Dampness in construction

Dampness control is fundamental to the proper functioning of any building. Controlling moisture is important to protect occupants from adverse health effects and to protect the building. The unwanted moisture enables the growth of various fungi in wood, causing rot and mould health issues(Airborne mold concentrations have the potential to be inhaled. Asthma is also triggered by the sensitization of dust mites accruing humid, wet regions of a structure, moisture accumulation and related problems, such as mold growth and material degradation.) The highest airborne mould concentrations are found in buildings where significant mould infestation has occurred, usually as a result of severe water intrusion or flood damage. Another health effect associated with structural dampness is the presence of bacteria in an indoor environment. Bacteria require water to grow and multiply and certain species can cause disease in humans, thus the intrusion of water into an indoor environment can place the health of occupants at risk from bacterial infections. Water removal and drying of wet building materials within two days will likely prevent mould and bacteria growth, therefore reducing occupants’ vulnerability to disease.

Measurement

• Relative humidity is commonly measured using hygrometers, which can be electronic devices or simpler instruments like psychrometers that use wet and dry bulb temperatures.

A Visual Guide to Damp, Mold and Indoor Pollution states that:

Excess moisture leads – on almost all indoor materials – to growth of microbes such as moulds, fungi and bacteria, which subsequently emit spores, cells, fragments and volatile organic compounds into the indoor air. Moreover, dampness initiates chemical and/or biological degradation of materials, which also causes pollution of the indoor air. Exposure to microbial contaminants is clinically associated with respiratory symptoms, allergies, asthma and immunological reactions. Dampness has therefore been suggested to be a strong and consistent indicator of risk for asthma and respiratory symptoms such as cough and wheeze.

An acceptable humidity level in indoor spaces ranges from twenty to sixty per cent year round. However, levels less than twenty per cent in the winter and levels higher than sixty per cent in the summer are deemed unacceptable for indoor air quality

Condensation: The moisture in the air condenses on cold surfaces, sometimes inside the walls called interstitial condensation. Out of circulating air patterns, a situation where air movement or flow deviates from expected or normal circulation pattern, either in a building or on a global scale. In indoor environments, being “out of circulating air patterns” means air is not moving effectively through a space, leading to stagnant air. In many cases, the insulation between the roof and wall is compressed, leading to a decrease in thermal resistance. Due to the lack of thermal resistance, condensation occurs, which leads to water damage in the indoor environment. (When insulation is compressed the air pockets are reduced. This compression decreases the insulation’s R-value and thermal barrier effectiveness.)

How dehumidifier works, by operating cooling coils below the dewpoint and draining away the water that condenses. Condensate dehumidifiers use a refrigeration cycle to collect water known as condensate, which is normally considered to be greywater but may at times be reused for industrial purposes. Some manufacturers offer reverse osmosis filters to turn the condensate into potable water. Desiccant dehumidifiers (known also as absorption dehumidifiers) bond moisture with hydrophilic materials such as silica gel. Cheap domestic units contain single-use hydrophilic substance cartridges, gel, or powder. Larger commercial units regenerate the sorbent by using hot air to remove moisture and expel humid air outside the room. The moisture removal efficiency of dehumidifiers can vary widely. However, “passive” humidity control, such as increased natural ventilation, has been used since ancient times.
Details in types:
Thermal condensation: Since the saturation vapor pressure of water decreases with decreasing temperature, the water in the air condenses on the surface, separating the water from the air.
Thermal condensation-refrigeration cycle(electric)