Merino comes in different weights. Even in very cold weather I will typically wear only two layers of merino--both long sleeve, the lighter 150 wt crew neck and the heavier 200 wt turtleneck--under a windproof she'll. I don't find anything more comfortable than merino. Another advantage is that I can wear it daily for weeks without washing since it doesn't hold odor. Synthetics stink if you look at them sideways for 5 minutes.