Energy > Heat Heat
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Remember, if your house looses heat in the winter you don't need to heat it all. Drain your water pipes and let the majority of the house go cold.
If you have one, your basement will be the least cold part of the house (notice I didn't say warm).
Your first line of defense is going to be to insulate yourself. Long johns, a full set of clothes, plus outside winter wear including hats and coats may be needed while you are awake.
Space blankets reflect 80-90% of your body heat, so break the out if you have them.
If you can, bring a large bed into the basement as a 'family bed'. Bring the bed frame down as well, as the cold will come up through the concrete floor. Kids get colder faster than adults so keep them in bed with you.
** test this out below **
If you have extra blankets you can section off a smaller part of the basement to keep warm, either with body heat or candles. Just be sure to leave a space for air to enter and exit.
Tea light with people around using space blankets.
What not to do:
Start a fire if you don't have a fireplace.
Use propane or natural gas if it's still available.
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Kids feel colder than adults (link this to an opening link below). Keep an eye on them in any emergency situations involving cold.
Remember as size doubles, mass quadruples. I remember reading an article about mice that are so small that have to eat all the time during winter just to stay alive. Elephants on the other hand have been known to break holes in ice and have a bath.