I bought a thunder blue regular fit REI co-op Sarahara Shirt. Planning to do some light hiking in the White Mountain area of New Hampshire this month and Banf, Canada in September.. I bought a size M and it fits comfortably with only a bra. My questions are:

Is this a layering piece? If so under or over?

I tried a short sleeve wicking t under and it felt too hot and constricting.

Should I think about going to size Large to wear it more roomy?