Moiture Control Flashcards
What is condensation?
Condensation is the process where water vapour in the air forms water droplets on contact with colder surfaces.
What are the sources of moisture that can cause condensation in buildings?
Local weather conditions, building occupants, activities such as cooking, devices, and plants.
What are the two types of condensation?
Surface condensation occurs as a film of moisture on the outer surface of floors, walls, ceilings, and windows. Interstitial condensation occurs within the construction, where moist air passes through the construction at or below the dew point.
What is the temperature gradient?
The temperature gradient is the change in temperature through the layers of the construction of a building element.
What is the dew point?
The dew point is the temperature at which a fixed sample of air becomes saturated. If moist air is cooled so that it can no longer hold the vapour that it contains, the temperature at which this starts to happen is referred to as the dew point temperature.
What is the difference between a gas and a vapour?
A gas is a substance in a gaseous state that cannot be turned into liquid by increasing pressure, while a vapour is a substance in a gaseous state that can be turned into liquid by increasing pressure or a further drop in temperature.
What is the difference between evaporation and condensation?
Evaporation occurs when no liquid remains and more energy is given to molecules, while condensation occurs when less energy is given to molecules and the dew point is reached.
What are the causes of condensation and the associated risks?
Moisture in the air, contact of moist air with cold surfaces, lack of appropriate ventilation, lack of vapour control/Damp Proof Course (DPC), and lack of insulation can all cause condensation. The associated risks include deterioration of building fabric, health issues related to mould growth, and loss of thermal performance.
What is interstitial condensation?
Interstitial condensation is the same as surface condensation, but it occurs between layers in construction. It can’t be seen until too late, is hard to understand and calculate, and poses a risk to the longevity of the building.
What caused the timber-frame housing market to crash in 1983?
The use of plastic vapour barriers (usually blue) in timber-frame construction created an impermeable layer that prevented moisture from getting out of the wall.
What are the principle steps required for testing condensation risks?
The principle steps required are to measure/calculate the moisture in the air (relative humidity or RH), measure/calculate the dew point, and measure/calculate the surface temperature.
What is the normal threshold for condensation risk?
70% relative humidity is considered the normal threshold for condensation risk.
What is the difference between a survey and simulation when testing for condensation risks?
A survey is used for existing buildings, while a simulation is used for new buildings.
What is the purpose of measuring relative humidity?
Measuring relative humidity helps to determine the actual amount of moisture in the air and the maximum possible amount, which is necessary for assessing the risk of condensation.
