When most people think of measures to improve the comfort and energy efficiency of a building, they think of insulation. An equally important technique is air sealing. It is actually easier for heat to escape with air leaking through holes and cracks than through solid objects.
In Passive House construction, we achieve a much higher level of air tightness. Also, in buildings that are not properly air sealed, air and moisture are constantly moving through the floor and walls, often depositing water inside the framed assemblies, creating conditions for mold growth and wood rot.