Why do my car windows fog up on the inside overnight?

Friday, 01 October 2021, 11:00

Most - if not all - drivers have at some point encountered condensation on the inside of their car windows; those mornings where you head out the door to go to work, only to find condensation has built up in your car overnight, or where you’ve popped into a shop, only to come back to your car to find the windows have developed condensation inside whilst parked.

Why is it important to clear condensation?

Clearing condensation can be time-consuming - but it’s essential so you can see other road users and do not fall foul of the law.

The Highway Code states you must have a clear view out of all your windows. This is backed up by Section 41D of the Road Traffic Act 1988, which stipulates that you must have a clear view of the road before you move off – Which you won’t have if you have condensation on the inside of your windshield and windows.

So, what causes condensation inside your car?

Here we explain, and also look at how to get rid of condensation inside car windows.

Why does my car get condensation inside: How does condensation develop?

Condensation forms when the temperature and moisture levels inside and outside of your car are different.

Because your car's cabin is enclosed, dampness and heat can build up.

When warmer moisture particles come into contact with a cold windscreen, visible vapour converts to tiny droplets of water, which forms mist on the inside of the glass.

One of the main causes of condensation inside your vehicle is people breathing, although naturally this only occurs when a journey is undertaken.

However, condensation can also build up overnight or while the car is otherwise parked up.

If you’re wondering how to stop condensation on car windows overnight or when parked up, the following pointers can help…

What contributes to condensation inside your car?

Damp in car can lead to condensation building up inside your car.

Remove unused items

Certain items you keep in your car may increase the level of dampness. These include dog blankets, umbrellas and coats.

It's a good idea to remove items like these whenever you are not driving, to help reduce condensation inside car when parked.

Remove rubbish

It’s also a good idea to ensure you remove any rubbish from inside your vehicle as soon as possible as drink receptacles such as old coffee cups and opened water bottles can also contribute to the build-up of condensation inside your car.  

Over time water from these open containers evaporates and the resulting moisture gets trapped inside your car.

Leaks

Leaks can develop in various parts of your car, which can also contribute to condensation forming inside your car.

Leaking doors or a leaking sunroof can allow rainwater to get into the car and cause damp.

Find out how to fix your leaking sunroof

A leaking heater matrix can also cause damp, which further aids condensation on your car windows to form.

How to locate a leak in your car?

It can be challenging to pinpoint any such a leak, but a trained mechanic should be able to help.

In some cases the only way to locate a leak is to strip down a vehicle in its entirety, which might not be economically viable - especially for an older car.

Older cars are more susceptible to leaks

As your car ages, the chances of a leak developing increases, especially after the six-to-seven year mark.

Combating condensation inside your car

To help combat condensation inside your car, wash down your windows, both inside and out.

As grime accumulates it gives moisture something to adhere to - so removing it could be beneficial.

Use a specialist automotive cleaner and polish, along with a cloth or newspaper.

Cleaning your car on a regular basis will help keep dirt and grime at a minimum and therefore reduce condensation.

Absorb moisture

Tackle condensation inside the car by placing moisture-absorbing items in your vehicle.

One effective way to help reduce condensation on windows is filling an old sock or pair of tights with cat litter. The clay particles in cat litter are excellent at absorbing excess moisture from inside your car.

Air out your car

If the weather is relatively dry and it is safe to do so, leave your windows open to allow your car to air out.

Clear condensation before moving off

As mentioned, you are legally bound to clear your windows before you begin your journey - So what’s the best solution to clear your car windows of condensation?

The most common way to get rid of condensation inside car windows is to turn on the heater or heated windscreens:

• If your vehicle has heated windscreens, turn them on.
• Turn on your blower full-blast but without heat to begin with and aim it at your windows. Gradually increase the heat over a few minutes.
• If you have one installed, use your air-conditioning system to dry the air.

Problems with your car heater not working? Our handy article explains what to do if your car heater isn't working.

Why does the inside of my car fog up overnight?

How do I stop my car from fogging inside?