What Stephen suggests makes a lot of sense at the colder temps as the dew point is moved outside of the down inner by the presence of the synthetic outer.
Dew point = that zone inside the bag where the humid air is cooling enough to condense. Ideally this would be the skin of your tent. However at lower temps this moves much closer and can become inside your sleeping bag.
A standard approach using down is simply more down with a bigger distance between inner and outer skins, but as the down does its insulating job means the temperature where human sweat is condensing moves with a thicker down to somewhere inside the down itself where it then freezes and accumulates, reducing insulation progressively nightly.
What Stephen is suggesting is not using as much down but a bit less and layering over with synthetic. This will move the zone where the dew point is into the synthetic overlayer. The down will experience far less temperature difference between its inner and outer faces, as the synthetic overlayer is insulating, and the outer layer of the inner down bag is still above the dew point, the vapor then passes to the synthetic overlayer. Synthetic has far less insulation per unit (of anything, distance, weight, you name it) so the dew point is forced into the synthetic from where it condenses, freezes but synthetic is not reducing its insulation anything like as much as the down plus the synthetic is allowing the down under it to stay so warm (practically body temperature) it never gets wet.
Stephen is also pointing out any wet from external sources will touch the synthetic, protecting the down under.
The counter argument is also interesting, each layer of a unit item is adding 2 layers of non-insulating fabric of little value.
I do my own kind of that theory, I didn't for colder situations go with a thicker single down item but I over-layer the down with synthetics I "throw" over the top, its a little more precise than that, I use a synthetic vest I zip over the mummy foot section, encouraging the coldest part of the down bag (the foot section) where condensation will be maximum, to pass its moisture to the synthetic overlayer. Think about that – the synthetic gilet over the foot of the sleeping bag is making the temperature at the boundary, the outer layer of the sleeping bag warmer – that reduces the probability water will be condensing inside the sleeping bag. That works to boost a down sleeping bag into colder temps but as the net additional insulation is slight, is only a basic hack. For a more lower-temp hack, I recently bought a 2nd down bag. It is overlayering an inner down bag, the inner down bag has a waterproof breathable outer layer. If the dew point theory is correct, the outer down bag will have the dew point somewhere inside it and the outer down bag will become water-saturated, probably ice forming, and run-back later which may get back to the inner down bag will be liquid water and run off the outer layer of the inner bag. The outer bag night-after-night will become less-warm but it won't be as intrusive as if the two separate bags were one bag.
Those people who are considering moving to synthetics from down due to accumulating damp issues, might wish to consider dew point and hacks to move it to outside of their sleeping bag by throwing over some light synthetic insulating items.
My most recent synthetic insulation has been bought with a little of this hack in mind, my last synthetic over-trousers, they unzip into something I can place over the middle section of my down sleeping bag and my last jacket also with this in mind. The synthetic items are no additional weight, I'm wearing them at times during the day, typically inactive times outside the tent. The same idea would be using down clothing OUTSIDE the down sleeping bag.
There are downsides to think about using synthetics over down. One is in general synthetic is at least twice the volume and weight as down for a given amount of insulation, that will tend to drive BPL towards down. However if you tend to move your synthetic decisions toward clothing and focus your down decisions towards sleeping it will tend to minimize the weight and make your clothing more rain-friendly. The guess is the 24hour period has above-freezing rainy conditions followed by below-freezing sleeping conditions?
