Thermal fleece (often called Roubaix fabric) is designed for cold-weather riding, typically when temperatures drop below 10°C (50°F). The brushed inner surface traps warm air against your skin, while the outer face often resists wind—making it ideal for winter tights and cold-weather knickers.
The trade-off is warmth itself. Thermal fleece adds bulk and isn't suitable above roughly 12°C (54°F). If you wear them in milder conditions, you'll overheat and the moisture management suffers, leaving you damp and uncomfortable. For those in-between fall rides, a lighter-weight tight or layering approach usually works better.
If you regularly ride in genuinely cold conditions and want to stay comfortable without piling on extra layers, thermal fleece is an excellent investment. Just reserve it for the right temperatures—your legs will thank you.