First, there were sleeping bags with as much insulation on the bottom as there is on the top. When the insulation in those bags became lighter and more delicate, we realized that the weight of a body would compress the insulation, rendering it useless. As a response, along came quilts, which did away with bottom insulation, altogether. If that insulation is not utilized, why not get rid of it and make the system lighter?
While that is all true, it’s not that simple. Most of that insulation on the bottom is being wasted, but it does do something. When a body is lying on a surface, typically there are points of contact that support most of the weight. Insulation under these points is pretty much entirely negated. However, there are other points of the body that might be suspended a little bit by those points. In these places, the fill might remain somewhat functional and provide some insulation. That said, these gaps are likely only 1/4″ – 1/2″. It is pretty inefficient to carry enough fill to loft 3″ when there is only 1/2″ available.
The other thing that fill does is create a good seal between the top insulation and the bottom insulation. Problem is, again, you don’t need 3″ of loft to fill 1/2″ gaps. You also don’t need the fill way underneath, to fill a gap over on the side.
In my view, there are a couple ways to look at this. One could say it’s a justification to have bottom insulation, but adjust the amount to something more appropriate for the job. Or….we just defer the entire job of bottom insulation to the pad and create a good seal between it, and the top system. The fill that is creating some small amount of insulation in that 1/2″ gap is doing something, but that something is likely a relatively small amount compared to the amount the pad is already providing.
That leaves the gap filling pro of bottom insulation. This seems to me to be the most legitimate reason to argue for bottom-side insulation, but if a top-side insulation system can prevent drafts, then there is no need for bottom fill to do this job. There are many examples of top-side insulation systems which do this job effectively. There are many examples of top-side insulation systems which do not do this job effectively. Some of the effective top-side insulation system examples, can do so in a lighter manner than having moderate bottom-side insulation, like some false bottom designs. Some of the other effective top-side system examples seem like they are around about the same weight as simply having maybe an inch of bottom-side loft, like edge tension systems or zippers.
I think if you can achieve an effective seal with top-side insulation, then having bottom-side insulation connected to the top-side doesn’t make much sense, and I think this is a pretty achievable goal. If you have a poor seal between top and bottom, with a pad strap or something, then using a full wrap bag would be an improvement.