My Ground Effect Median top has been my summer mountain bike top of choice for about 5 years. It’s a blended fabric:
‘A bi-component thermal fabric combining 51% merino wool and 49% polyester. Superfine 18.5 micron New Zealand merino wicks sweat away from your skin while the polyester provides a durable, non-pill outer. Lightweight at just 170gm/m2 and machine washable. Developed by Ground Effect and knitted just for us.
Composition: 51% merino; 49% polyester’
The fit is loose and the performance is always spot on. My two Ground Effects Tops have lasted years despite numerous washes. They are well made pieces of kit.