Air leaking in and out of a building carries moisture. If that water vapour collects on surfaces inside a wall, floor or roof system, there's bound to be mold growth. And mold is terrible for your health!
Why does mold growth happen?
Mold spores like warm, moist air.
When moist air condenses on cool surfaces, mold can start growing
Mold needs an organic base to grow on, like dust, wood, building paper, etc..
Poorly ventilated spaces allow excess moisture which allows mold to grow and flourish.