Is it a particularly hot day? What's the temperature outside the enclosure? Is he in the same room as a computer or something else that produces quite a bit of heat?
I wouldn't think a day at 84 now and then would be detrimental, provided it does get cooler. I have 84 days at room temp now and then without refusals, but my sample pool is only one animal. 2kdime would know better than me (and I believe he's talking about constant temps at 84, so if it cools down tomorrow or whenever it shouldn't be an issue).