Doors/windows.
Our 1960s house had Andersen wood windows of a newer vintage, but had two sliding patio doors that were single-pane aluminum units that must've been original. There was a security system sticker on one of them with a police car of that era...and the company folded in the '80s They were so cold, I took pictures from 2013 of frost on the INSIDE of the glass.
After the fire remediation, we got new double-pane sliding patio doors with vinyl exterior. Needless to say, our gas bill went down by a couple hundred a quarter, and that was with the thermostat turned UP from it being so cold.