Louis Garneau's sleeveless shirt is thin enough to be worn underneath a regular bicycle shirt, and just thick enough keep the bib shorts' shoulder straps from rubbing against your skin. When it's slightly chilly in the morning (55 degrees), the base layer provides just enough warmth. By the afternoon, when you would normally unzip the bicycle shirt, this base layer wicks away moisture easily. I wear it underneath a bike shirt and bib shorts with no problems in 85 degree heat.