Tha is not correct. It is entirely possible to have a super warm synthetic parka (in fact I have one, Patagonia DAS parka, and before that, even warmer, Golite SixMonthNight parka) that far exceeds many down jackets in warmth (I have one of those too, a 6 oz down filled pullover).
Achieving high insulation value is solely a question of using enough fill.
Down (especially high fill power down) does have some significant benefits over synthetics:
Because of the last two, most brands choose to make their super warm parka’s with down, in order to keep weight lower and make them more comfortable and supple to wear.
- It lasts much longer
- It is MUCH lighter for the same amount of insulation
- It is more supple and compressible for the same amount of insulation
I suspect this is what leads people to say “if you want really warm, you need down”. All the really warm parkas they have seen are down, all the synthetic parkas they have seen (especially if they are used, so lost loft) are less warm, so they believe it’s the fill material making the difference.
I guess I can make a Parka as warm as my down OR parka using synth. insulation but it would weight twice as much at least... Combining wool and synth insulation might make it warm without much bulk but it will still be quite heavy compared to the down parka and basically unpackable.
Low Activity and Cold weather (0ºF) requires really thick insulation. I had the DAS parka. Awesome piece but not as warm as my OR Parka. Or as packable.