Mammut - WS Helm Cap - Mütze
Compact and warm helmet liner
Although this can be worn on its own, it is really designed for use inside a climbing helmet. Hence it has thin Windstopper over the crown and and thicker fleece around the ears and nape of the neck. It's a combination that works well and fits under a helmet without feeling bulky. Compact enough to keep in the rucksack as a back-up hat, in case you lose your main one.