Following up on the blog question posted yesterday. This is a photo taken between a vented attic and a conditioned space. The dirty line at the bottom of the insulation shows the path of air infiltration from conditioned to unconditioned space. This is a typical problem in buildings that do not have proper air sealing. Fiberglass insulation works well as an insulator but does not stop air movement. The insulation becomes an air filter without an air barrier. Air movement also brings vapor movement. Once the insulation gets wet it loses all effectiveness. There is also a chance that the moisture movement will be enough to support mold growth. If you find that your insulation is acting as an air filter and not insulating – making the space air tight will solve the air quality and energy-efficiency issue.