Dew Point
Why is it Important for Humidity Control?

The dew point (DP) is the temperature at which water vapour in the air condenses.

Water vapour in the air is called humidity. The amount of water vapor that can be "held" by the air rises and falls with increasing or decreasing temperature. Relative humidity is a measure of how much air the air is holding relative to how much it could hold at its current temperature. It is expressed as a percentage.

The amount of water air can hold decreases by half for every fall in air temperature of 20°F/10°C. A body of air, enclosed in a room, may have a relative humidity of 50% at 80°F. The air is "half full" of water. If the temperature falls to 60°F the relative humidity is now 100%, or "saturation". At saturation the air is now "full" of water. Saturation is the dew point, condensation will now begin to form.

The level of discomfort we feel in high temperatures is closely associated with the dew point. If the DP is as high as 60°F/16°C or above most of us feel uncomfortable and will fall increasingly so as the DP rises. This is because the body's cooling system, sweating, depends on the sweat evaporating from our skin and taking excess heat with it. If the temperature is high enough to make us sweat but the warm air is at or close to the DP that air is already "full" and our sweat can not evaporate quickly enough to keep us cool.

A DP of 80°F is extremely uncomfortable and even dangerous for people with certain health conditions, asthma for example. As the DP falls we are progressively less uncomfortable and between 50°F and 60°F is pretty much ideal for most.

Very low dew points, below around 25°F, are also uncomfortable and may lead to dry, cracking skin and irritation and discomfort in the breathing passages.

What does this mean for humidity control?

For our personal comfort humidity control is as important as regulating temperature. In very warm conditions many of us use air conditioning to cool the air inside our homes. Air conditioning also reduces humidity, lowering the DP.

Air conditioning is controlled by a thermostat, it cycles on and off according to temperature. If the desired temperature is reached but the DP remains too high we still experience discomfort.

The typical reaction is to reduce the setting on the thermostat but the use of a dehumidifier will be equally effective and may result in an energy saving. This will depend on the climate in your part of the world and the capacity of your air conditioning system.

If your air-con is "over-sized", that is say it is capable of effective operation in a home considerably larger than yours, it will run for less time to reduce air temperature to your preferred level than would be needed to reduce humidity sufficiently for your comfort. A whole house dehumidifier, integrated with your air conditioning but controlled independently of it by a humidistat, will almost certainly be a more energy efficient solution.

When it is cold we turn on the heating. This has relatively little effect on the humidity in the home so if it is too low, a DP below 25°F, we will be warm but not comfortable. In this case there is no alternative but to increase humidity artificially and a humidifier is an effective answer. Once again a unit integrated with your heating system will be the most effective and economical.

For the protection of your home and its contents a high dew point is particularly relevant if the ambient temperature falls significantly at night. If the temperature inside the home falls sufficiently to reach the point at which the air is saturated, water will condense on all surfaces. The fall in temperature may not be uncomfortable for humans but the resulting condensation may be. This is a particular problem in a basement. Because warm air rises and is replaced by cooler air the basement is usually the coolest place in the home.

A dehumidifier will remove the excess moisture from the air and lower the DP, preventing condensation damage, mold growth and controlling dust mites. If your basement is typically at a temperature of 65° and above a standard room dehumidifier, of appropriate capacity, will be adequate for your needs.

It doesn't have to be cold for condensation to form; what matters is the dew point.

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