How do I stop glasses, mirrors and windscreens fogging up?

Warm moist air meeting a cold surface condenses into thousands of tiny droplets that scatter light. Anti-fog treatments work by making the water spread into a clear film instead of forming droplets.

What you'll need

• Washing-up liquid (optional)
• Microfibre cloth (optional)
• Shaving foam (optional)

Step by step

1. For glasses, apply a tiny amount of washing-up liquid and buff it almost off.A very small drop, spread thin and polished with a dry cloth. It leaves a surfactant film so water spreads rather than beading.
2. Make sure a mask fits closely if that is the cause.Fogging with a mask is warm breath escaping upwards. A tighter fit over the nose, or a strip of tape, redirects it.
3. For bathroom mirrors, use the same technique.Washing-up liquid or shaving foam applied thinly and buffed off. It lasts a few days and is far cheaper than an anti-fog product.
4. For a windscreen, use the air conditioning.It dries the air, which heating alone does not. Air conditioning plus the heater clears a windscreen far faster than heat alone, in winter as well as summer.
5. Direct airflow at the screen and reduce recirculation.Recirculating moist cabin air keeps the humidity high. Fresh air from outside is drier even when it is cold.
6. Remove sources of moisture from the car.Wet mats, damp coats, snow on boots. A car with wet carpets fogs continuously and no amount of demisting fixes it.
7. Clean the inside of the glass.A film of grease and residue gives condensation something to cling to. Clean glass fogs noticeably less.
8. Improve ventilation for persistent household fogging.Condensation on windows indoors is a humidity problem. Extractor fans, trickle vents and brief vigorous airing address the cause.

Tips

• Anti-fog sprays and wipes work well and last longer than household solutions. For anyone who wears glasses daily they are worth the cost.
• A car with an air conditioning system that has not been used for months may need it running regularly to keep the seals in good condition.
• Cat litter or silica gel in a sock left in the car absorbs moisture and reduces fogging noticeably.

Common mistakes

• Using only the heater on a foggy windscreen — Heat alone raises the air's capacity to hold moisture but does not remove it. Air conditioning dries the air and clears it far faster.
• Leaving recirculation on — It keeps recycling humid cabin air. Fresh outside air is drier even in cold weather and clears the screen faster.
• Using too much washing-up liquid on glasses — It smears rather than forming a clear film. A very small amount buffed almost off is the technique.

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