Thanks to the Teeming Millions, I already know that keeping the AC turned on will keep my windshield fog-free during the warm months.
What do I do now, when it’s cold out?
When I get into my car after it’s been parked outside all day in freezing weather, how should I set the heat so that I can:
[ol]
[li]Keep the windshield clear?[/li][li]Stay warm?[/li][li]Keep Opal warm?[/li][/ol]
[list][list][list][list]Breathing
…
[sup]Oh![/sup]
Rain-X and a couple of other companies make some anti-fog stuff that you can put on the inside of your windshield. It doesn’t keep your windows from fogging up, but as soon as the air from your car’s heater hits it (warm or not), the fog will clear up immediately. Also, make sure your car’s ventillation system isn’t set on recirculate.
Ooh, do I know this subject well!
(mind out of gutter please!)
As already mentioned, keep the air on “fresh”, as 'recirculated is only good for heating the passenger compartment when nobody is in it.
No matter the outside temp., whenever your windows fog, turn on the AC, even if the temperature opting thingy is all the way in the deep orange zone, the AC will dry the air and obliterate the fog factor in a matter of seconds!
What ever you do, DON’T spit on the inside of the windshield of a VW in below zero weather!
It’s funny…some cars I have owned definitely show more of a fogging problem (inside) than others. In some cars, I have to keep the defroster (or defogger, as some call it) running all the time…blowing conditioned air from the dashboard onto the front windshield - and eating up all my gasoline in the process. Even with fresh air drawn in, as opposed to recycled air, the windows still fog up quicker in some cars than others. Also, I should mention that the windows have been cleaned - the relevance here is that dirt on windows grants more surface area for the fog to form.
Maybe some cars are more air-tight than others? Just asking as a supplement to the original posting…
Why should this be? Friends of mine have noticed this, too.
I swear I’m having deja vu right now, no joke…maybe I’ve posted this EXACT reply under someone’s else’s post on fogging car windows…? Not sure…let’s try to explain the fogging windows, first…then we’ll tackle deja vu! 
I use rain-x antifog works fine heater on or not. rain-x also makes something for the outside of the window to make the water beadup.
The AC on is the answer to defogging the windshield, even in winter. I have tried it in the past, and it works, but now I have an actual, three-dimensional cite: My new Honda. When I turn the control button to the defog position, the AC AUTOMATICALLY turns on. I can’t even turn it off usng the AC control button as long as the general control button is turned to defog. I’ve even done it when the car was cool (winter hasn’t set in just yet. We’re waiting…tick…tick…tick…tick…)and I’ve turned the temperature control to hot. It still works.
Can someone explain why using the recirc’ed air isn’t effective, but the fresh air is? I feel like this should be really simple to figure out, but I just don’t get it.
It took me a minute or two to figure this out when it happened to me in a car, so don’t feel bad. The outside air is generally drier than the inside air (because exhaled air has a higher moisture content), so fresh air can absorb the condesation on the windshield, whereas recirculated air is probably already saturated and is shedding some of its water vapor by condensating it on your windows.
See I told you to stop breathing. 
Tuckerfan I’ve wondered why and now I know. Thanks.
I use regular Rain-X (the one for the outside of the windshield) on the inside of my car and I find it works better than the anti-fog product.