How to defog the windows without air-conditioning?

March 30, 2010 2:21 PM

How do I de-fog my windshield without air-conditioning on a cold, rainy day?

And before you say previously, the air-conditioning in my car does not work. Most of those solutions deal with tweaking your a/c settings, and I can't. I don't mean that I don't have cold air, I mean that I don't have the dry air -- the compressor... or something.

It's cold and rainy today. I tried everything this morning -- rolling the windows down, blasting cold air on the windshield, air from the inside, air from the outside, and everything seemed to make it worse.

Also, my car doesn't allow me to have the "defrost" (air blowing on the windshield) and the "recirculation" settings on at the same time.

Please help! I have to drive home soon, and I'd like to be able to see.

Or, is there no solution and I just have to take the train+cab back?

posted by thebazilist to travel & transportation (28 answers total)

keeping a window open will help significantly. since you have no air-based avenues left to explore, you might need to resort to a chilly drive with a window down.
posted by radiosilents at 2:29 PM on March 30, 2010

Is a dry lint-free cloth not an option? Am I missing something?
posted by Solomon at 2:30 PM on March 30, 2010

oh, you mentioned you tried windows already. well, i'm out.
posted by radiosilents at 2:30 PM on March 30, 2010

This may sound crazy, but did you try hot air?
posted by misskaz at 2:31 PM on March 30, 2010

Turn the heat up, even if you're uncomfortable. Raising the air temp lowers the relative humidity, which is exactly what the AC does.
posted by jon1270 at 2:32 PM on March 30, 2010

Blowing hot air on the windows works for me. I've also heard that you can clean the inside with shaving cream and that's supposed to help, but I've never done it so YMMV.
posted by TooFewShoes at 2:34 PM on March 30, 2010

Oh, I feel your pain. I had a car that had no AC *and* no heat for a couple of years. In the summer it wasn't a big deal (it was a convertible) but the damp New England winters and springs were annoying and foggy.

It sounds like you're gonna have to drive with the window open today - the fog collects because the temperature inside is warmer than the temp outside. Once you're moving and air is circulating around inside, it should help in keeping it unfogged.

To avoid it in the future, you could try Rain-X on the inside. If you already have some Rain-X on hand, dry it off with a really absorbent towel and use that.

On preview: Yes! try shaving cream! That's what people do to keep bathroom windows from fogging up. Dry off the window as best as you can, and put some on a towel and rub it on.
posted by AlisonM at 2:37 PM on March 30, 2010

Probably not the answer your looking for but,

Maybe keep a towel on hand to wipe extra moisture off window.

I used to have an old car which had a slow leak in the firewall which would puddle water at the drivers feet. Towel worked well, and in the winter leaving the window a crack open while parked helped reduce moisture build up (which leads to icy windows) because there was no temperature gradient across the windshield. Try leaving your window open a small crack while it is parked (only if this will not allow any water into the car, which would cause a bigger issue)

firewall ended up with a rust hole an inch wide from driver to passenger side of car. had to ditch the car for safety reasons + electrical issues from water. gotta love beater cars.
posted by MechEng at 2:39 PM on March 30, 2010

If you can warm up the windows, that ought to do it. This is all the "defrost" setting on the rear windows does - those little wires that run through the window just heat it up.

The fog is the warm moist air inside the car hitting the cold windows & instantly condensing (just like the mirrors in the bathroom).

I've tried Rain-X on the inside, and some sort of Fog-Off type thing (same manufacturers, I think) and it didn't work.

If you can get hot air blowing on the windshields, you should be fine. Test it in the parking lot first if you're unsure you'll make it.
posted by MesoFilter at 2:43 PM on March 30, 2010

AlisonM: "To avoid it in the future, you could try Rain-X on the inside. If you already have some Rain-X on hand, dry it off with a really absorbent towel and use that."

Specifically, Rain-X makes an anti-fog solution. It comes in a black bottle, rather than the normal yellow. The stuff works wonders.
posted by andrewcilento at 2:43 PM on March 30, 2010

Thank you!!!

Maybe I didn't leave the windows down for long enough to give it a fair chance.

I'll also see if I can grab some Rain-X. Shaving cream won't really work for today -- I'm parked in a city garage and can't really sit there wiping shaving cream around inside the car (especially since I have no idea how I actually will accomplish that without making a gigantic mess).
posted by thebazilist at 2:46 PM on March 30, 2010

Okay so people suggesting hot air --

I had the heat on for the first half hour of the drive, and then I realized that I was probably making it worse.

Won't the hot air I'm blasting be humid and condense, too, since it's pulling from outside?
posted by thebazilist at 2:49 PM on March 30, 2010

No, because the air would have to cool below the dew point. I think. We never used AC to defrost back in the old days, just heat & lots of volume. Crank up the heat full blast on the defrost setting and open the windows so you don't melt yourself.
posted by chairface at 2:52 PM on March 30, 2010

I keep a squeegee by the front passenger seat and use as needed on the inside of the windows, but will quite happily try Rain-X myself.
posted by dilettante at 2:52 PM on March 30, 2010

Won't the hot air I'm blasting be humid and condense, too, since it's pulling from outside?

No. The amount of water that air can "hold" increases as it gets hotter. Cold air from outside will be nearly saturated, but heating up will significantly increase the amount of water it can hold, allowing the water on your windscreen to evaporate.
posted by Mwongozi at 2:54 PM on March 30, 2010

Hot air holds lots of water. Hot water holds lots of sugar.

As these cool, the gaseous vapor turns into liquid water, and the liquid sugar turns into solid sugar.

This is called Supersaturation and is why fog occurs near water. (London Fog = London River) Hot air that holds lots of water suddenly cools when it hits the cold air above the water.

In the case of your car, the hot air that holds lots of water hits your cold windshield & the water instantly condenses (turns into liquid again).

By heating up the windows, you're allowing the water in the air to remain gaseous when it hits the windows.
posted by MesoFilter at 2:59 PM on March 30, 2010

In Vancouver, not everybody has AC. It's just not needed very often here. We do, however, get an abundance of miserable, cold rain. What do we do? We roll the window down. Or use the the standard hot air defrost. Or we use the sleeve of our jacket to clean out a small space above the steering wheel. It's not really an issue. A pain occasionally, but not a big deal.
posted by cgg at 3:00 PM on March 30, 2010

Here's a dramatic example of supersaturation. Hot liquid holding a liquefied solid is allowed to cool so that the solid is floating around just waiting for something to attach itself to. You drop in a crystal & this happens. Again.

If things were kept hot, the crystals would never form because the (sugar, etc.) in the water would still be a liquid.
posted by MesoFilter at 3:04 PM on March 30, 2010

Windows cracked a bit, defroster blasting with warm or hot air. OK to not have the recirculate setting on. It works for me.
posted by fifilaru at 3:24 PM on March 30, 2010

definitely use rainex's fogex. works in florida, the land of 100% humidity and perpetual morning dew point
posted by Redhush at 3:32 PM on March 30, 2010

Shaving cream won't really work for today -- I'm parked in a city garage and can't really sit there wiping shaving cream around inside the car (especially since I have no idea how I actually will accomplish that without making a gigantic mess).

You don't need to use a ton of shaving cream, just a little bit. Put it on a towel and rub it around on the glass, kind of like you would car wax. Then, if you need to, wipe the excess off with a new, clean towel.
posted by AlisonM at 3:52 PM on March 30, 2010

Even without working AC, the defrost setting will still work if you set it to blow hot air on the windshield. I've had several old cars that either had no AC, or had AC that died long ago, and the defrost settings would defog the windows.
posted by zombiedance at 3:59 PM on March 30, 2010

Do you have a vent setting? I don't have AC installed in my car, but if I turn the dial to cold I can vent outside-temperature air onto my windshield, and that does the trick.
posted by croutonsupafreak at 4:52 PM on March 30, 2010

OK to not have the recirculate setting on.

Not just okay, but required not to have recirculate on. Recirculate will just make things worse. The moisture from you shoes, clothes, floor mats and breath must be able to exit the car.

Make sure your floor mats, front and rear are not damp from leaking windows or a leaking heater core. The warm air from the heater will pick up the moisture from the floor and condense it on your windows.
posted by JackFlash at 4:54 PM on March 30, 2010

Right. Turn it onto defrost mode so the hot air is blowing at the windshield and keep recirc off.

And as JackFlash says, look for something causing excess moisture in your automotive environment.
posted by gjc at 5:47 PM on March 30, 2010

I read somewhere that a slice of cucumber will temporarily fog-proof a bathroom mirror. Maybe pop open the lunchbox and give a shot? The worst that can happen is you'll look really silly & all of us will get a good laugh. And maybe some funky smell in your car.
posted by Ys at 7:17 PM on March 30, 2010

Hot air worked!! And the best part was that it kept the fog off of the back and side windows so that I caould see when changing lanes (I'm in a minivan).

It was quite a bit warmer this evening than it was in the morning, though, so I'll be doing another test tomorrow.

I guess it never occurred to me that constant hot air would heat up the actual glass. I was imagining the glass staying cold and the hot air, pulled from the outside and full of humidity just continuing to condense on the windshield.

I'd still like to try Fog-ex -- unfortunately Manhattan drug stores don't carry auto supplies.
posted by thebazilist at 7:25 PM on March 30, 2010

those of us with old VWs keep a handful of clean(ish) bandanas on the passenger and back seats, for exactly this purpose. bonus - if you give somebody a ride, they can keep their side wiped too!
posted by toodleydoodley at 7:38 PM on March 30, 2010