Warm air can carry away more moisture, so you need to increase the temperature if you have climate control. Not sure how manual AC works, so this might be applicable to those only. But making the glass too cold will definitely cause condensation both on the inside + outside.
This is what I call "Turbo Mode" and required only briefly to clear heavy fogging.
========================================================
is the same as this:
[ATTACH=full]204312[/ATTACH]
Water vapour in the air is condensing on a cold object.
The car's AC is the problem as well as the solution to OP's problem.
Fogging on inside/outside of windshield, near the vents only (bottom of windshield):
- The AC temp is set too low. The glass has become so cold that moisture is condensing on the cold parts near the vents
- Raise the AC temp to around the ambient/outside temp +/- 1°C (technically it should be above the dewpoint, which depends on temp + Relative Humidity, but external temp is a good/fast estimate)
If there is heavy fogging in a stopped car (AC off), use "Turbo Mode".
It works best once the engine is warm, since the heat comes from the engine coolant. If you had parked overnight and the engine is cold, this method won't work, so let the engine warm up first
- Crack open windows
- AC on
- External air intake (i.e Recirculation off)
- Temp to highest setting
- Fan on Max speed
- Airflow to Windshield vents
- This should clear up the glass in seconds
Once the glass is clear, you can lower the temp to a more comfortable range, and redirect the airflow towards yourself aka Regular AC usage. For reference:
On warm nights I set temp to Ambient - 1 °C, airflow to face + feet, Fan on Auto, which will usually spin at lowest speed)
On cold nights I set temp to Ambient + 1 °C, airflow to face + feet, Fan on Auto, which will usually spin at lowest speed)
If you observe condensation slowly forming near the vents, simply increase the temp a notch.
Never switch off AC in rain/high humidity conditions: else the glass will fog up.
Fogging on outside of windshield, over a wider area:
This may not actually be due to condensation at all, it could be:
- Streaking, where the wiper blades are old and leave behind a film of water. Replace wiper blades
- The washer fluid is too soapy, so it is leaving behind a translucent film: don't add too much soap concentrate to the reservoir. It should leave no residue at all
- The glass is extremely dirty with oily residue: wash it properly with soap + water, finish with IPA
- If none of the above works, your glass might have scratches that trap water, or scaling from hard water, in which case it might be easier to replace the windshield itself
In the TBHP link above, the haze pattern changes when the wiper moves over it, indicating it's on the outer surface.
Fogging on inside of rear glass:
Just turn on the rear defogger. It uses electric heater wires to warm the glass and remove condesation.
Usually not too important. If the AC is running, it will take some time, but will eventually remove moisture from rear glass too
And for those who like to ride in the rain, get this and apply on the inside of your visor 
While this works on car too it makes no sense since you need to run AC anyway.
