in such poor condition that the building has been closed to the public.

A similar process, but on a vertical plane, bombarded people passing under the Hirshhorn Art Gallery. The process is displayed in Figure 13 and explained in Figure 14. The warm air leaks out of this building not because of the stack effect but because the inside air is under pressure. This is to ensure that outside air does not enter by unauthorised routes to disturb the operation of the air conditioning.

?Figure 13: Office workers passing under the raised bulk of the Hirshhorn Museum in Washington DC are unaware of the danger from falling ice.

Figure 14: How the Hirshhorn makes ice. Warm, humidified air from the pressurised interior leaks out. Its water content freezes on the cold outer skin of the wall. The rising sun warms the concrete and melts the ice, which escapes through drain holes to the outside of the wall where it re-freezes because of evaporative cooling in the brisk wind of air of low relative humidity. Later, the growing solar radiation melts the ice where it touches the wall, sending slabs of ice to smash on the pavement below.

These buildings, or restorations, date from the middle of the twentieth century. Architects and their engineers now understand these threats to the building structure. Buildings are now much more carefully designed, even with the help of conservation scientists. Figure 15 shows the National Gallery of Canada in Ottawa, opened in 1988. The winter climate is severely cold so extreme precautions were taken to prevent condensation in the elaborately designed light wells bringing daylight down to the lower floors. Unfortunately they also bring the condensation down, as shown by the bucket behind the group listening to the architect extolling the merits of his building (figure 16).

?Figure 15: The National Gallery of Canada in Ottawa, opened in 1988. The architect is Moshe Safdie. Light is channelled through wells from the roof into the lower galleries. The elaborate sealing of the complicated skylights has not been entirely adequate to prevent condensation of water from escaping air.

Figure 16: A group of delegates from the 1994 IIC conference on preventive conservation is listening to the guide, one of the architectural team who designed the National Gallery of Canada. In the gloom to the right, a bucket collects drips of condensate.

Can we do better in the twenty first century? Success is far from certain. There are two factors working against establishing a good museum climate which is also safe for the building. The first is the architectural fashion for lightweight buildings and the second is the natural desire of architects, and their sponsors, to make the building itself a novel and surprising work of art. Figure 17 shows the 'Black Diamond', the 1999 extension of the Danish Royal Library. It appears to be of massive granite, but in reality the walls have to be lightweight to lean outwards. The outward leaning wall has rapidly become a standard architectural idiom, most recently exemplified in Cardiff's Millennium Centre (figure 18) with its copper wall surface.

?Figure 17: The Royal Library in Copenhagen, opened in 1999. Architects Schmidt, Hammer & Lassen. The outward sloping walls are covered with black Zimbabwe granite veneer and with large areas of glass.

Figure 18: The Millennium Centre, Cardiff, Wales, 2004. Percy Thomas Architects.

However, the advantages of massive walls in stabilising both the temperature and the relative humidity in buildings cannot be denied. Much of the RH variation outdoors is attributable to the daily temperature cycle, typically bringing the RH down to 60% during the day and up to 100% during the dew fall of the night. The actual water content of the air does not vary much at all. Keeping the temperature stable by having walls with sufficient thermal inertia to buffer the daily cycle will automatically stabilise the RH, without help from mechanical devices. If the walls are porous so that they can absorb and desorb water vapour and pollutants the climate is further stabilised and its long term stability can be maintained by ventilating only when the outside air is of suitable water content to push the interior RH to the desired value.

To appreciate the potential of these building techniques one has, for most examples, to turn to more sober architecture, designed for purpose rather than for pomp. However, I start with a notably pompous building, the castle of Segovia in Spain (Figure 19). This comes from a vanished age when prestige was expressed through weighty construction, so the cellar of this building (Figure 20) has a good natural environment for preservation (Figure 21). The climate in the archive is very steady but the RH is a bit too high for most archivists to accept.

?Figure 19: The castle of Segovia. Mainly 16th century with extensive rebuilding after a fire in 1862. The military archive lies behind the lowest row of windows.

Figure 20: The military archive in the castle of Segovia. The room has massive temperature buffering from the approximately 13 metre thick limestone wall on the left. The room is lit by small windows in the thinner, but still massive wall on the right. (Photo by Victoria Smith).

Figure 21: The climate in the archive over a three month period. Notice the daily dips in RH at point A, caused by the archivist opening the windows during the day. At point B, later in the year, the outside air contains more water vapour, so opening the windows now increases the interior RH. If a combination of sensors and computer could calculate exactly when to open the windows, a more even RH would be obtained, and also a lower
average RH.

An archive in Copenhagen (figures 22 - 24) illustrates one strategy for reducing the RH without using air conditioning. In this archive heat is borrowed from the surrounding office areas and balanced against heat loss to the outside, so that the interior temperature lies about half way between inside and outside temperature. This will give an annual average RH around 50% while keeping the average annual temperature below that comfortable for humans, but which gives slower chemical decay of the archived materials.

?Figure 22: The Arnemagnaean archive of early Scandinavian manuscripts occupies the windowless volume behind the facade of this building in Copenhagen University. KHRAS Architects 2004.

Figure 23: A cross section of the archive. On the inside there is a 50mm layer of moisture absorbing expanded cement covered with a single coat of porous silicate paint. The massive reinforced concrete box, designed to withstand a fall to the ground, functions as a temperature buffer. The insulation outside the concrete shell is, unusually, thicker towards the interior of the building and thinner towards the outside. This results in a temperature in the room which hovers midway between the nearly constant temperature of the inhabited parts of the building and the annual cycle of outside temperature.

Figure 24: The climate in the archive is very stable. The saw tooth trace of the carbon dioxide concentration can be used to check that the air exchange rate of the archive remains at the planned value, about one air change per day. Sensors outdoors and within the archive allow the controlling computer to pump air into the room only when it will move the RH to the programmed value, which varies slightly from winter to summer.

At this stage in the slow progress to low, or zero energy consumption climate control for historic collections, there is still much scope for progress. Calculation shows that it should be possible to lower the temperature of the archive to the annual average temperature, which is about 7°C in Copenhagen, while keeping the interior RH well below the 85% average RH outdoors. This means ventilating only in winter and relying on RH buffering by absorbent materials to tide the archive over the summer.

In public exhibition spaces, the ventilation rate has to be sufficient for breathing and removal of smelly chemicals. This will strain the buffer capacity of simple heat and moisture absorbers, so labyrinthine heat and humidity buffers will have to be developed. Building physicists are just beginning to make the framework of standards and computer programs that will give architects and engineers the confidence to design passive climate control, by way of an intermediate stage of mechanically assisted climate control. A further advantage of this design approach is that buildings designed to eliminate the need for air conditioning will also protect themselves against structural damage through condensation.