Hello, 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? ... View more