From a reader's email, quoted with permission:
...not sure how to put this delicately, but when I ride my bike in the cold I inevitably end up with a sweaty bra. Even if I am not exerting myself, the bra is soaking wet by the time I get to work and The Girls are not happy spending an entire morning waiting for it to dry. I've taken to stuffing paper towels in there, but was hoping you could share a better solution. How do you deal with this? Don't tell me you only wear wool bras?
Now approaching my third winter of cycling, one of the most valuable lessons I've learned is how to dress for the cold weather. Merely piling on layers can lead to overheating, then freezing underneath the sweat-soaked clothing when stopped at red lights. This is where choice of fabric becomes important. Wool and silk not only keep me warm, but regulate my body temperature - meaning that I sweat less underneath all those layers of warmth than I do wearing cotton or synthetic fabrics. And compared to technical synthetics, wool and silk do not retain body odor.
When choosing temperature-regulating fabrics, the key to the whole system working for me is to start from the inside out. If I am wearing a wool sweater with a cotton long-sleeve tee underneath, that cotton is going to be drenched in sweat; it's better to wear a wool baselayer and a non-wool garment on top of that. Similarly, underwear matters a great deal, since it is the first thing to contact the skin. Cotton or polyester underwear will end up soaked in sweat, causing discomfort even if every single other article of clothing I am wearing is wool.
So yes: In response to the reader's question, I only wear bras made out of fabric that regulates my body temperature effectively, which for me means wool or silk. Wool is the more durable and somewhat more effective option. But wool bras tend to be plain and sporty looking, and not everyone likes that. Also, women with larger chests often report that these bras do not offer sufficient support. If you prefer a look and feel that is more lingiree than sportsbra, real silk bras are available with everything from decorative lace to underwire support and nylon stretch. After having tried a number of manufacturers, I have settled on Ibex for wool underwear, and on Winter Silks for some fairly inexpensive silk bras. I also like to wear Icebreaker leggings instead of stockings once it gets cold, and always Smartwool socks. There are other excellent options out there. But as long as it's wool or silk, there should be no need to stuff your bra with paper towels before cycling to work.