Frost on *inside* of Windshield

I actually have more interior moisture problems here, near Seattle, than in my previous life in the much colder winters east of the mountains.

Over there, much of the winter was below freezing, causing a very low absolute humidity. As water evaporated inside the car, and pass-through ventilation flushed it out, the replacement air from outside was very dry. With little retained moisture, the morning ice on the inside was usually very thin.

Here, with much of the winter 2-15F above freezing, outdoor air can hold far more moisture. With the incoming air almost as wet as the outgoing air, far more moisture stays inside the car. In the morning, the liquid or occasional ice condensed on the inside surface is much thicker, taking much longer to clear.

 

My suggestions would be:

A) Make sure your set on the Front Windshield Defrost
b) Make sure your system is NOT in Recirc Mode
C) Set Temp to HI and Blower to Hi
D) Make sure A/C is set to OFF

Try these if you already have not, and let us know.

If the problem continues, A dealer visit may be in order to check it further. Good Luck!

 

Several,

Like any internet forum you always have to assume there will be a few bad apples in the bunch...it's too easy for some to reveal their evil side behind the anonymity. But most will be good people with helpful, considerate things to say. I find PC to be particularly helpful and friendly.

Now, the only "snarky" tone I have read in this thread so far, have been from you.

"Reread your post from someone else's perspective, remembering that text has no tone of voice. it was rather snarky."

 

Are you sure that the air intake is in ON position and not OFF in which case the air present in the car could be wet by your own breathing ?
Have a look at the switch !
This answer, only trying to help you and not thinking you're an idiot.
yours comments were not very nice towards paprius4030 !

 

Sure. East of the mountains is desert. There isn't much of a moisture problem in the desert. However, even with moisture, if your temperature is above freezing, you won't have any problem with ice on the inside of the windshield. Fog, yes, but not ice.

Our location combines the worst of both worlds: cold weather and lots of moisture. As you point out, the worst time for frost is during transitional weather, when warmer moist weather turns cold. When that happens here cars become encased in ice. People have to climb in from the hatch, or use a bucket of hot water to open the driver's door. It can be a mess.

Eventually, usually in February, the lake becomes so cold that little moisture is added to air. Only then do things dry out and we get to see some blue sky again.

Another poster hit on another key element, which is moisture from our bodies. Humans are warm, wet things, and we kick off a lot of hot damp air. Trapped inside the small confines of an automobile, that alone makes for a defrosting issue. I see this in spades when my brother and I drive back from playing hockey: two damp people with two bags full of soaked hockey gear. It's a good test of the defroster.

Tom

 

I have to agree with Bob and Tideland -- I frequently see cars running around this time of year with windows completely fogged up. I'd be willing to bet a paycheck they were driving around with their recirc buttons engaged. If there's no dry air coming in and it's below the temp cutoff for the compressor (or the A/C button is switched off), the humidity level inside the car will exceed the dewpoint with surprising speed.

I haven't seen it confirmed yet; however with most other vehicles, the cutoff for compressor operation is around 40°F OAT. Below this temperature, the compressor will not operate, regardless of the setting of the A/C button on the climate control panel.

Having said that, I have noticed myself frost on the inside of the windshield frequently in the mornings. This is not unusual behaviour for my vehicles in my area. There is also an inordinately large amount of schmee from outgassing coating the inside of the windshield, which probably makes it a perfect surface for condensate to form and subsequently freeze.

I'm not concerned vis-a-vis my vehicle. I would be interested to know the OPs outcome, however.

 

From the manual:
When outside temperature approaches 32 °F (0 °C) [FONT=Arial,Arial][FONT=Arial,Arial]The air conditioning system may not operate even when is pressed[/FONT][/FONT]

Several cars I have had will only allow a manually selected RECIRC mode to stay on for 10 or 20 minutes, but the Prius makes no such claim, so it relies on the driver to do it, even if the partial defrost mode is selected. Only with the full defrost mode button will it switch to outside air and bring on the a/c for dehumidification. Most other cars seem to bring on the compressor in the partial defrost/floor vent mode. I guess keeping the a/c off is done in the interest of fuel economy.

 

Reviewing a few basics that many of you may already be familiar with.

Dew or frost generally occurs on clear nights with outdoor humidity close to 100%.

The mechanism is loss of radiant heat to the dark night sky dropping the surface below the dew point.

It occurs on roofs rather than walls due to their greater exposure to the night sky.

Where windscreens are flattened out to reduce wind resistance (eg Prius), they are more exposed to the sky and lose more radiant heat than more upright windows, hence they are more subject to condensation and frost on both outside and inside surfaces.

Indoor condensation and frost tends to start at the bottom of the window because the window is cooler than the indoor space and this causes a downward movement of progressively cooler air on the inside of the glass which in turn results in the inside glass surface being cooler at the bottom than the top of the window.

All of the above may help explain why the OP is experiencing more frost with the Prius than previous cars, why it is occurring at the bottom of the windscreen, and why it happens in dry weather rather than in wet weather when night skies are cloudy.

There could of course also be some additional moisture source or some other problem that increases the degree to which this natural phenomenon occurs.

 

This is what I am thinking... perhaps during all the rain they got, a leaky seam caused the underside of the floor carpet to get wet. I have seen floor boards rusted out due to this (much older vehicle in a very damp climate). The owners were unaware because the top sides were relatively dry.

 

Additionally, the fact that the vents are likely underneath the area that gets this frost, as the 'warmer air that is still in the system eventually works it way up, and is met by the cool glass, causing the humidity to condensate on the windows. Since the window eventually gets to a minimum of 32 degrees (0 for my Canadian and other friends), that condensation freezes.

This has also happened to me on any car that I have owned since I was 16.

 

A lot of good reasoning and explainations. However, I just don't feel they are the cause of dew or frost inside the car. Actually lately I have been quite puzzled about this. In my Honda Legend the Glass inside the car has always been bone dry. No matter what weather. My friend lend me his car over these few days and while my legend is parked on trickle charge.
Every time I drive his car I have to use something to wipe off the excesss moisture inside the car whilst there are none in my Honda Legend (Acura RL).
So I belive the car is the single attribute to this problem because the Honda Legend I felt is very tight when all door closed. Where as my friends car (Toyota Carina 15 years old) is not as air tight as the Honda. Both cars were parked at the same place under the same conditions but does not come with same results.
So I don't realy belive it has anything to do with sky or temp diff or etc but with the car being well built or not.

 

My cars will do this even when parked inside a garage. I'm pretty sure my garage does not leak. They also develop frost on the inside in the winter, when there is no liquid water outside to leak into the car.

I suspect you are over-simplifying the cause of inside moisture.

Tom

 

I get the frost inside quite often, just because the ambient air is moist enough.

I've solved the problem with a 500w electric heater, that I plug into an extension cord, which is connected to the house on a timer. I have it run an hour before I leave in the morning.

Also gives me a warm steering wheel and seats, very useful in sub-zero C temps. I can wait 5-6 minutes of driving, getting to the highway, before turning on the Prius heater.

First time *ever* I see this subject come up in over three years. Probably because I have *always* had inside frost with *all* my cars.

 

That sounds nice... just curious, how do you get the cord into the cab?

 

If you go the baking soda route, look for packages they make specifically for using in your refrigerator. They may be spill-proof.

 

Simple it may be but the results are louder than words or just theory.
My friends car's glass inside the car is wet every day. Even when you parked for a few hours outside it will have moisture inside and it is very annoying. He actually have a sponge just for the job to wipe the glass dry.

While I had a cloth inside my Acura RL (Sold now), it was dedicated to use to wipe my glasses clean and not for glass for the car.

Only after I had parked my Acura RL for a month without starting it up it start to mist inside. I even asked my friend before he lend me the car how come the car can have mist inside. So you can see I never had that problem for 4 years.

So I can only conclude that the car or the built of the car contributes more to the misting inside your car more than anything since the results show it.

Different car on the same road, under the same weather conditions and with such a huge different results shows that the built of the car causes more effect to the dew or mist of the glass inside the car more than other factors.

BTW my new Prius doesn't dew or mist inside. Atleast not yet.

To be frank with you Honda makes much better cars in terms of quality and the material used. Can't wait for the Honda CRZ hybrid and then I will trade this Prius for it unless I can get my hands on a Prius plug in before the CRZ.

 

If he kept this sponge inside the car, the water will evaporate from the sponge and re-condense inside the windows the next night.

Unless the operator takes some action to move the water from the interior of the car to the exterior, it will keep reappearing on the windows.