When you get into your car on a cold morning and the windows mist up, you can simply switch on the heater or air con and deal with the problem in short order. When your double glazing at home mists up, however, it can be a more difficult issue to deal with.

Misty double glazing – why it happens and how to deal with it

Misty windows are annoying and, as American author and evangelist Vance Havner once said, “It’s hard to be optimistic when you have a misty optic.” Where older single-glazed windows used to mist up all the time, newer double-glazed windows are less prone to the problem but it can still happen.

There are a number of reasons why your double glazing can mist up and it’s important to understand the causes before you can properly deal with the issue.

Condensation and how it works

You probably remember from school science lessons that condensation occurs when water vapour turns back into a liquid. This happens when warm air – which is pretty good at holding water vapour – hits a cold surface such as glass, causing the water to be deposited on the surface.

This is why the bathroom mirror mists up when you take a shower, the kitchen window mists up when you’re cooking, or any window mists up when you breathe on it. This is all to do with the relative differences in temperature between the air and the surface, so condensation is less likely in summer when the windows will be warmer. Of course, it works the other way too, cold air hitting warm glass can also cause condensation, but more of that later.

Okay, that’s the science dealt with, let’s move on to the practicalities of condensation in the home.

Why is my window misty?

There are a number of reasons as to why you might see condensation on your double glazing. The most common is simply because the warm air inside the house, for example, from cooking or taking a bath or drying clothes, is hitting the cold glass and forming condensation inside the window.

On cold days you might sometimes see condensation on the outside of the window – often in a small arc at the bottom of the pane. In this instance, condensation is good because it shows that the window is warmer than the cold outside air and therefore your double glazing is doing its job.

On older double glazing, you can sometimes get condensation between the panes where you can’t get at it. This is particularly annoying and is caused by the seals between the glass panes having broken down, allowing moisture into the gap where it gets trapped. So when the glass is warmed by your heating on the inside, or the sun on the outside, you get condensation between the glass.

Curing condensation

Condensation from household activities such as cooking and bathing is not inevitable and is fairly easily dealt with. The key is ventilation, open a window or use a trickle vent when carrying out these tasks and use an extractor fan if you have one. The trick is to establish a flow of air so it doesn’t have time to linger and deposit its water load. You should also keep the kitchen or bathroom door closed in order to prevent moisture-laden air from spreading to other rooms.

Condensation can cause water to pool at the bottom of the window. You should mop this up as soon as possible. If left it can cause mould and if it gets behind the window seals it can shorten the life of your sealed units.

Condensation on the outside you can safely ignore. You’ll most likely see it first thing in the morning when the heating has been on after the house has cooled overnight and it will go away as the outside temperature rises.

Condensation between the panes is where things get difficult. As we said above, this is caused by the seals of the double glazed sealed units breaking down, allowing moisture to enter between them. It can also happen in windows that have been poorly installed without a gap at the bottom between the frame and the glass to allow water to drain away.

The only real answer to this type of condensation is to replace the failed sealed unit with a new one. There’s a positive to this because the latest sealed units are much more efficient than older ones. This is because they have a bigger gap between the panes making them more efficient insulators because it’s harder for cold air to bridge the gap. The gap between the panes is also filled with inert gas rather than just air; again this makes for better insulation, keeping in the heat from your room.

Modern glass also has a low emissions coating. This means that heat from your room gets bounced back and also that any heat from the sun during the day is allowed in and retained rather than wasted. How does it know the difference? The heat from the sun is of a shorter wavelength which is allowed in, whereas a warm room generates longer wavelength heat that is bounced back into the room by the microscopic coating on the glass.

Didn’t we tell you there was a test?

So, what have we learned from all of this? There are some simple points to help you understand how to deal with misty windows.

  • Ventilate your kitchen and bathroom when they’re in use
  • Don’t allow water to stand at the bottom of the glass
  • Condensation on the outside is fine
  • Moisture between the panes means that you need to replace the unit

If you need any additional help or advice, why not call our friendly team today?