It’s an all-too-familiar ritual on a winter morning. You climb into your vehicle, eager to be on your way, only to be greeted by a thin layer of frost coated on the inside of your windshield. You frantically try to scrape away what is obscuring your view so you can carry on with your morning.
But why frost on the inside when it’s the outside temperature that dipped below freezing overnight? Let’s take a look at why it happens so you can stop it from happening next time.
Understanding Frost Formation
If there’s frost on the inside of your windshield, it usually has to do with a battle between temperature, humidity, and ventilation in your automobile:
- Temperature changes – Air has a tipping point, known as the freezing point, whereby the temp drops low enough that some moisture in the air precipitates out of a liquid state into a solid form, that is, your frost. This usually occurs outside nighttime temperatures dropping below freezing, and the inside not being warmed up yet.
- Moisture/humidity – Think of the inside of your car as a miniature weather system. Moisture can intrude into that system from a number of sources, and that will affect the relative humidity (the amount of moisture the air molecules inside the car can hold). In high humidity conditions, as the temperature drops, there is less room for moisture, which condenses on the windshield.
- Ventilation/circulation – If your automotive ventilation system is not functioning properly, or if you shut it off too rapidly, that holds humid air inside. If you don’t get a chance to circulate it and bring in drier outside air, that damp air cools and deposits its moisture as frost.
Causes of Internal Windshield Frost
If you are experiencing frost on the inside of your windscreen, it may well be due to one or more of the following reasons:
#1. Too Much Moisture.
Sometimes the inside of the car can be like a mini rainforest! Bringing rain and/or snow in on our shoes and on clothes, spilling drinks, or even your own breath can all raise the level of moisture in there.
Try to keep things as dry as possible inside. If you spot any water on your floor mat, grab a rag or some paper towels and soak as much up as you can.
#2. Leaks & Broken Seals.
Sometimes your car is leaking, and you don’t even know it! Check your car’s door and window seals for any signs of damage. Water getting in there may well be the cause of your frost on the windshield problem.
A clogged sunroof drain tube is often to blame as this water often drains onto your floorboard.
#3. Poor Ventilation Systems.
It’s important for air to move properly inside your vehicle. If ventilation is not up to the job, moisture can’t escape properly, causing excessive accumulation and frost on the inside.
Make sure you are getting decent air flow from your vents (including windscreen defroster) and that your A/C is working properly. In most vehicles, your A/C is being used in temperatures above zero degrees when you turn on defrost mode, as it acts as a dehumidifier.
Make sure to check your cabin (A/C) air filter as well, clean and/or replace as necessary. A clogged filter is often the cause of poor ventilation inside. Costs typically run between $10-$20 and are often easy DIY replacements on most vehicles.
#4. Outside Weather Conditions.
Now Mother Nature can be blamed too! Winds with moisture and drops in temperature can effect the interior climate inside your vehicle.
It’s all about balancing the temps, and trying to keep your cars interior and exterior temperature close will help prevent the ice from forming on the inside.
Preventative Measures
Keep that windscreen clear of internal frost by lessening moisture and improving your car’s frost-fighting capabilities. Pursue these to keep your mornings frost-free.
Reduce Humidity Inside Vehicle
You already know that moisture = frost.
So grab some silica gel packs (save those packets from the packaging of your other purchases throughout the year) or a moisture absorber and drop ‘em in your ride. They’ll start hoovering up that humidity.
Rice or charcoal briquettes? Go ahead, poor dude dehumidifier. Even cat litter can help. Just throw a sock full of one or the other under your seats.
Sealant and Insulation Improvement
Sometimes the little buggers are the sealants cause all the trouble. Inspect your car’s window seals and door gaskets. Spotted a gap? A silicone-based sealant will plug it for the time being if you don’t have the time and money to replace the seal.
Insulate Your Windshield
Fitting a windshield insulation cover will prevent a sizeable temperature differential between the inside and outside of your car. It serves as a barrier protecting all that lovely warmth inside your car from losing the battle to the outside cold. Even a car cover will be of help in winter.
Less of a temperature differential means condensed moisture won’t freeze into frost that takes a shovel to remove.
Daily Habits to Keep Your Windshield Clear
You can remove frost or build up inside your car by adopting a couple of daily habits that will give you a clear view of the windy road ahead even on frigid mornings.
Effective Window Cleaning
Keep your inside windshield clean since a clean surface resist frosting up quite a bit. You can use Rain-X (inside and out) as a protective layer against frost.
Shaving cream will also work in a pinch; just spread a thin film on the inside of the glass and wipe off. The soapy film helps prevent condensation.
Turn on Defrost
The defrosting mechanism in your car is there for a reason. When it’s chilly, start your vehicle a few minutes early and turn on the defrost. Be sure to adjust that air recirculation setting to pull in fresh, dry air, cutting down the moisture in the cabin.
Note: It should go without saying but please don’t start up your vehicle then go back in the house while it warms up with car doors unlocked. Either use your remote start feature or lock that car doors. Some places, just DON’T leave your vehicle running unattended — ever.
Don’t Turn off Your HVAC System
We sort of hinted at this importance with our airflow discussion. Even so, many drivers go with simply turning off the ventilation in their car once they feel the inside temperature is agreeable.
Don’t do that! In the cooler months, your car’s HVAC system always should be on at least to its lowest speed to draw in outside air to the cabin.
Speaking of which, be sure the system is NOT “recirculate” (or the equivalent) because that’s simply moving around (humid) air already there in the cabin. Driving by a dairy cow farm? Turn on recirculate. The other 99% of the time? Turn off recirculate.
Other Absorbent Methods
Sometimes you need a little help. As we’ve already mentioned, throw one or two silica gel packs or dehumidifier pads in the interior of your vehicle to help soak up moisture.
Sprinkle a bit of salt on a damp cloth and wipe your windows (it lowers the freezing point and helps keep the frost at bay—just like ice melt does on your driveway!).
Dealing With a Frosty Emergency
Clear visibility while driving in winter weather is essential. If you’re pressed for time and discover your windshield is frosted over on the inside, here’s how to deal with it.
Fire up your car and turn your car defroster to full blast to gradually soften the ice. Meanwhile, use a plastic scraper to gently scrape away at the frost; a small scraper is ideal for negotiating the curves of the inside windshield.
To keep this from happening tomorrow, make up a batch of three parts vinegar to one part water, and wipe it on the night before to prevent build-up. Just take care not to damage your car’s finish.