Which ventilation system should I choose for a new house with airtight windows?

How many rooms do you have, and is it possible to run ducts through the attic or basement?

With airtight windows and no ventilation, you're definitely in for trouble - air gets stagnant, humidity goes through the roof. A supply-and-exhaust system with heat recovery is the right call; it'll return the heat/cold from the exhaust air back into the house. But keep in mind that these systems need regular filter cleaning (roughly every month or two), otherwise efficiency drops and you get that musty smell.

If a full-blown supply-and-exhaust setup isn't in your budget yet, start with a compromise: install supply valves (even simple mechanical ones) in the bedroom and living room, and run exhaust from the kitchen and bathroom through the wall or duct. It won't give you the full benefit of airtightness, but it'll improve things right away. You can always upgrade to a full system later if needed.

Here's the main tip: when you're installing ventilation, run not just the main ducts, but also a spare pipe for humid rooms - down the road it'll be way easier to upgrade or replace individual sections than to find space for new channels in an already finished house.

Before you invest in expensive heat recovery ventilation, check whether you actually need it - it all depends on your region's climate and humidity. In a dry climate, sometimes a simple supply system with exhaust fans in the kitchen and bathroom is enough, and a heat recovery unit makes sense if winters are harsh and you want to keep the heat in.