When you choose plastic windows for rooms, then purchase with those with systems that will provide an exchange with street air. There are system designs. These are supply ventilation valves, climatic and other. The best solution is a system of automatic supply valve, which themselves open and close depending on humidity. And then you need to start the fight against the fungus. Remove the fungus traces with a metal brush with a metal brush. Then we treat the surface with an antiseptic or fungicide.
