On dry cold mornings the glass is clear, but if it rained the day before I get a full fog on the inside that takes ten minutes of blowing to clear. Same car, same parking spot, same temperature roughly. I keep the windows shut and I have not spilled anything. It seems obvious that rain is involved but the rain is outside and the fog is inside, so what is the actual route the water takes?
You carried it in on your feet.
Wet shoes on carpeted mats put a surprising amount of water into a sealed cabin. Overnight it evaporates, and in the morning the glass is the coldest surface in the car so that is where it condenses. The rain outside is irrelevant except that it wet your shoes and the pavement.
Things that help, cheapest first:
If it fogs even after dry days, then you have a leak and that is a different conversation.
The recirculate point catches a lot of people. It feels like it should keep the damp out, but it just recycles the wettest air in the car back over the glass. Fresh air plus AC clears fog several times faster.
Cleaning the inside of the glass is the step everyone skips and then insists their car is special. Do it with a proper glass cleaner and two cloths and the difference is embarrassing.
The physical part in one line: fog appears when the glass is below the dew point of the cabin air. Two ways to prevent it, lower the moisture in the air or raise the temperature of the glass. Heating the glass is what your demister does and it is slow. Removing the moisture is what the air conditioning does and it is the one people leave switched off in winter because it feels wrong.
Small thing that made a real difference for me: leaving the car with the windows cracked a centimetre when it is parked somewhere safe and dry overnight. It equalises the humidity instead of trapping yesterday's wet air with you.
@stropping_sal · 3mo ago · 3 replies
If drying the mats does not fix it, check the drain under the windscreen where the wipers sit. Leaves block that channel, water backs up, and it finds its way into the cabin filter housing or the footwell. The tell is a damp carpet under the passenger side that people never notice because it is under a mat.
Press your hand flat into the carpet in both footwells. If it comes up damp, that is your route, and no amount of AC will fix it.
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@mise_en_mess · 3mo ago
Push hard into the carpet rather than patting it, and lift the mat while you are down there. Mine felt dry on top and the underlay underneath was soaked.
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@slowtrainjo · 3mo ago
Also worth checking the boot or spare wheel well while you are at it. Water gets in through a tail light seal surprisingly often and then evaporates into the same cabin air.
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