Windscreen fogs up on the inside
Condensation on the inside of the glass means the cabin air is holding more moisture than the glass temperature allows. Persistent fogging that returns every morning or a greasy film that smears is not normal weather; it means water is getting into the car or coolant is leaking from the heater matrix. Smell the mist on the glass: sweet means coolant, musty means wet carpets or a blocked drain.
Urgency
Safe to drive, fix soon
Possible causes, most likely first
- Leaking heater matrix
Sweet smell, a greasy film on the glass that is hard to wipe off and the coolant level slowly dropping. Press the carpet under the dashboard on the passenger side; damp carpet with a sweet smell confirms it. - Water ingress through seals, drains or the scuttle
Wet carpets, especially front footwells, and a musty smell. Check the windscreen scuttle drains, sunroof drains and door membranes; pour water over the suspect area and look for the entry point. - Blocked air-conditioning evaporator drain
The evaporator sheds litres of water on humid days; if the drain tube is blocked, it ends up in the passenger footwell. Find the drain under the car near the bulkhead and clear it with a soft wire or compressed air. - Recirculation stuck on or cabin filter saturated
Humid air is being recycled instead of replaced. Make sure the recirculation button is off and check the cabin filter; a wet, dirty filter also feeds a musty smell. - Air-conditioning compressor not engaging
Without the dehumidifying effect of the evaporator the glass fogs even with the heater on. Press the AC button and listen for the compressor clutch and a change in idle; no engagement means the system needs checking for refrigerant or a faulty clutch. - Wet carpets or mats from spilled liquids or snow
Simple but common: moisture released slowly from soaked carpets or rubber mats holding melt water. Lift the mats and press the carpet in all footwells and the boot floor. - Interior film from off-gassing plastics or cleaning products
A thin film that fogs quickly when cold is residue from vinyl dressing or cigarette smoke. Clean the inside of the glass with a glass cleaner and then a dry microfibre cloth.
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What you can check yourself
- Smell the fog on the glass and wipe it: sweet and greasy means coolant
- Press the carpets in all footwells and under the dash for dampness
- Check the recirculation button is off and inspect the cabin filter
- Press the AC button and listen for the compressor engaging