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Dew Point

The Role of Dew Point: Environmental Conditions

When most people think about modifying and controlling indoor environmental conditions they typically consider temperature and humidity as the principle factors to adjust. Important as temperature and humidity are, a critical variable is often overlooked: dew point. A better understanding of the dew point concept itself—what it is and how it relates to other environmental measures—will help clarify its key role in the management of indoor environments.

What is Dew Point?

In practical terms, dew point, expressed as a temperature, is the point where the air can no longer retain all of the water vapor within a given unit volume. When the dew point is met, some of the vapor must be expressed as liquid water. That liquid water is usually expressed outside in the form of dew on grass or plants, fog, clouds and so forth.

Dew point is occasionally confused with relative humidity, a more widely reported measure of air moisture. Befitting its name, relative humidity is in fact relative; warmer air is capable of retaining more moisture than cooler air, so relative humidity changes as temperatures rise or fall even as the amount  of moisture per unit of air remains constant. Dew point, on the other hand, is an absolute measure. Assuming constant barometric pressure, the dew point is a function of the actual mass of water vapor per unit of air.

Controlling Dew Points in Indoor Environments

Managing moisture levels, along with temperature, is a key to obtaining appropriate conditions for all sorts of materials that require protection for preservation purposes. As a general rule, it’s far easier for climate control systems to regulate temperature than moisture.

This is where controlling the dew point plays a role. Based on the way mechanical heating and cooling systems typically work—circulating outside air through the system and heating and cooling that air as needed—adjusting the inside dew point to the desired level can be a real challenge.

For example, in temperate climates, winter dew points are often below freezing. When the heating system circulates that air in an indoor environment after warming it, the air is extremely dry and, unmodified, would produce humidity levels that would be extremely uncomfortable for people and all kinds of problems for art objects and artifacts. The dew point of this circulated and warmed air must be raised significantly to reach desired levels.

In the summer, the problem is reversed; dew points are naturally higher in the summertime. This moist air is circulated by cooling systems and dew points will be far too high when pumped into internal environments after being cooled. The system’s challenge in this instance is to lower the dew point to an appropriate level, since high dew point levels can promote the growth of mold and have a direct negative impact on paintings, photographs and other forms of visual art.

The Bottom Line

Dew point control is the often overlooked key to managing indoor environmental conditions. While temperature receives most of the attention, obtaining a heating and cooling system that can control the dew point is imperative for anyone dealing with art objects and archival materials.

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