Just a t-shirt. if it's cold out (sub-40) perhaps a long sleeve t.
If its a Cotton T you wear, instead look for a thin Wool
or the wicking tech wear stuff mentioned above .
since cotton absorbs and retains sweat,
when its 90F out that evaporates , unless its 90+% humidity out.
50f less so.. Cycle rainCape or open the pit Zips