Our Lord in the Attic: A Case Study

Visitor comfort

donwload Climate data canal room (XLS, 1.2 MB)

donwload Climate data sael (XLS, 1.2MB)

donwload Climate data in church (XLS, 2.0MB)

c Outdoor climate data (XLS, 1.1 MB)

donwload Thermal comfort report (PDF, 88KB)

Thermal comfort (RH)enlarge

The predicted percentage of dissatisfied people (PPD-value) plotted against an increasing RH

Thermal comfort (T)

The predicted percentage of dissatisfied people (PPD-value) plotted against an increasing T

Thermal comfort (air movement)enlarge

The predicted percentage of dissatisfied people (PPD-value) plotted against an increasing air movement

Some visitors complain feeling dizzy in the church, especially when attending events in the summertime, when it is hot. In recent summers, the staff noticed that conditions throughout the house can get rather stuffy as well. In the summer of 2006, which was the hottest summer on record in the Netherlands, mobile fans were placed throughout the building to increase ventilation and create a cooling sensation for the public. Visitors were also given a personal paper fan.

CO2 concentration data was collected and thermal comfort measurement were taken to help establish the cause of the discomfort . The thermal comfort of an individual is determined by environmental conditions that allow a person to retain a constant core (body) temperature. It is affected by environmental factors, such as air temperature, air speed, relative humidity, and radiant temperature. Other factors include a person’s physical activity, clothing, physical differences, and recent thermal history. A person generates varying amounts of heat depending on the level of recent physical activity. Heat is also gained from or dissipated into the environment through conduction, convection and radiation. The larger the temperature difference, the more heat will be transferred between a person’s skin and the surrounding environment through conduction. A higher air speed will produce a higher rate of heat transfer through convection. Lower relative humidity air will allow a higher rate of heat removal through latent heat loss via perspiration. And layers of clothing will certainly determine a person’s level of exposure to the environment.

ASHRAE’s comfort standards (ASHRAE 55-1981) suggest that the optimum operating temperature (a function of the mean radiant temperature, the mean air temperature, the mean skin temperature, and air speed) of 24.4 ºC in summer for visitors with light clothing.  A temperature of 28 ºC is considered to be the upper limit, requiring an air movement of 0.8 m/s to provide an acceptable comfort level.

During warm summer days with relatively high RH, visitors to museum Our Lord in the Attic often expressed discomfort from high temperatures, especially in the church. A museum staff member noted a visitor fainting in the church on a hot day.  The predicted percentage of dissatisfied people (PPD) is presented as a function of RH (at air temperature of 25 °C and radiant temperature of 22 °C), temperature (at 55% RH, with increasing radiant temperature) and air movement (at radiant temperature of 22 °C, air temperature of 28 °C and 55% RH). The plots clearly indicate that increasing temperature and increasing RH cause more discomfort, while increasing air movement (fans) lead to more satisfied visitors. A PPD value of more then 10% is generally considered to represent an uncomfortable thermal environment.

Since levels of CO2 were found to be less than the critical level most of the time, it can be assumed the reason for visitor discomfort is attributable to high temperature. The canal room is often warmer than the church; however, visitors are still relatively comfortable there as it is normally the first or second room they visit. Visitors must climb several additional stairways to reach the church. By the time they reach the church, this climb will have certainly raised physical activity levels beyond the “sedentary condition” on which comfort standards are based. The effects are more prominent on the higher floors, such as the floor level and the gallery levels of the church, especially on the canal side where wall temperatures can often reach into the 30’s ºC. These warm walls further reduce a person’s ability to disperse heat by radiation. These compounded conditions can make the visitors’ experience quite unpleasant.

For three days (10% of the time) in June 2005, the temperature in the museum reached above 26ºC; and reached 28ºC for several hours during the period January 2005-January 2006. During the heat wave of July 18—25, 2006, the church’s daily temperature reached above 30ºC and remained above 28ºC throughout the day. Interior surface temperatures of the Church’s walls closely followed that of the air. The wall facing the canal daily reached temperatures above 31ºC.  Both the air and surrounding wall were too hot to provide any heat removal from visitors’ skin. And, these high temperatures were considered to be beyond the range of temperature that could be relieved even with high speed of air movement. The museum may expect more hot summer days in the future as a result of global climate changes.

© J. Paul Getty Trust / Netherlands Institute for Cultural Heritage / Museum Ons' Lieve Heer op Solder