They're closed back, but kind of on the ear, not completely covering it as a cup might...
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The places I wear headphones, like my home and rehearsal spaces are air conditioned. I've worn them while mixing in hot spaces, but I only have them on for very short periods during that use. I don't wear them while cycling or sunbathing at the beach.
Anything sealed will be warmer than anything that won't, but if rejecting some outside noise, as well as keeping the headphone sound in, is important, it's something to live with.