Hi Neil - I'm a member of the heavy sweater club and bought the MH Featherweight after reading the great results you and others have had with it. On the MH site, I think it is called Transition Zip T, is this the shirt you have used? I ordered my usual size and it's a bit loose, certainly not form fitting. I was wondering if I should go a size smaller or if you've ever tried layering under it. It seems like on some of the colder mornings, I might want an additional layer and my thinking was that it would be better to layer something under it, rather than over it, but maybe I'm off on that. I'm also concerned that since it is a bit loose it won't perform as well as it should. Given the high price tag, I'd really like to get the maximum performance from it. I've tried getting in touch with MH directly to see if they would recommend going a size smaller, but have had no luck. Any recommendation on sizing or experience with layering?