Friday, 7 June 2013

Condensation on the windows

People have asked about condensation on windows. I heard a story from someone who moved into their new house and complained to the architects about condensation on the windows. It seemed like quite a serious problem. The architect's solution was to wipe the window with a cloth. 

So, do we get any condensation on our windows? Well, the answer is yes. But not on the inside and not in winter. We get it a few times a year on cool mornings in early summer. 

Condensation happens when the temperature of an object is lower than the temperature of the air. The object forces the temperature of the air down, which reduces the amount of moisture it can suspend and can lead to that moisture being deposited on the object. It's more likely to happen when there is a larger temperature difference and when the humidity is high. It's unlikely to happen inside a house if you have triple- or good double-glazed windows. Recent increases in airtightness, and use of non-porous insulation materials such as expanded (or extruded) polystyrene in Japanese houses often mean more condensation on windows. We rarely saw condensation on the windows of our old house, even though the windows were single-pane. The the heat was racing through the windows so quickly and the air was just far too busy getting through drafty gaps to worry about depositing its water molecules.

Anyway, I think the condensation is appearing on the outside of the windows because of radiation. Not radiation coming into the house from some nuclear power station, but heat radiating away from the house into the stratosphere. On a clear night, there is nothing to radiate the heat back, so the temperature drops below the ambient temperature. 

Meanwhile, what's happening in the bottom bit of the atmosphere as a whole is that the temperature is dropping, because it's night time. The air has the same amount of moisture, so as the temperature drops, the relative humidity goes up, since relative humidity is the amount of moisture in the air compared to the maximum it can hold, not an absolute measure of moisture. It was around 90% while the condensation was going on. 

This probably wouldn't happen on less well insulated windows, since the heat from inside the house has more effect on the temperature of the outside window pain. It's most apparent on our large south-facing window, and the condensation is away from the edges. With triple panes, the window insulates better than the frames, so the biggest possible temperature difference will be in the middle of the pane. Also, since it's radiating in all directions, the middle of the pane has less obstruction and will radiate more.

The other condition for this phenomenon to occur is that the shutters must be up. If the shutters go down, then the windows aren't going to radiate, their temperature is not going to drop, and they won't attract that moisture from the atmosphere.