Why does my Windscreen Freeze 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.
How to stop or prevent it
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 (not gel) 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.
When you are close to your destination, turn the heat down and open the windows to regulate the temperature in the vehicle and remove moisture. This can help with the moisture build up.
Check when your cabin filter was last replaced, many manufacturers will advise on a recommended replacement interval, if yours hasn't been changed in a while it may be holding moisture and not allowing fresh air into the cabin.