9 SETTING HUMIDITY LIMITS

An informal standard exists for the upper limit for humidity within a building during the heating season: humidity should be low enough to prevent running condensation on windows. This standard is difficult to use as a design value for new construction or remodeling, but it is quite useful for operation and control. There is a growing consensus among building scientists that during the heating season an indoor relative humidity of 50% is too high. Humidity control during heating seasons is actually somewhat easier than humidity control during air conditioning seasons.

Air conditioning consists of passing air across a chilled coil. The coil lowers the temperature of the air, and it removes moisture. Lowering the temperature is called sensible cooling, and humidity removal is called latent cooling. Providing the appropriate ratio of sensible to latent cooling is a matter of sophisticated system design, control, and operation. Correctly balanced sensible and latent cooling cannot easily be achieved with temperature control only.

Most collections would benefit from humidity control, that is, from providing a humidity sensing device that can override the operation of the air handling unit. Humidity control could prevent collections from being subjected to extreme dryness during the coldest days and extremely high relative humidity during the hottest days. (Most systems fail mechanically or fail to deliver the proper air at those times.) One simple humidity control strategy consists of having the humidity sensor shut off the temperature control once a critical level of wetness or dryness is reached. Suppose that during the winter a critical low indoor relative humidity level is reached. The sensor could shut off the heating unit, the temperature would fall, and, consequently, the relative humidity would rise. During the cooling season, if indoor relative humidity rose above a critical value, the air conditioning could shut off, the indoor temperature would rise, and the relative humidity would decrease. In both cases, staff and visitor comfort would be sacrificed for stability of the collection.

Setting appropriate indoor humidity levels via control is important. But more important is to ensure the safety of the collection during the days when the equipment cannot deliver the intended supply conditions. These days are not only statistical possibilities but actual calendar days, and they are often times of great distress for museum professionals.