Down is the natural choice for lightweight insulation. The Jack Wolfskin Men's Helium Ice Jacket is a classic style down jacket. It is very light and keeps you nice and warm, even during reduced activity. It features a high-loft duck down fill. The down is specially impregnated to protect it from damage caused by moisture. And the windproof outer fabric provides you with additional protection from wind chill.
When you no longer need it, the jacket simply stuffs into its own hip pocket and takes up very little space in your pack.