I was considering harvesting the down from an older sleeping bag for a new project, but since it's probably 600 fill power rather than the 800+ that's available today, I've been trying to decide whether it's worth it, weight-wise.

So, this made me start re-thinking about down. Down works by stabilizing air. Air is a poor conductor of heat as long as it can't move around (convection). So, that means that the down that stabilizes the air best provides the best insulation.

Now, if you had some imaginary 10,000 fill power down, it would be very light, but the air spaces would be relatively large, and the air wouldn't be stabilized that well. If, on the other hand you had some, say, 50 fill power down, it would be heavy and very dense, and there wouldn't be much air in it to stabilize.

So there's a "sweet spot" somewhere in between. Is it possible that 600fp down is actually slightly better insulation than 800+fp down? Obviously, the higher fill power results in a lighter sleeping bag (or coat or whatever), but how does 2" of 600fp compare to 2" of 800+fp?

I have noticed that some of my older down articles seem warmer, but there are construction differences that make direct comparison difficult. For instance, I have an old Camp7 duck down jacket with 65/35 cloth for the external fabric that's warmer (it seems, anyway) than my Montbell Alpine Light jacket, which is puffier and has "better" down.

It may be that I have too much leisure time Anyway, I'd be interested in your thoughts.