Do merino wool base layers need to be skin tight? I have bought a Merino wool base layer for all the obvious reasons, they're raved about and have been sworn by by many a rider, and I was getting not only chilly in the winter weather but also wearing a cotton t-shirt and getting caught in the rain caused it to very quickly go cold, damp and heavy.

So I bought small and medium and settled for the small but even that is quite loose on me and a baggy fit. It certainly keeps me warm, no complaints there, but then I very quickly get too warm. They're supposed to be magic - keep you warm in winter and cool in the heat - but if it can't stop me from overheating when its cold outside, I struggle to believe it being any better in the summer.

My theory is that it needs to be a skin fit in order to achieve its wicking capabilities, and because it's loose, it isn't able to wick the heat direct from my skin, but rather heats up the air between my skin and the base layer causing a reserve of hot air which keeps getting hotter and thus causing my own mini personal sauna effect. 

Am I right or totally wrong? I don't mind being wrong, but just want to understand the reasons I seem to be too hot and what I can do to resolve it.

I am also wearing a waterproof jacket over it which isn't very breathable. This could just as much be the problem, but I don't want to leave the jacket in case it rains as I don't know how Merino wool base layers - especially when baggy - cope when wet? (Another question maybe?)

Thanks.
I am also wearing a waterproof jacket over it which isn't very breathable


Well there's your problem. 

The most wickable, breathable material in the world isn't going to achieve those properties if you put a plastic bag over it. If you don't want to be caught out in the rain, keep the raincoat in a backpack/messengerbag/pannier/whatever until it's needed.

Regarding how tight the base layer should be, I would say it should be pretty tight, especially when you first put it on because it will shrink up in the wash and loosen up considerably when you sweat in it. However, if it's a little baggy it's not the end of the world, it just wont wick moisture away as well.

Also keep in mind that everyone has different heat/cold tolerances. Two different people may require entirely different layers at the same temperature based on exertion, weight, preference, etc. If you're a hot blooded type, it might need to get a little colder before you put on a wool jersey. I know I'd catch on fire for anything above around 55&deg;F