I will only use the ziploc bags again if it is "dry" food like a sandwich.* While I do reuse ziplock bags to a point, I will not wash them out to reuse them if they've held food. It simply grosses me out.
Wet foods like fruit and meat get new bags.
I use ziploc bags to freeze meat in so as soon as it gets cooked, the bag gets thrown out.
While I will don another jumper or sit under a blanket, I don't scrimp on heating when the kids are involved. In winter I always put the heater on for 30 minutes before their bedtime to warm their rooms up. Then the heater gets turned off and I jump under a doona on the lounge. lol