What should be worn underneath this jacket to keep warm? It appears as if a couple of additional layers are needed as the material is very thin and light. However, if for instance a down jacket is recommended, what's the point of wearing this heli alpine in addition?

I use the bootcut heli mid pants on my legs, and on my upper body I wear an Icebreaker t-shirt, an Icebreaker longsleeve 200 (optional, only if it is cold), my back protector and then the PP heli mid. If it is very cold, I might swap the heli mid for the Arcteryx Delta SV high loft jacket. Due to the body mapping, the jacket is warmer than most GTX shells. I've never used primaloft or down under the shell, that is to warm for me.

yep, hey there Karl...obviously not a component jacket. I paired mine up a couple different ways. First I used the Peak insulated hoody which was wayyy warm underneath. Then I went with something not quite as warm but just about perfect the Atom LT from Arcteryx underneath. The CLO insulator Jacket from Norona is the best for me so far. Its warm without being too warm. It does not have the hood but a very nice collar which I prefer. This Peak Heli is by far the best I've owned. Better than any shell from Arcteryx or Norrona (that I have tried) and I own Norrona now (I like Norrona's bibs). No doubt Karl, expensive for two pieces meaning Shell and insulator but the material, fit and craftsmanship are worth it. Maybe look at the Peak Performance Massive which is a seperate shell and insulator combo. It looks really clean and has a seperate hood from the collar which I like. You get both for the same price as this individual Heli Alpine Jacket.