Why does my windscreen freeze on the inside? what causes ice inside a car, how to clear it and prevent it

Why the inside of car windows ice over and tips on how to stop it, including using cat litter and shaving foam

An iced-over windscreen is one of those things that most drivers have to deal with at this time of year.

The combination of low temperatures and wet conditions means that cars are vulnerable to a layer of ice forming on the outside of the glass, leaving owners searching for the best way to quickly de-ice their car. Here’s a hint - it’s not hot water.

However, for some people the problem goes further than that and as well as ice on the outside of their car, they face frost on the inside of the windscreen. Not only does this mean more work to defrost the car but it’s also a sign of a bigger problem that needs fixed. So here’s why the inside of your car might ice up and how to stop it happening.

What causes a windscreen to freeze on the inside?

Ice forming on the inside of a windscreen is caused by exactly the same things that causes it on the outside of the glass - moisture and temperature. When the interior temperature of your car gets low enough any moisture in the air will condense and start to cling to surfaces, including the windscreen and windows. Once it makes contact with cold glass it will then begin to freeze.

Excess moisture inside a car is caused by a variety of things. It can be as simple as a wet coat, pair of shoes or umbrella left in the car overnight. Alternatively, a window accidentally left open could allow rain or snow to get in. Even having the heater up high just before parking can leave additional moisture in the air.

More serious but sometimes harder to spot are problems with the car’s drainage or ventilation. Anything that lets water into the car or stops it draining properly can lead to the car being constantly damp and vulnerable to icy windows in winter and fogged-up windows in warmer weather. Look for signs such as wet floor mats or seats and check for any obvious signs of damaged door or window seals or blocked drain holes in the bottom of the door.

How to remove ice from inside a car window

If you come out to find the inside of your car windscreen iced over you should use the same methods to clear it as you would for the outside - heat and the right tools. Use your car’s heating system to aim hot air at the windscreen and an ice scraper (not a credit card or CD case) to gently scrape the ice clear. Make sure to lay a towel along the dashboard before you start to soak up the water, just try not to cover the air vents.

How to stop a ice forming inside a car

The best way to stop your windscreen freezing on the inside is trying to remove as much moisture as you can from the car’s interior. If there’s an obvious source - be it wet clothes or dodgy door seals - fix that first.

If the cause is less obvious you can use special dehumidifier pads to suck moisture from the air. These reusable bags contain silica, are widely available and only cost a few pounds. A quick alternative is to place a tub with some salt, rice or cat litter in it in your car as these will all drawn moisture out of the air. Just make sure to replace it regularly.

If you park in a garage, leaving a window open a crack can help damp air escape the car. You can also try treating the windscreen. First thoroughly clean and dry the glass - water particles cling to dirt. Then apply a thin film of shaving foam to the glass with a microfibre cloth before buffing the foam off to a streak-free finish. The detergent in the foam helps create a barrier so the moisture cannot stick to the glass.