Correct ventilation ensures that indoor pollutants and moisture are allowed to escape in the wintertime, and has a cooling effect in the summertime.
In New Zealand, we have poorly-ventilated houses and many are damp. Perhaps the first issue to address is moisture build-up from cooking, bathing and drying clothes. Having a controlled means to ventilate areas where moisture is generated is essential.
For good ventilation strategies, see Smarter Homes.