Dampness is one of the biggest factors that can deteriorate the structure of a building. Even if there is no water ingress through the exterior, e.g. roof or rising dampness through the foundations, dampness can occur through the use of the building. Damp surfaces can attract mould and fungus infestation, but also the proliferation of dust mites. These can have serious effects on the health of the occupants.
Our daily activities, such as cooking and using the shower or bath release moisture into the building. With every breath we also exhale a certain amount of moisture. On top of this water evaporates from every wet surface and indoor plants. The moisture content of the air is usually measured in relative humidity (rel.h.) with its peak at saturation point, which is 100% rel.h. At this point the moisture forms visible droplets (steam or droplets on surfaces). However in most cases dampness on walls is not visible. Often it is only discovered when black mould has formed.
How does condensation occur?
Water’s capacity to retain moisture is dependant on its temperature. Warm air can physically hold more water than cold air, e.g. at saturation point, 20 deg. C warm air can hold 17.3 g of water per cubic meter; at zero degrees this drops to 4.8 g/m3. If warm air cools down, the absolute moisture content stays the same, while the relative humidity increases. In the example below we have 20 deg. C. warm air at 50% rel.h. containing 8.7 grams of water in each cubic meter. If this air cools down on a cold surface, it reaches its saturation point (100% rel.h.) at about 8 deg. C. If the air cools down even further moisture falls out on the cold surface as condensate. You will have seen this effect when water droplets form on the outside of glasses with cold beverage.
Dampness from internal humidity occurs typically on cold surfaces, e.g. windows, window reveals or external walls. The risk increases if there is little ventilation in a certain area, e.g. behind curtains or furniture.
How to counteract such dampness problem?
Appropriate ventilation will help to expel excessive moisture.
MVHR systems will additionally reduce the indoor humidity, through the de-humidification effect of heat recovery. The more outside air temperatures are colder than inside air temperatures, the incoming air will increase in temperature and thereby decrease in relative humidity. E.g. Zero deg. cold air with 50% rel.h., raised up to 18 deg. C. will be introduced into the dwelling at less than 20% rel.h. As such it reverses the condensation effect mentioned above. Choosing the right amount of ventilation through the MVHR will help to keep the humidity levels in the right bracket.
Careful consideration of an appropriate ventilation strategy needs to be applied for both existing and new built housing stock. Existing properties that are being upgraded pose a special risk, as the building is being made more air tight and added insulation can often not be applied as consistently as in new buildings. Dampness can then easily form on the cold spots of the thermal envelope. According to Dr Sterling Howieson of the University of Strathclyde the risk of black mould triples for such renovated dwellings.