What causes windows to fog

Generally the AC reduces the fog especially in cars. Many cars force the AC on in defrost. The AC removes the moisture in the air.

Dewpoint is the magic number. When the surface of the glass is below the dewpoint temperature, you will get fog on the windows. Bathrooms are particularly bad. See:

SUPERTHERM - Dewpoint Chart - super therm paint ceramic (http://www.eaglecoatings.net/content/dewpoint.htm)

If home windows are double or triple pane, then it's possible there is a leak between the panes of glass.

what causes windows to fog

https://www.askmehelpdesk.com/cars-trucks/when-asking-about-your-vehicle-303903.html

Is the fog/moisture build-up on the inside of the glass, or is it on the outside of the glass?

Condensation forms when warm, moist air comes into contact with cooler air or surfaces, like the car windows, a can of cold beer or soda, etc. That excess moisture will form into droplets. That collects and then needs to be removed from the environment before it has a chance to evaporate back out into the surrounding air.