Condesation
Water comes in three states: solid, liquid and vapour. The building envelope (walls, windows, roofs etc.) separates the interior from the exterior. Generally, we want to keep solid and liquid water outside, and find this intuitive. What about vapour? Whenever we exhale, vapour is released into the air. Other activities like cooking or showering can increase humidity levels inside. Some humidity inside is comfortable, and harmless. Problems begin to crop up when vapour inside the building or within the building envelope can transform into water or ice.
Vapour becomes liquid when it contacts a cold surface and condenses. If the surface is very cold, it may freeze. To avoid accumulation of water and ice, we can do three things:
Control the temperature
Control the humidity
Make provisions for drying
Condensation occurs when air becomes saturated with vapour. This is called the dew point, and it changes depending on the temperature of the air. As air decreases in temperature, its capacity to store water vapour decreases – this is why we see dew and frost at night when the temperature drops and the humid air from the day is no longer able to hold as much moisture. We also tend to see condensation and ice build-up on other cold surfaces where relatively warm, moist air has passed by. For example, condensation may occur on the outside of cold water pipes, or the underside of attic sheathing where warm, moist air from the interior is getting into the attic.
Let’s imagine a balcony supported by a steel beam which extends into the building where the surrounding air is warm and moist. Over the winter, this steel beam will conduct cold temperatures into the building. This is a design flaw called thermal bridging: not only is it inefficient from an energy perspective, it can damage the building. If the beam is cold enough, condensation can freeze and build up layers of ice over the winter. This is the same phenomenon that happens inside a freezer that needs a good defrosting. When the weather warms, this ice can melt all at once and give the misleading impression that a leak has occurred.
Vapour often moves with air and can travel any path which air can travel. Water vapour molecules can also diffuse through some materials which are impervious to air. The building envelope should be designed to control the flow of vapour so that water or ice is not able to accumulate, and so that humidity levels are maintained below 60% relative humidity to prevent mold growth and deterioration of moisture-sensitive materials.
We can control the humidity using vapour barriers and the temperature using insulation, so that we don’t reach the dewpoint inside our buildings. As a third line of defense, we should also allow the building envelope to dry when it becomes wet (whether from water ingress or condensation). A small amount of condensation can become a big problem over time, when moisture is trapped inside the wall. Sometimes leaks aren’t quite what they appear – condensation and ice buildup may be the culprit.