ASHRAE Journal - October 2009 - 44

ASHRAE59–2009 YE A R S JOURNAL made over the centuries to restore the approximately 2000 m2 (22,000 ft2) of surfaces, typically using some sort of cleaning process and then applications of a varnish or glue which temporarily brightened the surfaces but eventually turned dark brown. After many layers of dirt and varnish had nearly obscured the priceless frescoes, the Vatican finally decided in 1960 to begin a drastic restoration process. The restoration involved a careful and thorough removal of all the accumulated layers of grime and sealers. First the lower walls were done, then Michelangelo’s work on the upper walls and ceiling, and finally the altar wall, which was completed in 1994. The results have been truly outstanding, exposing bright coloring and artwork details which were hidden for centuries. Prior to the installation of the air conditioning system, the Chapel was ventilated in the spring, summer and fall by natural air flow through open windows and doors and by exhaust fans located under the south windows. This introduced moisture, dust and other pollutants into the space. In winter, heating was provided by a forced-air heating unit in a sub-basement with two outlet grilles in the floor. The heating unit provided very limited heating, but no humidification. The Vatican commissioned a survey of interior conditions in the early 1980s.6 These surveys showed that: a.) In summer, interior temperatures ranged between about 25°C and 35°C (77°F to 95°F) and relative humidity was about 50% to 60%; b.) In the spring and fall, temperatures were between about 18°C and 25°C (64°F to 77°F) and RH about 45% to 55%; c.) In winter, temperatures ranged between about 15°C and 20°C (59°F to 68°F) and RH was about 35% to 45%; and d.) Vertical temperature gradients of as much as 3°C (5°F) were common. Using these results to validate it, a dynamic simulation (described below) was used to study interior conditions at all times of the year. Figure 1 shows typical temperature contours for a day in July. Considerable variation is noticeable in both air temperature and humidity, due principally to the presence of people, during the time when the Chapel is open to the public (10 a.m. to 2 p.m. is shown here. Recently the summer occupancy period has been expanded to be 9:00 a.m. to 5:00 p.m.). Note that the wall temperatures (at the interior surfaces) are very close to the air temperatures, except when the large people loads occur. In addition, significant vertical temperature stratification was observed. These wide ranges in temperature and humidity conditions were clearly unacceptable for the safety of the frescoes. 44 ASHRAE Journal The Sistine Chapel is inside the Vatican. It was completed in 1484. Prior to the restoration, however, the fresco surfaces were somewhat protected by the layers of glue and varnish that had been applied over the centuries. The restored fresco surfaces, on the other hand, have not been protected with any sealant. They will remain in the original state. The Vatican thus recognized the need to protect these art treasures for posterity by creating a “microclimate” around them and sought to provide a suitable environmental control system. The objective of this microclimate system was to prevent the deposition of any kind of particles or substances on the surface or within the immediate subsurface layers. Without this action being taken, the walls and ceiling would gradually become contaminated once again, principally from the increasingly polluted Roman air and the numerous visitors, as many as 8,000 each day. Seven system requirements were established before the system was designed: 1. Constant air relative humidity; 2. Controlled fresco temperatures; 3. Still air at fresco surfaces, limited air motion at floor level; 4. Minimal noise level; 5. Minimal pollutants from visitors and outdoor air; 6. Invisible to occupants; and 7. Exceptional reliability. Constant air relative humidity. For a plaster surface, it is the air relative humidity, not absolute humidity or humidity ratio, that determines its moisture content. Temperature has only a minor influence. This is true of most materials found in museums and art galleries. For the present system, it was decided that relative humidity would a s h r a e. o rg October 2009

ASHRAE Journal - October 2009

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