I remember always being cold fasting, fun times. Here is a tip.

Empty all air, breathe hold, 1 in 1 out quick like 3 seconds total, breathe hold on empty again, repeat.

You will be very warm within 2 minutes. Recently used this to spend an hour outside on a dark and windy 50ish degree night in jeans and a tshirt after my kiddo stole my coat.