Why does my windscreen freeze inside? What causes ice inside a car and how to prevent it

It’s an accepted fact that at this time of year your car windscreen will end up covered in ice.

The low temperatures and seemingly constant rain or snow means that most of us have to spend some time deicing our vehicle before we set off.

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However, for some drivers the problem isn’t limited to the outside of their car and cold mornings can mean scraping the inside as well as the outside of the windscreen.

Here’s why car windscreens ice up on the inside and what you can do about it.

What causes a windscreen to freeze on the inside?

The problem of ice 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.

If the temperature inside your car drops low enough any moisture in the air will condense on surfaces such as the glass and eventually begin to freeze.

Moisture can be caused by a number of things. It could be as simple as a wet coat or pair of shoes 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 difficult to spot are problems with the car’s drainage or ventilation but these can lead to the car being constantly damp and vulnerable to icy windows in winter and fogged-up windows in warmer weather. Check for clues such as wet floor mats or upholstery and look for any obvious signs of damaged door or window seals or blocked drain holes in the bottom of the door.

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How to stop a windscreen on the inside

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 and 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 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.

If you do find your windscreen is frozen, follow our guide here which details how to clear ice and condenstaion from the inside and outside of your car.