Why do my windows fog up in my car

12 Jul 2018

Advice

If winter motoring is blurring your vision, these five tips will help you clear the air

If you live in the southern part of Australia, chilly winter months can make driving tricky.

If it’s not wet or icy roads that are throwing you a curve ball, it’s windows that fog up at the drop of a hat. Indeed, poor vision from fogged up windows can make driving downright dangerous. And still we’ve all seen fellow drivers in cars that look like a sauna!

Fogging is caused by warm, humid air inside your car forming condensation on the cooler windows. It’s the reverse of the water droplets that form outside of an ice-cold can of drink on a warm, humid summer’s day.

Modern cars are especially susceptible to fogging. The tightly sealed cabins that keep us free from noise and drafts can also trap unwanted moisture.

But the remedies for steamy windows are really straightforward. Your car is fitted with a number of features aimed at keeping your vision clear, as our five top tips reveal.

Use the aircon

It’s a common misperception that the air-conditioner is only used to cool the air coming from your car’s ventilation outlets. But it can also be used in conjunction with the heater (or at least a neutral temperature) to dry the air inside the cabin, and prevent the accumulation of water vapour.

If your windows are ‘fogging up’, try leaving the air-conditioning switched on with your heater set at a comfortable temperature. You’ll be surprised how quickly the fog clears.

Hit the demister

Using your car’s demister function is an often overlooked way to clear the windscreen of condensation. In most modern cars the demister will engage the air-conditioner and the fan to force dry warmed air on to the windscreen.

Climate control systems will also open the vent to draw fresh air into the cabin, and often engage the heater for a short time to warm the chilly glass. Warm dry glass will stay free of condensation a lot longer than glass that’s cold to the touch.

Let the outside in

Many of us use our car’s recirculation feature to keep fumes and other contaminants out of the cabin – great idea! But recirculating the cabin air over and over again also serves to prevent any moisture accumulated in the cabin from escaping (in fact, it compounds), and it’s this moist air that forms as condensation on your car’s cold glass.

So open the vent, or wind down the window for a little while. The fresh air entering the cabin will replace the stale, humid air and help prevent fogging up.

Press defrost

In addition to the front windscreen demister, your car is also equipped with a rear window defroster. To keep the glass clear of condensation, the defroster uses an electric element to warm the glass sufficient to evaporate accumulated moisture. It’s a system that works as effectively as your windscreen’s defroster, eliminating one of the two causes of foggy windows (remember, one is warm, humid air, the other is cold glass).

Some cars even have almost invisible elements in the windscreen for the same effect. In others, the defrosters also serve to clear your car’s wing mirrors – another very handy feature!

Keep it clean

The oil, dust and other contaminants (such as volatile compounds from dashboard vinyl, etc.) that builds up on your windows gives water vapour more surface area on which to hold. Clean glass reduces the amount of area these tiny droplets love, meaning your car’s glass will take longer to steam up in the first instance, and be quicker to clear with the methods above once foggy.

There are also additives that can also be applied to clean glass to help prevent window fogging. These should be used with caution on tinted coatings and rear window heater elements. Check your vehicle’s manual for details.

Our team of independent expert car reviewers and journalists

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