A topquilt is similar to a sleeping bag. The idea is that any sleeping bag insulation that you are laying on top of gets flattened by your bodyweight and as result loses it’s insulative properties. This is true with any sleeping bag, but is especially true with highly compressible bags such as down or highly compressible synthetic insulation. This is why you see some sleeping bags with no insulation on the bottom side at all, but rather just a pocket/sleeve for the sleeping pad. In a hammock, a topquilt works even better than it does on the ground. The hammock topquilt can generally be narrower and lighter than a quilt meant for ground use. In a hammock, your bottom insulation, weather it’s a pad or an underquilt, will wrap around your backside and usually continue up the sides of your body. The 40″ wide Yeti for instance, provides bottom insulation several inches up past the sides of your shoulders/arms, so insulating your back and sides is covered by your bottom insulation. A topquilt will then overlap that bottom insulation. This is as simple as tucking it in along your sides which is very easy to do in a hammock since the hammock body itself rises upward on both sides of your body. A topquilt is very similar to a sleeping bag from the knees down. This part is fully enclosed by the quilt, but from the knees up it acts more like a blanket you’d use at home in bed.
There are several benefits to this approach. First, there is a signifigant weight and bulk savings over a full sleeping bag. The topquilt is narrower since it doesn’t need to extend all the way underneath you body like a sleeping bag does. Also, there are no zippers and so there’s also no need for a draft stopper for that zipper. There’s also no hood. You’ll just want to use some sort of warm hat, but if it’s cold out you’ll be carrying this anyway so it serves double duty. The other main benefit of the topquilt is that it’s alot less constrictive to sleep in and to get into and out of than a sleeping bag. In fact, after a few nights in a hammock, many folks quickly realize that it’s much easier and more comfortable to use their sleeping bag as a topquilt by unzipping it to the knees and simply covering up. This does work. Many do find an opened sleeping bag to be excessively wide in a hammock, and there’s of course no weight or bulk savings, but you still get some of the comfort/convenience benefits of using a topquilt, and it can be a great way for a new user decide if they want to invest in a dedicated topquilt.