It's insanely warm. My 3 year old goes to a forest school, we live in Kentucky so it's not terribly cold but the temp drops to 10(F) and his teachers actually asked me to stop wearing this shirt at all because he was crying he was hot and sweaty in 15 degrees and 5 inches of snow π . So far it's holding up great. Runs a little small, or maybe very true to size, other than that it should work with the base layer. It fits him well now, but I factored in room for growth and not a lot of extra space.
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