An interesting thing that I was made aware of recently, is that the natural gas piped into our homes produces a surprising amount of water vapour when it combusts.
With modern boilers, this is not a problem since they are well sealed and vented via flues to the outside. However, where there are gas fires, gas hobs or gas ovens, these might contribute to the total amount of moisture in the house, thus raising the humidity & potential for condensation.
Most of the work I do revolves around fixing doors and windows. Draught proofing them is often what people come to me for but the door or window in question is often malfunctioning. It usually has some form of wear or damage that is stopping it from performing it’s primary function thus creating the draughts in the first place.