Why are my car windows frosting up - on the INSIDE?

Winter has suddenly arrived in Chicago, and my car windows are not only frozen on the outside, but they are moist on the insides as well! Not from my moist breath; it happens before I even get into it. This never happened in the last ten winters I’ve had it. Now I have to run the defroster for five minutes (or wipe it with a towel) before I can even drive.

My theory is that the rubber molding that is supposed to seal the doors is breaking down. It’s a ten-and-a-half year old Scion with very low mileage, but that doesn’t keep the rubber from deteriorating. Does my theory make sense? Or is there something else I should be looking at? Thanks, car-mavens!!

I don’t know what it is like in Chicago, but it’s been fairly humid around here in Pennsylvania lately. When humid air gets colder, it can’t hold as much water, so the water condenses out.

Typical winter air isn’t so humid (because it’s usually cold all the time) so this normally isn’t an issue.

ETA: Open your windows for maybe 30 seconds or so to let the humid air out and the (colder) less humid air in, then shut the windows and turn the defrost up on its hottest setting. If your car doesn’t automatically turn the A/C on when defrosting, turn it on, as the condenser coils will help draw moisture out of the air. That will help your car defrost and dehumidify a bit faster.

If anything it’s because your seals are doing an exceptional job. When you drove your car last it could have been high humidity and once the car cools off it condenses on the inside.

What you have to look at is the transition from a rainy day to temps below zero. 100% humidity followed soon after by a drop in temperature below 32F will always produce frost on the inside of the windows.

There are other things that can cause this. You’re airbox drain could be clogged and any condensation in the box is trapped. That’s a very common occurrence. the last time this happened I had to use a coat hanger to unplug it. Whatever blocked the drain exit could not be unclogged with a pipe cleaner. Be warned. You really don’t want to poke an airbox drain line with something as stout as a coat hanger because you risk poking a hole in a heater core.

Thats dew. Just like you find it on the grass, you find it wherever humid air is cooled down. When the air in the car dropped below freezing, any humidity in the air is going to turn to water vapour , and on those cold windows, freeze.
If the vehicle has the fault (caused by age, not driving distance… the mechanicals wear out with distance … ) , the fault that causes this isn’t the door seals, its the air conditioner/ heater system leaking or blocked, so that water is either leaking into the system, or unable to escape the system.

The fan flows air through the air conditioner (cooler) first, and then the heater second, the heater which has hot coolant water from the engine in it.

The air conditioner’s cooler is small, and very cold, and it makes for very cold air…
Now very cold air can contain no water vapor, so water collects on the cooler and drips out below the car… But if it can’t leak out, the water may be forced through to the heater.

The heater may be leaking, and this will then cause the air in the car to become humid.

Well this is all conjecture, I am leaning toward the problem being that its not a severe winter there and you have warm humid days.
The main solution is to ensure the air in the car is dry before stopping.

Two solutions:

  1. the air if you are driving around in freezing cold air, eg at night open the vent ( Some vehicles show it as vent open, and some as “NO recycle”) and turn up the fan - you can have heater set to any temperature you like, its irrelevant to the absolute humidity in the car … This Can’t work if the air outside isn’t almos completely dry (freezing temperature…)

Heated air may feel drying when its warm, but it can still have a fair amount of water in it… this is the difference between relative humidity (how “drying” it is at the temperature), and absolute humidity (how much water there is in the air, irrespective of temperature… When air is freezing cold, it contains very little humidity, and it wont be dropping what it has as frost or dew in your car .

  1. Drive with the " Vent closed" or “Recyle” and air conditioner on.
    You can heat, if your car allows AC and HEAT at the same time (some cars will turn off AC when you require temperature to go up too hot… perhaps you just need to stick to warm air rather than cooking hot air…) ; the air conditioner acts as a dehumidifier, and the water drops out below the car… And then the heater warms it up to be dry warm air…

Actually you can do step 2 with the vent open but I can’t know if it works well enough in your circumstance. (depends on the humidity of the air outside and the flow rate of water from the cooler…)

Yep. My newish car has the same issue so it’s not the seals. Blasting the defroster with the AC on helps a lot.

Ha, this happened to me for the first time today too, and I heard someone at work mention this. No idea.

One big source of water vapor in your car is puddles of snow melt water on your floor mats. I got good results by mopping it up with a big towel I keep in the car. When the towel gets really wet/dirty, take it indoors.

This problem is gonna happen occasionally, so keep an ice scraper close at hand for your own safety.

Since it is a new occurance, your heater core could be leaking a little.

Thanks for all the advice! I’m still experimenting.

The first occurrence was indeed on a very cold (high teens, low 20s) day following several warmish (40s) rainy days, so the “leftover humidity” theory makes a lot of sense. And I did drive around yesterday blasting the fan with the vents wide open to get fresh (dry) air into the cabin. Today, the effect is greatly lessened, but not yet gone. The experimentation will continue, though. Will have to try the A/C trick to dry things out even further.

I always have this problem at my parents’ house - when I go there around 5 PM as the sun is setting, and leave well after the sun sets.

For me the problem isn’t warm air from driving over there (they live about 1 minute away) but it’s parking in the sun, which warms the hell out of the cabin, and then the sun goes down and everything freezes up.

I always wonder if a sun shade would help in this situation. Too distracted to try, tho.

This happens to my car, but I do know why: I don’t have a garage, so I use a car cover. It’s impossible to remove every bit of moisture from it. If I don’t have time to walk it back to the house before I leave for work and put it in the basement, it goes into the car. Given it’s a hatchback, there’s no way to really seal that moisture out of the rest of the car like a real trunk would. So, I get windows that frost inside sometimes. I try to make the time to bring the cover back to the house.

This just happened in my wife’s car, because the sun roof has been leaking. The wet floor mats were our first clue.

I recently had the same problem. I went to amazon and got a car dehumidifier

I put it on my dash board overnight and in the glove box when I’m driving. The first morning after putting it in my car overnight I had no more problems with fogged up windows.

My guess as well, always looking for the worst case scenario. :slight_smile:

Rue this out so the issue can be one of simple moisture control–check your coolant level.