Why are your house windows always foggy?

If the fog is between the panes it is the result of the seal being broken and the dry gas between the panes having escaped. If the fog is on the interior surface of a single pane window, that comes from high humidity inside the house condensing on the cold glass. High humidity can come from inadequately ventilated gas heaters or stoves or it may simply be that you don't run your exhaust fan when you shower.

If the humidity is coming from burning natural gas, humidity is not your problem. If you don't have good exhaust, you will likely be poisoned by the carbon monoxide.

It is recommended that the exhaust fan be left running for a half hour after a shower. They sell timer switches at the hardware store for that purpose.