If the windscreen keeps fogging up again minutes after you clear it, the cause may be under the carpet rather than in the glass. Blocked cabin drains can trap rainwater or air-conditioning condensation, leaving the interior damp enough to mist the windows day after day. These drains are simply small channels and tubes that carry water away from places like the base of the windscreen, the sunroof frame and the air-conditioning box.

Where the water gets in

The most common trouble spot is the panel at the base of the windscreen, where leaves, dirt and tree debris collect. When its drain holes block, water can rise and spill past the pollen filter housing or nearby seals, then run into the front footwells.

There are other hidden routes too. Sunroof drains can back up and send water down the roof pillars, while a blocked air-conditioning drain may drip into the passenger side after a drive. Because the foam under modern carpets can hold a lot of moisture, the top surface may feel only slightly damp even when the underlay is soaked.

Signs it is a drain issue

The clue most drivers notice first is not a puddle but persistent condensation. A wet cabin raises humidity, so the glass fogs easily on short trips, cool mornings or after rain. A musty smell is another strong hint. If the moisture feels greasy or smells sweet, though, that can point to a different leak and is worth checking quickly.

  • Windows mist up more than usual, even in otherwise dry weather
  • Carpets feel damp near the pedals or front passenger footwell
  • A stale or mouldy smell appears after rain or after using the air-conditioning
  • You hear water sloshing or find moisture under floor mats days later

Drying it properly

This is why simply wiping the glass or changing mats rarely solves it. The blockage needs clearing, and the carpet underlay often needs time to dry properly as well. Gentle cleaning of leaves and debris from the windscreen base is a sensible first step, but forcing wire or high-pressure air into drains can detach hoses on some cars.

If the problem keeps returning, ask a workshop to check the scuttle drains, sunroof drains if fitted, the air-conditioning drain and the pollen filter area for water tracks. It is also worth drying the under-carpet foam, not just the visible carpet, because trapped moisture can lead to mould, corrosion and electrical faults around seat wiring or floor-mounted modules.

If your windows fog unusually often and one footwell always seems damp, do not assume it is just wet weather. A small blocked drain is often the real cause, and catching it early is far cheaper than living with a soaked interior.