For subzero, I use 2 different ones. The wide walmart waffle blue and a section of the stiffer walmart blue pad cut to fit the width for my torso. Not sure of their actual thickness 1/2 and 3/8 I think. I also hang my poncho underneath as an extra windblock and put my pack liner garbage bag in it as a garlington insulator. I sometimes add in the sunshade reflector pad for the years when I haven't gotten accustomed to the cold yet. If the temp is expected to be significantly below zero, I may bring an additional 3/8 ccf for ease of mind as -20 is nothing to mess around with especially when on a solo trip 5 miles in. CCF is bulky but light.
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