Why do glasses become opaque?
Glass surfaces inside a car fog when warm, moist air meets cool glass. It’s the same idea as a cold drink in a fridge getting a foggy bottle. When several passengers step into a humid car wearing wet clothing, the cabin humidity climbs and the windows fog up more, especially in rain. The moisture carried by breath also condenses on the glass, leaving a milky film.
So what can be done to stop this from happening?
1. Heat the air effectively
Direct the full flow of warm air toward the glass and set the heater to a high temperature. When the engine is cold, performance is lower, so increasing the fan speed helps. Air conditioning is a strong ally in defogging, provided the system works well in the vehicle. A quick one- or two-minute blast with the AC can clear the glass.
Make sure recirculation is not active. Recirculated air is already humid, which makes fogging worse.
In some Renault models the recirculation control can feel loose. Don’t rely on recirculation in wet weatherestimates—drawing air from outside is the best way to keep the glass clear.
2. Use electric heaters
Modern cars commonly feature rear window defrost. Activate it to improve rear visibility; fog on the rear window clears quickly. Front windshield defogging is standard even in affordable cars like the Lada Vesta. In many trims it’s included, while price-conscious replacements of cracked glass may omit heating elements.
Some vehicles have an electric heater integrated into the heating system, delivering warm, dry air to the glass as soon as the engine starts.
3. Inspect the air intake and path
The heating system draws air through a box under the windshield, which can collect leaves, seeds, and dirt. In rain, this debris becomes damp and humidifies the air, contributing to foggy glass. A simple yearly clean-out of the air box improves cabin air quality and keeps the glass clearer.
Air must circulate both in and out. Most cars rely on openings in the trunk area and under the rear bumper to vent air. Blocking these gaps with objects can trap moisture and worsen fogging.
Air outlets run along the sides and include check valves to regulate flow.
4. Check for leaks
Fogging often appears when water enters the car through leaks. Damaged seals, a leaking sunroof, or worn weatherstripping can let moisture in. A leaking heater core or a faulty evaporator drain can dampen soundproofing and mats. If the undercarriage is rusty or plugs are missing, water can seep in from below.
Driving through deep puddles without underbody plugs can soak carpets and cause persistent fogging on the windows.
5. Consider ventilation upgrades
Some owners upgrade their ventilation system in the trunk area to improve airflow, which helps reduce fogging. These tweaks can slightly lower the risk of condensation on the side windows, though raindrops may still enter through door liners that protect the cabin. Overlays can help ventilation but may reduce visibility and aerodynamics.
If the rear window defrost fails, a portable 12-volt fan on the parcel shelf can provide warm air and help clear the glass.
6. Apply coatings and simple remedies
Anti-fog products applied to clean, dry glass can limit condensation, but the effect may not last long. The coating wears off as sliding windows move and water droplets wash it away. Some people use basic home methods, like wiping the glass with a soap-water cloth, though this can freeze on contact with cold glass.
There is a frost-resistant homemade defogger: mix 10 ml ethyl alcohol with a few drops of dish soap. Some people substitute antifreeze for alcohol. These mixtures are easy to apply and can be effective.
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