Windscreen fogging is a common issue, especially during early mornings or late nights when temperatures drop and humidity rises. This phenomenon occurs when warm, moist air inside your car meets the cooler surface of your windshield, leading to condensation. While this is typically a temporary inconvenience, persistent fogging can indicate underlying problems that may require attention.
What is the reason
Windscreens fog up due to condensation, which occurs when the moist air inside the car comes into contact with the cooler glass surface, causing the water vapor to turn into tiny droplets. This usually happens when there is a temperature difference between the inside and outside of the vehicle—typically on cold or rainy days—combined with poor ventilation. Activities like breathing, wet clothes, or bringing in snow or rain contribute to the humidity inside the cabin, accelerating the fogging process.
Inoperative Air Conditioning Problem
A foggy windscreen might also be a sign of an issue with your air conditioning system. In cooler weather, when you switch on the heater to clear the windscreen, the A/C often activates automatically to help remove moisture from the air. That’s because air conditioning not only cools your car—it also dries out the air inside. If the A/C isn’t functioning properly, moisture stays trapped in the cabin and continues to fog up the glass. Common A/C issues that may lead to this include a faulty compressor, a leaking evaporator, a blocked receiver/drier, low refrigerant levels, or even something as simple as a blown fuse. In some vehicles, a flashing heater control light may indicate a malfunction in the air conditioning system.
Potential Problem With Heater Core
What looks like fog on your windscreen might actually be something more serious—engine coolant. Your vehicle’s cabin heat comes from the heater core, a small radiator-like component that circulates hot coolant from the engine. If the heater core develops a leak due to corrosion or damage, it can release a fine mist of coolant through the air vents. This mist can settle on the inside of your windscreen, leaving a hazy film that doesn’t clear easily. A telltale sign is a sweet, syrupy smell inside the cabin. You might also notice dampness or a coolant puddle on the passenger-side floor, and in some cases, your low coolant warning light may turn on.
What should I do with a Foggy Windscreen?
If your windscreen fogs up, turn on the car’s defogger or demister to blow warm, dry air onto the glass, which helps evaporate the moisture. Use the air conditioner to reduce humidity inside the cabin, even in cold weather, as it dehumidifies the air. Open the windows slightly if needed to improve airflow and allow moisture to escape. Avoid wiping the fog with your hand, as this can smear the glass and reduce visibility. For a long-term fix, keep the windows clean and use anti-fog products or silica-based moisture absorbers inside the vehicle
