In reply to JayPee630:
Thank you for your replies. I'll provide some more details.
My trek last year was in the beginning of December. Altitude was around 2000 metres at the start and base camp was at 4130 metres. Generally the temperatures during the day were quite warm, even hot while in the sun. In the shade they dropped markedly and I had to put on extra layers. I was moving quite fast apart from when we had to climb (seemingly endless) flights of steps.
I had a simple Rab DryFlo 120 longsleeve shirt which I wore most of the time - and that was sufficient. In the morning sometimes I added a thermal layer underneath/and or a Patagonia R1 hoody fleece. If it got windy I put on my Marmot Precip.
I didn't need anything else apart from very early morning starts and when we got up beyond 4000 metres. Then I used a (probably fake) down jacket I borrowed from my tour operator. It was quite bulky and generally it made me too hot.
Basically I'd like to replace this item and have my own. I'm aware the Rab and Arc'teryx are midlayers but the reviews and comments I've read indicated they can handle temperatures down to 0 degrees C if accompanied by additional layers (which I have).
I found this info on another website (
https://www.reddit.com/r/Ultralight/comments/57hlaw/how_warm_is_the_patagon...
"The Patagonia Nano Puff has 60g/m2 of Primaloft Gold insulation. It has a theoretical clo (unit for measuring garment warmth) of 1.5, which means it should keep you warm down to approximately 40 degrees when paired with standard baselayers while sitting around. It would be warm to significantly colder temperatures while hiking/backpacking, though most hikers I know don't wear insulation other than a light fleece while hiking, unless the weather is really frigid (<15 degrees or so for me)."
I know it refers to another jacket but both have the same weight of Primaloft Gold.
I'm *fairly* confident that this jacket, plus my other layers, would be sufficient. The other option would be a 100g one but that might be simply too warm. I might be better off layering two 60g synthetics instead.