The term relative humidity is a measure of the amount of water vapor held in a given volume of air compared with the maximum amount of moisture the air could hold at the same temperature. The ratio is usually expressed as percent relative humidity. Thus, 50 percent relative humidity means that the air contains half the water vapor it is capable of holding at that temperature. At 100 percent relative humidity, air contains the maximum amount of vapor it can hold, and such air is said to be saturated. The temperature at which saturation occurs is called the dew-point temperature.
Since psychrometry is the branch of physics relating to the measurement or determination of atmospheric conditions, particularly regarding the moisture mixed with the air, humidity is measured using an instrument called a psychrometer. It involves measurements of dry-bulb temperature and wet-bulb temperature.
The dry-bulb temperature is simply the air temperature and is sometimes called ambient or sensible temperature, commonly measured using a thermometer. The wet-bulb temperature is the temperature indicated by a wet-bulb transducer (thermometer), at which liquid or solid water, by evaporating into air, can bring the air to saturation at the same temperature. The results are plotted on a psychrometric chart, a graph drawn to represent the thermodynamic properties of moist air. A psychometric chart provides data to analyze the interaction between dry-bulb temperature and relative humidity. This allows experts to predict whether moisture condensation may occur on a particular surface, given the air temperature (dry bulb), the relative humidity, and the temperature of the particular surface. Knowing the air temperature and relative humidity, “they” also can predict the condensation temperature necessary to convert water vapor into liquid water.1 2 
There are two simple facts to keep in mind:
When it occurs in our dwellings, we refer to it as steaming or sweating on windows and other surfaces. Sweating can be found easily as a dew formation on a surface such as a cold glass of iced liquid or on the inner surfaces of window panes. Sweating or condensation also can occur in walls, roofs, ceilings, and floors of buildings. When it does, it can make insulation inefficient; cause rot, decay, or mildew on wood framing; and even result in deterioration of masonry materials. To make matters worse, this type of damage occurs inside the wall cavity, where it is hidden from sight and can go unnoticed for long periods of time.