How can I prevent glasses fogging when cycling in the cold with face covered Related to this question: need cycling eyewear, but fogging is a big problem but that is more focused on the eyewear itself, whereas I'm interested in the face covering material.

I cycle to and from the train station every day. Only about 20 minutes each way but in the cold that's still quite a time to have a freezing cold nose / cheeks. So I have experimented with scarves, snoods, balaclavas etc when it's cold. However all of them cause my glasses to fog up due to the hot air of my breath condensing inside the glasses. Contacts aren't an option for my type of vision; it's glasses or nothing.

I don't want to go for some separate cycling glasses or big goggles. I have enough stuff to carry with me as it is. But I am open to an alternative way of keeping my face warm.

What should I be looking for in a product that allows me to wear glasses and still keep my face warm?

I'm not necessarily after specific product recommendations, more about materials / equipment or a specific feature to look for that can solve this issue. Because currently my options are: 


a) see where I'm going, but have a freezing cold face. Or 
b) Be all warm, but not be able to see so well.


Should I look for woollen scarves? balaclavas with breathing valves? A different breathable material I am currently unaware of?
What you are looking for is a product that covers your face (to keep it warm) seals along the upper edge to your face (to keep breath from leaking up under your glasses) and uses a design to direct your exhalations away from your face (so your exhaled moist breath doesn't fog your lenses).  

There are actually several products on the market designed to do these things.   Many people do cold weather outdoor activities (fat biking, skiing, snowboarding, snowshoeing, etc) have these issues and they are attempted solutions in various manners.  Chemical lens treatments, electric heat and or air movement, and design directed airflow are all out there.  I'd suggest checking with a local sports store and seeing what they support/use/recommend.