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Frosty windscreen

What causes frost to form on the INSIDE of the windscreen after sitting overnight? I have a '97 Subaru Impreza that has exhibited this problem occasionally since day one. I hardly ever use the recirculate switch, which I understand could cause moisture to remain in the cabin. I wonder about the integrity of the windscreen seal. Thanks for any ideas.

I’d like to know too. My '06 Ford Escape hybrid has the same issues and no moisture intrusion through any seals. All we could assume was the moisture we might bring in with us on snowy shoes or trap inside on high humidity days etc. but we could never prove it out. I put moisture absorbant in and around the dash area and it seems to help. Keep those little packets of silca gel that come in products you buy and use sticks of chalk to absorb, and clean the inside with vinegar & water.

High moisture inside the car and cold air outside.

As Ken suggested snow or rain on shoes or clothing as well as blocked drainage in the A/C or using the re-cycle rather than fresh air when driving can increase the problem.

Local weather conditions can also be a cause.

Maybe your recirc door is stuck in the wrong position. On some cars, you can see them work from inside the cabin near the passenger’s right foot or behind the glove box. Is your coolant level stable? Maybe you have a heater core leak.

Some moisture is getting into the car. It’s either on your shoes, stuck in the HVAC system (clogged drain) or leaking in somewhere. It doesn’t take much moisture to form frost on the inside of the windows on a cold night. Leave a window slightly open on a sunny day and see if it helps dry out the interior.

You don’t need a leak for that to happen, there’s lots of water vapor in your car’s cabin from you and potentially the atmosphere. You respire water vapor all the time. You breathe out 100% relative humidity so that’s approx 5% water vapor with every breath. Your skin is respiring water vapor too. The atmosphere has water vapor in it, some areas a LOT of it, some not so much. The car interior is relatively warm when you’re driving it so the air can hold more water vapor and when you park, the gases are basically trapped inside along with the water vapor. It cools down at night and viola, you hit the dew point and condensate forms on the inside of the window.

Thanks to all for the replies, advice, and suggestions. It is not a major problem by any means…only occurs sporadically and is primarily a nuisance by causing a real mess on the dash when scraping it off. My A/C is out of commission(discharged) and has been for about 5 years but I have noticed the problem before then. Also occurs even when there is no snow outdoors, so I don’t think it is because of myself or passengers bringing moisture in. It could be the vent door is malfunctioning intermittently as it only occurs on the windscreen near the base and not other windows…but now I have some ideas to pursue.

What’s stranger still for me is the 06 Escape is the only vehicle this happens to. We’ve lived in the same house for 33 years so a geographical location is not a proving factor. Two other trucks are parked in the same yard…no frost inside on days when the escape did. Still baffled.