I have had a serious problem with Ice building up on the roof of my house. Going into the attic it appears to be quite well insulated (Fiberglass Rolls), but I noticed that the vent in which both my gas furnases use to exaust out of the roof have no fiberglass insulation around them. Is this because it would cause a fire or something the contractor overlooked when my house was built in 1973. My water heater also vents into this same duct. Winter is coming soon and I would appreciate some help!