Our 'furnace' doesn't run only to maintain body temp. To some extent it is always on. It's like a house in the middle of summer with the furnace running. With temperatures equalized there can be no thermal transfer at all and the only way we can dump heat is via convection (fluid (air) movement over a body) or evaporation. Add in still air and very high humidity and we have no way at all to dump heat.
That's why most people are comfortable usually when the temperature is in the 70s or so. That seems to be the ideal temperature differential so that our bodies can dump excess heat via thermal transfer, without needing to rely on sweating (evaporation), and without having to fight too hard to keep any in.
I lived in a tent for 18 months and going into it I really thought the winters would be the worst part. It wasn't. It happened to be almost a wetland so it was always very humid and had very dense vegetation which made for very still air. Needless to say, the summers were brutal. In those scenarios there's just not much our bodies can do to keep us from cooking ourselves.