I've been using the very light micro grid tops (crew necks) from both Patagonia (Capilene Lightweight) and Outdoor Research (Echo) for a few years now and they flat work. A couple of years ago Sitka (Core Lightweight) came out with what appears the exact same fabric, only with a hood. I loved the hood, but is only available in camo. About a year ago, Outdoor Research came out with a hood version that's really nice. They breathe extremely well AND dry quickly- the two biggest factors imo when looking at base layers. The shirts are light enough to wear as a sun shirt in hot conditions (you want a lighter color one for this). Light as in 4.3 oz for the hooded version! They are wearing very nicely as well, which given their light weight is a huge plus.