Driver Knowledge Tests

How to quickly demist your car’s windscreen

Trying to peer through that A4-sized patch you’ve hastily cleared as it gradually fogs up again is no fun in heavy traffic. Fortunately, your vehicle has some immediate settings you can use to clear your windscreen quickly.

A foggy windscreen will happen when the temperature outside is low and the air inside the car is humid. It’s worse if your windscreen is also dirty on the inside (the condensation forms around dust and dirt particles on the windscreen), if you get into a car in wet clothes (the heat from your body will evaporate the water from your clothes), or if you have a leak in your car that is keeping something wet, like a carpet. The important part is the humidity – if you can reduce that, your screen will clear. So, there are five rules for quickly defogging your windows:

  1. Aircon on – this dries the air
  2. Fan full blast and directed at the windscreen – this passes the maximum volume of dry air across the windscreen
  3. Air set to outside, not to recirculate – the air outside is less humid
  4. Temperature as high as it will go – warmer air evaporates the condensation quicker
  5. Windows down a crack (unless it’s raining) – this helps get fresh air, drier air, into the car quicker.

Your car might have a button that does the first three for you.

Once the screen is clear, turn the air conditioning off if you don’t need it because it can increase fuel consumption by as much as 10%.

If your car doesn’t have air conditioning, open the windows and use the heater on full blast.

Rear demister

The rectangle with the wavy lines is for the rear window

If your rear window is foggy, turn on the rear demister, but don’t leave it on any longer than necessary because it takes power to operate and therefore is wasting fuel. The rear demister is a fine wire heating element either on the glass or embedded into the glass which heats the glass and evaporates the condensation. Some cars also have this feature on the front screen.

Honda Odyssey air conditioning panel showing (on the bottom row) recirculate, front demister and rear demister

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

Posted in Advice