What is moisture?

What is moisture?

Moisture is the presence of liquid, usually referring to water. The term “moisture” often means water content in trace amounts, either condensed or diffused as liquid or vapor. Small amounts of water may be found, for example, in the air (humidity), in foods, and in commercial products ranging from cosmetics to building materials.

Moisture in the Air

What is moisture?Moisture in the air, combined with sunlight, is what creates all of our weather. These two forces form clouds to make rain and thunderstorms and to cause winds to blow. Without the sun or moisture, the Earth would have no weather.
The amount of moisture in air is very small in comparison with materials or in liguid. On a normal typical spring day there is about 1 gram of water per cubic metre of air. Water vapour is defined the following ways:

  • As a vapor pressure (kPa) – this gives the amount of partial pressure the moisture is exerting in the air. The more moisture, the higher the vapor pressure.
  • As a weight by dry air weight (g/kg) – this gives the absolute mass of vapor inside a kg of dry air (approximately 1 cubic metre)
  • As relative humidity (\%RH) – this is the percentage of water that is in the air divided by the amount of water that the air could possibly hold in its current state.


Even over the hottest deserts, the air is full of water. We are in fact surrounded by water all the time. The air behaves like a sponge and constantly absorbs water. This water is in the form of invisible vapor, and has been evaporated from oceans, lakes and rivers. The actual amount of water vapor in the air depends on how hot and dry the area is. This vapor, or moisture in the air is called humidity.

When you breathe out, you are filling the air with water vapor. If the air is very cold, the vapor will turn into lots of tiny water droplets and your breath looks like steam.

We give ‘moist’ days words like humid or muggy, and ‘dry’ days words like crisp or fresh. What we actually are feeling is the Relative Humidity (\%RH) of the moisture in the air. The reason we feel this is because at high humidity days (close to 100\%RH – ie ‘saturation’) when we perspire, our sweat does not evaporate very quickly or easily, because the air around us is already at ‘saturation’ and can not absorb any more water. Conversely, on a ‘dry’ day, the humidity is low (ie 50\%RH) and our sweat evaporates easily as the air around us has ‘spare capacity’ to absorb more water vapor.


Although this water is usually invisible vapor, there are times when we can see it. When air rises and cools, some of the water vapor will turn into drops of liquid, forming the clouds, mist and haze that continually surrounds the Earth. This process is called condensation. Water vapor will only form water droplets if the air contains plenty of dust, smoke, salt and other particles. These are called condensation nuclei and are needed as a base for the water to condense on to. If the air is very pure, there will not be enough nuclei and clouds and mist will not form.

Moisture condenses as air cools. This is because the cooler the air is, the less water vapor it can hold. The limit to the amount of water air can hold is called its saturation or dew point. Once it reaches this limit, water vapor condenses into droplets and this is why dew is formed. After a cool night, leaves, grass and spiders’ webs are often covered in dew drops, the water that the air could not hold.

Moisture Content

Moisture content is defined as the percentage of water in a product or object. In some cases, moisture is desirable, but in others, it can cause problems. In commercial products, the moisture content is important because it can affect the product’s:

  • ability to be stored
  • its weight
  • its conductivity (water conducts heat effectively)
  • its integrity
  • contamination with organisms that grow in a moist environment such as bacteria or mold.

Moisture Problems in Homes

The level of moisture in the home is important in achieving a healthy home environment. Too much moisture in the home contributes to numerous health impacts, health hazards, and structural home damage. Excessive moisture also causes or contributes to mold health hazards such as asthma and allergies.

Several common signs that conditions of excessive moisture exist in the home include:

  • mold growth
  • mildew odors
  • deterioration of other structural items
  • rusting metal
  • condensation on piping and/or windows
  • rotting wood
  • peeling or blistering paint and wallpaper
  • visible water leaks and/or stains

Be sure to read Understanding Home Moisture and How to Control Humidity for additional information about limiting home moisture.