I commute year-round, and 2 years ago I pushed my low temp thresh hold back down to 10F. I use a thin nylon balaclava. For me, even at 10F the main objective is to keep the wind off my skin as opposed to holding in heat. between 15F and 10F I cover my mouth and nose and breath out with an exagerated overbite to keep the moisture away from my glasses. I also clean my glasses with soap and water blow dry them which leaves a thin invisible film of soap which helps prevent fogging. I would assume there is a low temperature point at which I would need to hold in heat around my face, but fortunately it doesn't often get that cold here in Colorado Springs, and at this point I wouldn't want to ride below 10F, which it only does a few days each winter; not like the midwest where I grew up and you may have a week where the high temp doesn't break 0F.