To me, I'd be far more concerned about sweat & humidity than the temperature itself - therefore, if I was using CIEMs during the hot months on Japan, I would personally use a dessicant/dehumidifier each night, to keep the armatures and the sound tubes nice and dry.
That's just my personal opinion - I'm not stating it as an absolute factual necessity.
That's what i was actually talking about: humidity.
Thanks for all the answers so far!