Men’s size medium New with tags, never worn, just tried on. The best thing about synthetic insulation is that it stays warm even in extreme, wet conditions. Until now, that reliability has come at the cost of bulkier and heavier products. The Kelvin Aerolite Jacket harnesses the insulating properties of trapped air to create synthetic insulation that replicates the low-bulk, highly compressible characteristics of down, but with warm-when-wet performance. We designed this go-to, versatile layer to keep you warm when you need it, yet disappear in your pack when you don’t.