I'd get the equivalent in thickness/weight of Patagonia Capilene 2--light weight but not silk weight. The same is true of merino wool--you want light weight. Both are lovely materials, but both will bust your budget unless you can find a really good sale. In fact, I'd head for the athletic (not outdoor) department of a big box store like Target, KMart, Wallyworld and look there for lightweight wicking layers similar in thickness to Capilene 2.