Condensation is caused when moist air of a warmer temperature comes in contact with a surface of a cooler temperature. A benefit of the non-conductivity of PVC-U profiles is reduced condensation. When outside temperature is low, the PVC-U window frames inside can still be warm because PVC-U is a very good heat insulator.
Condensation occurs when humid air is cooled below the temperature (dew point) at which it can hold the amount of water it has absorbed. If the air is fairly dry, it can be quite cold before water starts to condense. The more water held in the air, the higher the temperature at which condensation starts, and in very humid conditions condensation might even be found on warm surfaces.
Aluminum frames are very poor in heat insulation. When outside temperature drops the inside surface temperature of aluminum window frames drops under dew point which causes condensation. Sometimes the aluminum window frames are so cold some people see ice on aluminium window frames when they wake up in cold winter mornings in the South Island.
Condensation on single glazed aluminum window
While PVC-U window frames are a very good heat insulator which can dramatically reduce the possibility of condensation on window frames condensation can still form on double glazing.
To prevent condensation on double glazing either warm the window surface or dry the air inside. Raising the room temperature can warm the window. To reduce the relative humidity in the room, control or eliminate moisture sources.
The chart above shows that when the room temperature is 20°C the aluminum window surface temperature is 5°C and relative humidity has to be lower than 30% to avoid condensation. On the other hand, at the same room temperature, when PVC-U window frame surface temperature is 15°C, there will be no condensation even when the relative humidity reaches 70%.