After crawling around in the attic recently, I felt cool air around the duct unit and figured I had leaks in the ducts. Well, it turns out it's worse than that. I'm no expert on what these things are called, but the big box attached to the furnace and blower with all the ducts running out of it wasn't sealed properly. On the bottom of the box is a gap almost 1" wide at one point that was left completely open and air is just pumping out of it. It's almost too big to try to put mastic and tape over it. I went to Home Depot to get some expandable foam to put in it but the "expert" there said that would make the unit sweat, plus I'm worried about how hot the unit may get in the winter. Expandable foam is combustible above 240F. This was a poor installation job and I'm ticked I'm just now finding it after almost 7 years. No telling how much this has cost me. Can anyone recommend the best way to fix this disaster?