Over Thanksgiving I discovered a torrent of cold air pouring behind our shower in the basement and tracked it to a large opening in our attic. That same day I went and picked up some additional insulation and proceeded to close the opening. Problem solved right? Nope, instead I found out by sealing this rather large hole I made our home too tight. The problem is that in worst case testing (all exhaust fans running and the clothes dryer) a significant enough negative pressure was created which prevented our water heater from drafting correctly.
If you have one or more atmospheric vented gas appliances, make sure to test after home improvements!