Bathroom Fan and Open Window?

[Ralph]

My wife is always concerned about damp and condensation in the showroom, so always opens the window despite have a humidstat fan in there (we don't have MHRV)

 

I've noticed that now the weather is colder, the inside of the open window (triple glazed) is running with condensation when the windows is open.

 

So are we causing problems leaving the window open while showering? Not to mention heat loss.

[SteamyTea]

The window is just hitting the dew point temperature, while also being is the path of humid air.

https://en.wikipedia.org/wiki/Dew_point

 

You run a risk of condensation forming inside the shower room as colder, external air, will be mixing with warmer, internal air. Remember that we tend to measure relative humidity, not absolute humidity which is the actual mass of water in the air.

So while the temperature and AH may seemingly only drop a small amount, it is enough to cause condensation.

You can get condensation forming at high temperatures, think of that puff of steam that comes out when you open an oven door. The local air temperature may be 120°C, and the kitchen temp 22°C, but there is enough difference to cause a phase change from vapour to liquid, in a fraction of a second.

 

Edited October 15, 2022 by SteamyTea

[Conor]

By having the window open, you're cooling the inside pane of glass (imagine a waterfall of cold outside air pouring in through the open window). This is enough of a drop for condensation to be formed. Better off having the window closes and having the fan womed for longer.

[Ralph]

Thanks folks, this confirms what I was thinking.