If it was 18*F outside and your furnace went out and your house got cold then there are other problems to worry about too. Insulation, seals around the windows, seals around the doors, leaky ducting in the attic or under the house or in the walls, leaks around the power outlets on the walls, leaks around exposed beams, leaks around the entrance to the attic, leaks around the water lines under the sinks (shower, toilet). Do you have blinds for every windows? What about curtains on top of the blinds?
In a properly sealed house if your furnace was working and say 68*-71* inside you should have been able to go all night and not notice much of a temp drop.
Double up the insulation in the attic, if you have insulation between the 2x4s or 2x6s put some going crosswise and double it up.
DO NOT block off dormer vents, or whirlibirds you need air flow in your attic, and you need it under your house too... need to breath so it does not sweat.
If you have a sliding glass door or other large windowd doors make sure they are covered. If you don't have dual pain or very very old dual pain this could be why too.
There's some ideas