My favorite underlayer for all but the very hottest most humid part of the summer here in Houston, is a light merino wool vest from Jones Ware (yes it is spelled that way) which I bought off of the web site. It keeps me warm when it's cold and pulls the sweat off in the warmer temps, plus I find it breaths better than my craft underlayer. Jones ware makes some nice wool stuff for almost every season and or climate.