Internal Frost Accumulations on my 2020 Prius Prime

Here is my write-up I just did in a different thread.

I have a few tricks to deal with internal frost on the front windshield.

First, I keep a thermometer/hygrometer on my dash to monitor cabin temp and humidity. When relative humidity is low, there is no worry about fogging up the window. I try to keep the cabin humidity below 70% all the time. For three short trips yesterday, while I was driving (indicated by the red lines in the graph below), the cabin humidity was lower than 60%. Humidity rise takes time (see the lag on the rise after my drive) and with a short trip of less than half-hour with only one passenger, inside window fogging is not a problem.



Second. I have treated inside windows of my car with an anti-fog agent. I use Rain-X anti-fog, but shaving cream would do the same job.

Third. I keep a few bags of desiccant on the dash and cabin. You can buy a reusable type like this. But rice or kitty litter in a small pillowcase or a sock would do the same job. I have been using one with activated charcoal which also acts as a deodorizer like this one. It works great to keep the air clean and prevent musty smells.

Forth. If the front windshield starts to fog up, cracking a window just a little is the quick and easy remedy to get rid of the fog.

Fifth. If it is too cold to crack the window open, then as a last resort, I turn on the HVAC manually set to temp 65F, AC OFF, and the lowest fan setting directed to the front window. With this setting, I can usually keep the EV mode without the engine turning on. BUT, albeit all the efforts, if the car decides to turn on the engine anyway, then I switch ON the front defrost. If the engine is already ON, then there is no reason to waste the good old combustion energy transformed into heat. Most of the energy in the petrol can not be used for propelling the car, so as might as well use it for the comfort of the driver.

BTW, I only did what I did yesterday as a part of my experiment. Normally, when the temp is below 14F outside, my routine practice is to turn on the car and immediately switch to HV mode to start the engine and warm up the cabin using the engine heat and save EV range for later use.

 

Just editorial: "upstate" location is where? And "7 degrees" is Fahrenheit?

Assume you're parking outside: any garage parking possible?

 

OP might chime in... but I assumed he is at upstate NY, and 7F temp is what he was experiencing when he posted yesterdat. BTW, today our low was -10F (-23C), I think OP is in a similar condition.

I don't know about OP's garage situation, but if it is unheated, my belief is that you still get internal frost if the inside cabin humidity is not controlled. Yeah, outside window frost and ice from precipitation can be avoided by sheltering a car in a garage. But for the internal frost and fogs, the enemy is the humidity inside of the cabin and dew point that can be reached easily when the outside and inside cabin temp differences are big.

I have never had an inside frost/fog problem on my car since I minimize inside cabin humidity and keep my car outside where the inside cabin temp and outside ambient temp are always very close. Thus dew point on the inside window is rarely reached.

 

It snowed off the inside of my Gen 3's windshield once. When I started the defroster to blow it around, it was a regular Interior Wizard-Blizzard.



That's all on the inside. White specks on the dash are snow.

 

I live a long way from a significant amount of water so humidity isn't a big issue for me. However, I still have a few rules to help keep internal frost at bay.

1. Minimize the amount of ice and snow that I drag into the car on my boots. It's just going to melt and then migrate to the windows.
2. Never use the recirculate function on the AC/Heater. That will just make things worse.
3. When I'm driving around town, always have a window open a bit to pull out any internal humidity.
4. If I have to clean snow off of the car, clear the air inlet at the base of the windshield before I turn on the heater fan. Otherwise, instead of sucking in outside air, it sucks in the snow.

Ed

 

Do you remove the wet towel from the car, to dry out elsewhere, and put in a fresh dry towel? If not, the moisture will just evaporate from the towel and re-condense on the window every night. Lather, rinse, repeat.

Do everything you to get moisture out of the car. Any wet towels, clothing, footwear, mats, etc., must be removed and dried outside the car. And it will take multiple cycles of this to draw out any moisture from carpets and upholstery that you can't remove for drying. Minimize any new water coming in, especially on footwear.

And remember that basic human respiration necessarily adds more, so must be flushed out. Don't leave your HVAC system set to Recirculate, as this retains moisture in the car. I nearly always have mine set for Fresh outdoor air, to help push that human-added moisture out.

 

I have this problem, too. I live in Ottawa, Canada, where winters are both cold and damp. Quite often there is snow on the car while it's charging with the cabin heater on, and I think some snow gets stuck in the grille at the base of the windshield and contributes to the moisture problem.

Like others have said, do everything you can to keep moisture out of the interior. I bought 3 dessicant packs and I put them on top of the dash whenever I park the car for overnight. They can be re-charged by microwaving them, and I think they do help.

I also have WeatherTech floor liners (the tray type) and I empty them out whenever there is snow melt in them, too

Honestly, I think the heating & defrosting with this car is marginal for Ottawa winters. I don't like to idle the car unnecessarily, but it can take a long time to get the car defrosted so I can drive safely. Maybe an electrically heated windshield, like the type available with some European cars would help, but they have their issues, too

 

And it (respiration) can be a lot more, depending on the cabin dew / frost points. If you notice the drivers side window / windshield fogging up you've got two choices, turn on the heat or open a window. The problem was batted around in the PiP threads 8 years ago.