I have single pane windows and one was broken this weekend. I covered it in plastic and found that I had a significant negative pressure in the house, drawing in outside air. After inspecting the duct work, I found a significant leak where the package unit heat pump (outside) connects to the duct work. Taping this up corrected the negative pressure in the house. This can cause significant performance problems, especially if the only source of air is drawn in from the attic space through the exhaust fans openings and other leaks in lighting or attic access doors.