How to Clean the Inside of Your Windshield (And Stop It Fogging)

Most people don't know the inside of their windshield is dirty until the first dark drive home in the rain. In daylight it looks fine. Then an oncoming car hits it with its headlights and the whole windshield lights up in a gray smear, and you're leaning forward trying to see the lane lines on US-2.

That's not the rain. It's a film on the inside of the glass, and it's one of the easiest things in a car to fix yourself.

Where the haze comes from

A few things build it up, and most cars have all of them:

• The dash itself. The vinyl and plastic in a dashboard give off an oily vapor as they warm up, and it settles on the coolest surface nearby, which is the windshield. It's worse in newer cars and in summer, and it's the reason the film comes back even if nobody touches the glass.
• Hands. Every time someone wipes the fog off with a palm or a sleeve, they leave skin oil behind. That's the smudge shaped like an arc you can see at night.
• Smoke and vape. Either one leaves a yellow-brown film that's sticky to the touch. If the towel comes off tan, that's it.
• The heater. Dust and grime from the vents blow straight at the glass every time the defroster runs.

The film matters for more than glare. Water condenses on dirty glass faster than on clean glass, so a hazy windshield also fogs up quicker and takes longer to clear.

How to clean it, step by step

Do this with the car parked out of the sun and the glass cool. Warm glass dries the cleaner before you can wipe it, and that's where streaks come from.

1. Get two microfiber towels. Clean ones, and not the ones that went through the dryer with fabric softener, which leaves its own film. One is for cleaning, one is for drying.
2. Cut the oily film first if there is one. If the glass feels greasy or the towel comes off brown, mix isopropyl alcohol and water half and half in a spray bottle and wipe the glass with that before anything else. Glass cleaner alone mostly smears an oily film around.
3. Spray the towel, not the glass. Spraying the inside of a windshield puts cleaner all over the dash, and it can spot the dash and the gauge cover. Use a cleaner that says ammonia-free, because ammonia can damage window tint.
4. Do one half at a time. Sit in the driver's seat for the driver's side and the passenger seat for the other. Wipe side to side, overlapping each pass, all the way into the corners.
5. Reach the bottom with the back of your hand. The corner where the glass meets the dash is the hardest spot in the car. Flip your hand over so your knuckles are against the dash and push the towel down into the gap with your palm facing the glass.
6. Buff it dry right away with the dry towel, before the cleaner dries on its own.
7. Check it from outside the car. Look through the windshield at an angle, or turn on the headlights of another car pointed at it in the dark. That's when anything you missed shows up.

A trick for finding streaks: wipe the inside side to side and the outside up and down. If a streak is left, its direction tells you which side it's on.

The back window: go gently and wipe along the defroster lines, not across them. Those thin lines are printed onto the glass, and a scrubbing pad or a fingernail can break one, which kills that strip of the defroster.

Why your windshield fogs up, and how to clear it faster

Fog on the inside is the moisture in the car condensing on glass that's colder than the air. Around here in the fall and winter there's a lot of moisture to work with: wet coats, wet shoes, a wet dog, four people breathing.

To clear it fast:

• Turn the A/C on with the heat. It sounds backwards, but the A/C pulls water out of the air. Most cars switch it on by themselves when you pick defrost, and that's why.
• Use fresh air, not recirculate. Recirculate keeps the wet air you've been breathing inside the car. The outside air on a cold day is drier.
• Crack a window for the first minute if it's really bad. It lets the humid air out while the heater warms up.
• Don't wipe it with your hand. It clears a spot for a few seconds and leaves an oily smear that fogs worse next time.

To keep it from fogging in the first place, the glass has to be clean and the car has to be dry. Pull the floor mats out and dry them inside after a wet week, because a soaked mat keeps putting water into the air all day. A carpet that stays damp under the mats is also how the musty smell starts. And if the defroster seems weak, check the cabin air filter: a clogged one cuts the airflow to the glass. Your owner's manual says where it is.

Anti-fog treatments for the inside of the glass do help on cold, wet mornings. They only work on glass that's properly clean first, because they bond to the glass, not to the film on top of it.

Common Questions

How often should I clean the inside of my windshield?

About once a month from October to March, and any time you notice glare at night. In summer every couple of months is usually enough, though a new car's dash can film the glass faster when it's hot.

Can I use Windex on the inside of my windshield?

The original blue Windex has ammonia in it, which can damage aftermarket window tint and isn't great on the dash. Use an ammonia-free glass cleaner instead, sprayed onto the towel. If the glass is oily, go over it with diluted isopropyl alcohol first.

Why does my windshield get foggy so fast?

Usually because the glass has a film on it and there's extra moisture in the car. Water condenses faster on dirty glass, and wet floor mats or a damp carpet keep the humidity up. Clean the glass, dry the mats, and use the A/C with the heat on fresh air.

When I do it

The inside glass is part of every Interior and Full Detail, and the outside glass is part of every Exterior and Full Detail. Since I only book times that finish before dark, I check it in daylight before I go. An interior detail is $249 for a sedan, $289 for an SUV or pickup and $329 for a van or 3-row, and the same price in every town I serve, from Arlington down to Bothell. If the glass has a smoke film or the inside fogs no matter what you do, text me at (425) 600-7897 and tell me what you're seeing.