Air contains moisture in the form of water vapour. The warmer the air, the more water vapour it is able to hold. Relative humidity (R.H.) is a measure of how much moisture the air holds in comparison with the maximum amount the air can hold at that temperature. For example, air at 50 percent R.H. holds half the moisture it is capable of holding.
As air is warmed, its relative humidity will decrease. For example, air leaking into a house from the outdoors in winter at 80 percent R.H. and at ¯10°C will have a relative humidity of less than 10 percent once it is warmed to 20°C. That’s why leaky houses can be excessively dry indoors in winter.
As air is cooled, its relative humidity will increase. For example, warm, moist air at 50 percent R.H. and at 20°C leaking out of a house and into an attic in winter will reach 100 percent R.H. by the time it has cooled to 9°C. At 100 percent R.H., condensation occurs, with water or frost forming on the nearest surface. This is why air leakage outward can cause moisture problems in attics and walls.
Air doesn’t need to pass into or out of the house to change its R.H. or to condense. Cold surfaces can also cause condensation. Think of the droplets of water that form on a glass of cold water on a humid summer day. The air immediately adjacent to the glass has been chilled to the point where its relative humidity is 100 percent, and condensation occurs.
Where is the coldest surface in a house during the winter? Usually, windows have the coldest surface, which explains why condensation – in the form of fogging or frost – is common on cold days. Cold surfaces can also occur at thermal bridges, which are locations where there is very little insulation or where structural members extend through the insulation from inside to outside. Examples of thermal bridges include exterior corners and wall/ceiling intersections.
You may be surprised to learn that condensation caused by cold surfaces can also be a problem in the summer. Usually, in late spring or early summer, the ground has not fully warmed up from the previous winter. On warm, humid days condensation can occur on basement walls and floors that are in contact with the ground. For example, outside air at 70 percent R.H. and 25°C will condense on basement walls and floors that are cooler than 19°C. Therefore, opening basement windows won’t always dry out the basement; in some cases, it can actually increase moisture problems.
The higher the indoor relative humidity, the greater the chance of condensation occurring on cold surfaces. There are many moisture sources that can contribute to elevated humidity levels in winter:
- occupants (30–40 litres/week for a family of four)
- summer moisture absorbed by house and released in fall
- (20–40 litres/week for about four weeks)
- drying of construction materials in a new house (25–35 litres/
- week for first 18 months)
- drying firewood indoors (15–30 litres/week for three cords/season)
- household activities: showering, cooking, bathing, washing
- (15–20 litres/week)
- damp basement, crawl space or sump pit (15–20 litres/week)
- drying clothes indoors (10–15 litres/week)
- humidifiers
- indoor pools, hot tubs, greenhouses, house plants and aquariums
Indoor humidity levels are also influenced by air exchange, which has the effect of replacing moist indoor air with drier outdoor air during winter. The rate of air exchange in a house depends on the use of mechanical ventilation, such as bathroom and kitchen exhaust fans or a heat recovery ventilator, and on natural air leakage. The combination of indoor moisture sources, air exchange rates and cold surfaces will determine how much condensation will take place in your home.
…To be continued February 22nd, 2012.
