I wanted to make a high warmth/weight garment for three-season conditions using synthetic insulation. Using synthetic insulation gives better moisture resistance, so the garment can, in a limited set of situations, be used while moving. Going synthetic carries a weight penalty, but the total fill weight is so small that the increase is minimal (~0.5 oz). In cold environments, skin temperatures (and therefore heat loss in the absence of insulation) are highest for the head, neck, and torso area. So the ideal garment from a pure warmth/weight perspective would cover these areas and no others. Neglecting the head and neck, which can be insulated with various hats, hoods, etc, a vest attempts to do just that. However, the armpits are a significant source of heat that a vest fails to insulate fully. With this in mind, I designed a puffy Tshirt that would insulate the core/torso and fully cover the armpits.
The feature set is minimalist. The full front zipper allows for temperature regulation and makes the Tshirt easier to put on and take off. A hem drawcord seals out drafts. Drawcords at the end of the sleeves help seal in warmth (and are removable). The total weight of the Tshirt is 4.3 oz, with a single sided loft of just over 1".