I have several Wiggy's bags, and so far I've never slept cold in one of them. I have two that are designed to "nest" together into one bag, and in that configuration it's rated for -40 F. Ratings are sometimes pulled out of the air and there's no objective way to test them but Wiggy's seem to be rated pretty fairly. If it's really cold I might also wear my Wiggy's Sun Walker booties in the bag, too.
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman