"When bottled or mains gas is burnt, it gives off water vapour ...moist heat."

So, that would be from the hob? I assume the exhausts from the furnace and water heater are vented properly.

That being said, there is far more water vapour produced from taking a bath or a shower or from cooking a stew on the hob, which also occur in my house which is heated with electricity.

I still don't understand how "moist heat" adds much to the humidity in a dwelling. Perhaps you could edify me. I really would like to grasp this concept in the full.