The goal of this ible is to share a few tips and tricks I have learned over the years to prevent frost from forming inside your car's windshield.
Step 1: There Is Cold, and Then There Is COLD
First off, why does this even happen?
We all know that when you have a cold bottle of water in the summer, condensation builds up on the outside. During the winter, the opposite happens and the humidity from inside your car builds up on the cold window.
When you start looking online, you will find a lot of drastic conclusions such as you have a leak in your car etc., and this may be the case, but the truth is that "cold" means a different thing to different people, and most don't live in places where winter tires are mandatory because it's suicidal not to have them because a clean road is only mostly covered with snow and ice. In fact, most of the guides talked of water leaking in from the winter rain.... However, the colder it is, the less humidity you need to start building up that frost, so if you live in a place where -10c (14f) is not that cold and -20c (-4f) is quite normal, you really don't need that much moisture. So this is for everyone who doesn't have water pouring in their car, but rather just haved a really cold climate to deal with.
Next up... Sources of humidity and what you can do about them:
Step 2: You Have Your Air on Recycle
So why does it happen? Here are the main culprits, and some personnal annecdotes from the past 15 years
Don't do this, use the outside air. That option with the little arrow pointing from outside to inside the car? You want that one. When I finally started driving for my commute for the first time in my life, what I had been doing wrong finally caught up with me. I would have the heat on max, no AC, and the air recycled, and now that I was no longer biking and bussing, it started becoming a problem. It got so hot that I would feel sick and had to keep my windows open at -25c with the heat blasting. Over the first two months of winter, it just got worse and worse, until I had an ice sheet to scrape off the inside of my windshield every time I needed to use my car. Not just frost, but an actual sheet of ice. Sufficient to say, I don't do that anymore. That brings me to point two...
Step 3: Your AC Is Off
If you have AC, keep it on! Many cars now automatically turn on the AC when you put the window defroster. There is a reason, you car will still get heated up if you have the heat and AC on at the same time. The AC is just there to remove humidity.
Step 4: Water on the Floor
Your boots are wet and you drag in snow. Try to shake your boots off outside while sitting on your seat, and if you can see a puddle on the floor lining, empty it out.
Using a plastic lining can go a long way for this, as fabric mats will suck up all the water. For the past decade I used the cheaper kind, and I did keep a piece of fabric under them, so that the drips would not go in the actual carpet, so at least I could remove it and clean it once in a while. However, this can cause issues. If you can afford it, getting laser cut cusom fitted plastic linings designed your model of car works best and are safer. When you use a generic plastic matt with a towel of carpet under it, it can slowly ride up and jam your pedal, which is exactly what happened to me one day when I had to accelerate quickly. Luckily nothing bad happened. I quickly turned on my four ways and shifted into neurtral to pull over with the engine revving like crazy. If you do have floor mats that are not attached, make sure to pull them back now and then.
Step 5: You!
Your breath brings a lot of humidity with it, but I don't recommend you stop doing that. It is good to know though that it may be worse for you not because you are doing anything wrong, but because you are you. My wife barely ever has condensation next to her in the car, but whereever I sit, the windows fog up like it's that scene in The Titanic! Talking makes things worse, and so does exhaling from my mouth, but even if I do neither of those, within 10 minutes the windows fog up. Not much to do, but knowing it makes the troubleshooting less confusing.
And now on to generic solutions :
Step 6: Manually Remove Humidity
There are a few other things you can do that are not related to a specific source of humidity.
Once the car is hot and the frost melts, wipe it down with highly absorbant lint-free rags. These shop towels are the best, as micro fibre cloths tend to not be absorbent enough for the little bit of water you need to pick up, and paper towels or tissue paper will leave a huge mess. I discovered these towels when I needed to use a rub in wood finish, and I just can't immagine not having then in my workshop.
Step 7: Clean the Car
Remove whatever could be holding humidity like rags, crumpled paper in the bottom of the car, and even dust. These are all things that will soak up water and keep a nice fog going in your car.
Step 8: Use Sillica Gel
And last but not least, what works best for me at preventing that window fog from coming back, sillica gel packs.
Silica gell can actually be dried out and re-used, so you can effectively transfer the humidity out of your car that way. I got one of these little units. You can plug it in to "recharge" (dry) it, and I like that you don't have to keep an eye on it in the oven, or have to deal with a bunch of small beads that could be dangerous to children and pets if you spill them. The unit is self contained and even has a colour coded window that lets you know when the beads start getting saturated.
The pack will absorb what it can in short trips, but longer trips (45 minute +) where the car warms up and stays warm is where this will be able to really make a big difference.
A side note on this, if you drive an electric car, you may not be turning on the heat for your longer drives because they are better insulated and usually have other features to minimize the use of climate control such as heated seats and steering wheel. Unfortunately, this means that you probably don't even run the defog and AC enough to start with. My solution to this is to simply turn on the heat on full blast for about an hour once per month, with the sillica pack resting on or near the windshield vent so that it gets good air circulation.