Heating, Cooling and HVAC

Humidity is the condition of the atmosphere in relation to the water vapour it contains and is a fairly complicated subject to deal with fully, but a few brief notes will help you to understand enough of the subject for our purpose.

Water vapour is always present in the air in varying amounts, the amount that the air can hold depending on its temperature, the higher the temperature the more water vapour it can hold. The dew-point is the temperature at which air containing a certain amount of water vapour becomes saturated; any further reduction in temperature would result in condensation.

The Role of Humidity

Because of the temperature gradient, humidity decreases rapidly with altitude. The extremely low humidity at 12,000m is responsible for the uncomfortably low humidity in commercial aircraft. The ground-level outdoor humidity is constantly changing. During the day, water vapourises from forests, fields and lawns at about 1mm/day, about the same rate as from lakes. As moist air rises to cooler air levels, clouds are formed. Also, there is always a humidity gradient between sunny and shadowy spots on the ground. Even a slight wind is effective in transferring moisture to a cooler spot, where condensation can occur. Traces of atmospheric dust or other matter, such as the leaves of some plant species, are capable of inducing from saturated air condensation that then drips to the ground and waters the roots.

During the diurnal cycle, the water content of air increases while the sun shines. During the night the temperature drops and dew forms and recycles moisture to the soil. In coastal areas, the humidity can approach 100% at night on a regular basis.

Indoor climate

The indoor climate differs in several fundamental ways from the outdoor climate, because air inside of buildings is confined in a comparatively small volume. In fact, it is often insufficient to maintain human moisture and pollutant effusion below a noticeable level. Furthermore, indoor air is not part of the biological and climatic air cycles which purify ambient atmospheric air and disperse pollutants. As a result, the quality of indoor air undergoes tremendous variations in a short time. This is reflected in indoor humidity trends. The moisture content of a closed room increases rapidly because each occupant continuously adds moisture to the air in the form of perspiration and with every breath day and night at a rate of several litres of water per day. This water readily condenses on cold walls and windows, but since there is no soil or surface to absorb it, every effort must be made to replace the water-laden air by ventilation.

Thus, if buildings are not carefully ventilated, the indoor habitat becomes saturated with moisture, causing not only air quality problems, but eventually structural damage.

Source:Vent Axia Ventilation Hand Book