>I'm not sure what you're considering "heavier" clothes, but my layers work just fine to about -20C. It was a bit chilly the times it was <-40C, but I don't think that's what you meant?
What I mean is that even for 0C, I'd take a heavy padded jacket which would fill 1/2 to 2/3 of the backpack by itself.
I wouldn't be letting it "a bit chilly" with mere layers in -20oC.