Every winter, a large number of home owners are experiencing fogged-up windows, water running down from windows, frost buildup, and staining and mould on ceilings and walls. Unknown to the owner, there may also be significant wetness within the exterior wall system and deterioration of the wood structure.
The culprit is the excessive water vapour in the interior air. On the other hand, cracking furniture, scratchy throats, and static electricity buildup are symptoms of too little water vapour in the air. Both of these problems can be corrected, although it is more difficult to lower the moisture level than to increase it.
What is condensation?
Condensation problems in residential buildings occur because air can hold only a limited amount of water vapour, an amount that varies with temperature. Cold air is able to hold less water than warm air.
When air at a given temperature contains all the water vapour it can hold, it is said to have a relative humidity (RH) of 100%. The temperature at which the relative humidity becomes 100% is known as the dew point temperature. If the temperature drops below the dew point temperature, some of the vapour is forced to condense as water or frost, depending on whether the temperature is above or below freezing.
Air cooled by contact with cold surfaces, such as windows, will therefore deposit some of its water vapour on the glass or the sash whenever it has more water vapour than it can hold at its new and cooler temperature. This surface condensation is an indication of excessive water vapour in the air.
Two types of condensation
Basically, there are two types of condensation. Surface condensation on walls, ceilings, floors, and windows is caused by high humidity inside the building. This can occur even if the building is well built.
Concealed condensation, on the other hand, is caused mainly by the movement of moist air into the building envelope, primarily through air leakage. The amount of air leakage is dependent upon the tightness of the exterior building envelope. A vapour barrier is used to reduce moisture entering the wall or ceiling by diffusion. If the vapour barrier is well sealed against air movements from within the house into the wall or ceiling cavity, it acts also as an air barrier.
While the vapour barrier is important, it should be recognized that in most instances, the amount of moisture entering the wall cavity by air leakage is far greater and more damaging than that from vapour diffusion. Therefore, and effective air barrier is an essential part of the building envelope.
Sources of moisture
The principal sources of moisture in a home are household activities. These vary with the living habits of the occupants. Typical sources of moisture are the occupants, cooking, dish washing, bathing or showering, and clothes washing. Much more moisture will be generated if there are pets, plants, and aquariums in a home. The generation of moisture in a small house may range from 7 to 9 kg each day, and up to 23 kg per day on washing days. During the course of one week, this may amount to 65 kg of moisture. As liquid water this would be 65 liters and if spilled on the floor, there would be a lot of mopping up to do.
Electrically heated homes
Electrically heated homes typically have higher indoor humidity than homes with fuel-fired heating systems because electrically heated homes do not require a chimney or air for combustion. The problem of high humidity is further aggravated in newer homes because the air barrier has been made more airtight over the years. As a result, there is less chance of air leakage and thus the likelihood of higher indoor humidity. Condensation problems are particularly noticeable where baseboard electric heaters are the only source of heat, and where there is no forced air movement in the house.
To overcome this problem, it is necessary to vent these homes and to provide a controlled intake of outside air. Since the cold outside air will result in heat loss, and air-to-air heat exchanger should be installed.
Some humidity is necessary for comfort and health, but a balance between desired comfort and surface condensation must be reached. The first step in solving condensation problems in a home is a willingness by the occupants to reduce humidity.
Windows are often the coldest component of a building enclosure, and hence they can be an indication of humidity problems.
The following table provides a guideline for acceptable levels of indoor relative humidity in relation to outside air temperature. In rooms with poor air circulation, these RH values should be 5% lower.
|Outside Air Temperature||Indoor Relative Humidity (RH) less than|
Therefore, the two basic actions which should be taken when excessive humidity problems exist are: firstly, to reduce the indoor humidity and secondly, to ensure that the air barrier is tight. Since air and vapour barriers are not perfect, there will always be some minor air leakage and vapour diffusion and for this reason, it is also important to provide ventilation in attics and unheated crawl spaces.
While the recommended remedial measures may be costly in some cases, they are likely small compared to the damage caused to the building envelope and the structure over the long-term. Also, the heat loss through wet insulation can be significant.