Negative teens is brutal, even a well insulated house loses a lot of heat if you want it comfortable but there’s a big range between uncomfortable and dead. -15f to 30f is the same difference as 30f and 75f.
which means they'll provide a few minutes of heat at best. You'd be better off using them as blankets or letting some water freeze them into scoop shapes and turning them into makeshift shovels.
They also aren't going to burn nearly as cleanly as wood, meaning the burn will be far less efficient.
Depending on the size of area that you're heating, getting from -15 to 30F will take at least a solid hour of burning cleanly.
Really though it’s not about how good an energy source this is, but just how much old junk people have. I have seen people toss several hundred pounds of old clothes that don’t have any real value. It’s the same with old books they don’t burn that well but when you have a half a ton you might as well burn em vs toss them in a landfill.
But that's not the point, the dire situation with little support is the point.
My hands lose all dexterity entirely in winter. With sufficiently cold weather my hands are either cold, stiff, and in pain, or I’m wearing gloves that are too bulky to be useful for anything other than crudely gripping large objects or swatting things away. These people are experiencing cold beyond what I’ve ever experienced, so I can’t even begin to imagine how much worse it is.
If people are burning things for warmth, they are absolutely not going to be holding a needle.
...really -7C is cold beyond what you ever experienced ? It's not too bad. I'd imagine the main problem is if those temperatures are rare just nobody have proper clothes to handle it
-20 C is ~ -7F
I hate that fucking stupid unit
But yeah, that's "my face hurt being outside" kind of bad ;/