SURVEY -VS- MONITORING
Moisture Monitoring can be a critical component in the overall evaluation of a building. The first step is to observe obvious conditions, such as puddles around the building after a storm, condensation on window glass during cold weather, cracks in plaster that appear in winter and disappear in summer, and the smell of musty odors during hot, humid weather.
These observations are of a survey type; they are made at a point in time to record the current condition. Monitoring, in a formal sense, is a series of surveys done in a planned fashion in order to assemble a more complete view of existing conditions through time.
Moisture-monitoring projects need a holistic approach to investigating material deterioration, including moisture-related problems. The use of instruments to document conditions and detect changes too subtle for human observation can be part of the monitoring process. Instrumentation can indicate changes in relative or absolute measurements. Measurements taken during a monitoring project are usually most useful as relative readings. They indicate if a condition has changed for the better or worse from a previous reading, and if the change can be tagged to an event. The experience of the observer, the background information available, and the observation of conditions as they change, makes it possible to determine the cause-and-effect relationship causing deterioration.
Monitoring instruments that automatically take readings provide ongoing observation, even when a person is not on site. The key is to be able to translate the information from the collected data and relate it to a desired mitigation program. For example, are the ground moisture sensors showing that the surface regrading, which was done to eliminate ponding water, has been successful in eliminating damp basement walls? Has the HVAC system been properly regulated to compensate for outside humidity when large numbers of museum visitors enter the structure? Are the wooden walls sufficiently dried out to accept primer and top coats of paint? By comparing readings from a baseline number over a period of time, the drying out of a wall or the stabilization of a climate control system can be charted.
The use of automatic data-logging instruments, by themselves, can collect a lot of information - but without a reference point the data is not useful. A temperature and humidity monitoring unit in a room may have no real value if there is no way to tell how many people are in the room at a given time, or what the weather is like as readings change, or if the room is on an outside wall of the building (north, south, east, or west), or when and how often the HVAC system is running. The measurement of a condition must be tied to other relative information in order to be able to make sense of it.
An example of a successful monitoring program was presented at the 1995 annual meeting of the Association for Preservation Technology International, held in Washington, DC. Erhardt Winkler, from Notre Dame University, described the monitoring of the Egyptian Obelisk in New York City's Central Park. The stone monument was showing signs of rapid decay in the urban environment. An initial hypothesis was that the polluted air was causing chemical decay of the stone. Dr. Winkler felt it was important to review the whole environmental history of this monument to determine why it was showing recent decay when the monument had already lasted over 2,000 years. Measurements and a review of the monument's history revealed heavy salt contamination due to its years in Egypt exposed to the flooding of the Nile River. It was subsequently moved to the Mediterranean Sea coast where it was exposed to sea spray - a direct source of salt contamination. The relatively stable humidity levels in that part of the world did not activate the salts so as to cause deterioration of the stone. When it was moved to Central Park, however, the wide swings in relative humidity caused the salts already in the stone to migrate; the wetting and drying forced salt crystals to the surface where spalling and erosion could occur. The monitoring and history of this monument, coupled with the experience of the observer, identified the source of the deterioration. This will give conservators data with which to develop a conservation approach.
Instruments will not provide answers by themselves. Other information needs to be taken into consideration. Also, be sure that the instrument is recording information that is clean or uncontaminated. An example would be reading the moisture level of a masonry wall surface. If an electrical resistance moisture meter is being used, is it measuring the dampness of the surface, or is it measuring the dampness and salt content of the surface? Changes in the salt concentration on the surface may make the moisture reading unreliable.
The use of instrumentation in a moisture monitoring project is usually essential. However, make sure the system is well thought out. Instrumentation is a valuable part of the process, but human experience and understanding is the most important part!
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Updated: June 11, 2002