As a warmer and arguably more comfortable alternative to their well-placed and beloved Prosecco Tech base layer, the Castelli Core Seamless Long-Sleeve Base Layer is well…seamless. The soft and synthetic blended fabric form-fits to comfort and work under more substantial winter riding jackets. Castelli provides the Core with exceptional wicking power and warmth, and it's rated to perform down to 39 degrees F — a four degree reach lower than the Prosecco Tech. A lighter weight weave is used on the side and the back to provide uniform insulation when considering you'll tuck this under bib straps that have a certain level of insulation built into them. And for those who are part of the gram-watcher club, The Core does register 43 more grams on the scale than the Prosecco, which may help in determining which Castelli layer best suits your goals.