Does the material cause you to sweat or...
Does the material cause you to sweat or is it breathable when used as a poncho?
I know Sea-to-Summit uses eVent in some of their products; this doesn't appear to be one of them. I would check out their webpage to see, but eVent is ridiculously breathable--purportedly more than Gore-tex. It's expensive, but worth its weight in gold.
This is made out of silnylon, which is not a breathable material but since it is a poncho and not a jacket, it vents better than a jacket would.