If you hold a measured loft constant, but you add more fill to it, density goes up and insulation goes up. That is misunderstood as higher density is warmer. If we reverse it and hold the fill quantity constant but reduce loft, density goes up and insulation goes down. The correct conclusion would be that, if quantity is constant, low density is warmer. A material is warmest at its highest loft. The highest loft it can achieve is at its lowest density.
These are kinda the rules that tell the full story. If I fill an open top box with down fill and let it loft to its maximum potential, that is the absolute maximum insulation that amount can achieve. If I compress it within that box, density increases and R-value decreases. If I close the top of that box but keep stuffing more down in, density and R-value will increase. If I then open the top and let that amount loft to full potential, R-value will increase even more, with the increased loft, while density goes back to zero.
Nature shows us this truth everywhere. The very birds that grow the down we use, have a set amount of down on their bodies. They cannot add more fill to create more insulation, but they do regulate their insulation through a process called piloerection. When they are in warm weather, their feathers and down sit more flat and compressed to higher density with less insulation. When it is cold, their pores contract and lift their down and feathers up, which decreases density, increases the loft, and increases the insulation. We see this all over nature with most creatures that have fur or feathers. In fact, humans do it. Most of us lack the fur to lift, but that doesn’t stop our goosebumps from trying to increase the loft around us.
The same thing happens with down gear. If we use narrow chambers where the baffles are holding the shells against puffing out, and we add high density fill at lower loft, we get much less insulation from it than if we use wide spacing that allows a lot of “puff”. That puff where the shells arc out is allowing the fill to expand, lowering density and increasing loft.
Below are two panels that are filled to the same exact fill quantity (calculated loft). They have the exact same amount of fill volume in them. The one on the right uses 3″ spacing between baffles, and those hold the fill at high density, but achieves little loft. The one on the left uses 6″ spacing which allows expansion and loft at lower density.
If we’re viewing this in a vacuum, an amount of fill has the most insulation for the least weight at its lowest density. If we take everything into account, a certain amount of density is required to maintain coverage and make up for compromising circumstances, but that density always comes at a cost to warmth to weight efficiency. Ideally, density should be catered to the use case so that there is enough to maintain coverage, but not so much that warmth to weight is reduced too far.
If density is very low, in the real world, the fill will shift around and leave gaps. These gaps will introduce convection, which will remove more heat than is gained by the increased warmth to weight efficiency of low density. If density is very high, the amount of insulation you get from an amount of fill is going to drop. You’ll have a large buffer for getting it wet or putting it into a lot of compression, but you just won’t get a lot of insulation from that amount of fill.