I’ve tried a great variety of clothing for winter riding (“winter” here in the San Francisco Bay Area means anything from 30° F to 70°F). To date, I’ve had the best results layering a Merino wool IbexWear full-zip Shak top over a summer-weight jersey (a snug-fitting one, such as those made by Castelli), followed by a windbreaker or vest or jacket as temperatures dictated. The tight fit in the base layer is important to wick moisture away and to eliminate that clammy feeling caused by damp, loose fabric contacting the skin. The Shak jersey offers the perfect blend of wind-resistance and breathability—and it’s comfortable too. And it’s wool, so it keeps you as warm as can be reasonably expected, even when you’ve perspired heavily.
The Shak jersey gets my absolute highest rating, being one of the most comfortable and most useful tops I’ve ever owned, not just for cycling, but everyday wear, hiking, etc. I haven’t yet been able to persuade IbexWear to add cycling-style pockets—otherwise it would stand out as a first-rate cycling jersey all by itself. It still baffles me why companies that cater to hikers and climbers don’t understand the merits of pockets as found on cycling jerseys—I often wear a cycling jersey while hiking just for that reason.
As much as I love the base layer/Shak combination, I decided to try the Castelli Fortuna Themal Jersey. Wow! This jersey has a soft inner layer which is very pleasant on the skin, with a snug but comfortable fit. While the fabric is relatively thin, it offers the warmth of a double layer, with superb wicking and breathability. It’s not as warm as the base layer/Shak combination, but it’s perfect for the 52° - 62° temperature range, and a lightweight sleeveless vest extends that down to 46° or so. For colder temps, throw the Shak over it, and you’re comfy.