Investigation into impacts of large numbers of visitors on the collection environment at Our Lord in the Attic

Our Lord in the Attic is a historic house museum located in the historic center of Amsterdam, The Netherlands. It is a typical 17th century Dutch canal house, with a hidden Church in the attic. The Church was used regularly until 1887 when the house became a museum. The annual total number of visitors has grown from 36,000 in 1990 to 75,000 in 2005; this trend is exponentially increasing. The museum had between 100 – 650 visitors each day in 2005; they typically stayed in the building for about an hour. There were two visitation peaks: one in the mid-morning and another in mid-afternoon. On two separate days each year the museum records large numbers (well over 1,000) of visitors. Masses, weddings, and music concerts, attended by 50 – 150 persons, are regularly held in the Church. The museum has consequently expressed major concern over the impact of the growing number of visitors on the various collection environments. A historic indoor climate of 10% - 95% relative humidity was estimated, from when the central heating system, which may have caused the majority of damages to the collection, was installed in the 1950’s to the installation of humidifiers in the 1990’s. However, between January 2005 and January 2006, in spite of a few central heating system malfunctions and large visitor numbers that particular year, the recent climate in the museum building did not reach harmful levels for the collections. Direct impacts of visitors were documented as increases in temperature, relative humidity, and CO2 concentration when the museum opened each morning as well as during special events in the church. However, relative humidity peaks remained less than 75%, and the value decayed quickly after the visitors had left the rooms or near the end of the museum’s visiting hours. As confirmed through air exchange rate measurements in the church, a high infiltration rate of outside air was the reason for reduced peaks and fast decay of the relative humidity and CO2 concentration. Daily maxima of CO2 concentration exceeding 1500 ppm were recorded whenever the daily total of visitors exceeded 500. Daily averages over the 08:00 – 17:00 period exceeded 1000 ppm whenever more than 600 visitors were recorded. However, the high infiltration rate completely diluted the CO2 concentration to an ambient level by the following morning. From the consideration of the CO2 and moisture accumulations, 600 visitors per day, well-distributed over the museum’s operational hours, can be safely accommodated in winter. The number can be increased to twice this during the summer by opening the entrance door and a window in the attic for increased natural ventilation. However, other important issues, such as safe levels of floor loading and vibration, as well as overall visitor comfort and experience, will have to be taken into consideration and may well result in allowing fewer visitors than what is recommended based on these calculations