We often get quizzed about the difference and purposes of double layers and single layer hammocks and this post is designed to cut through the red tape and get to the heart of the matter.
First of all, we see two main purposes for a double layer hammock: 1) warmth and 2) support.
Let's get to brass tacks. Warmth. Warmth is a huge concern of hammock campers who suffer from convective heat loss. This essentially means that air is circulating around you and there is no accumulated warmth below your body. Keeping your back warm is critical to hammock comfort.
The mistake I made -- along with almost any other hammock camper who started out -- is that I thought a sleeping bag would work fine to keep me warm. Not so.
A sleeping bag is a nightmare to enter in a hammock with all the writhing, wriggling and tossing necessary to get it under your back. Additionally, the compression of the loft in a sleeping bag causes the insulation barrier to hold in the warmth to become thin. Paper thin. It doesn't take long before you realize that a sleeping bag won't do in cold weather. While you may be willing to tolerate it on cool nights, most folks find it's a real hassle to deal with for the benefit it provides.
An aside: the purpose of "loft" in a blanket was something I never thought about. We all know fluffy means warm, but until I got into hammocking, I never thought about why. The short answer is that your body warms the the loose fibers/loft contained by a wind resistant barrier (the outer lining of the blanket/quilt). The wind barrier doesn't allow the air trapped within the fibers to circulate much. Thus those fibers get warm from your body heat and they stay warm (mostly) because of a cover that blocks circulating air/convective heat loss. With air circulating through those fibers and cold air blowing over it, it wicks away the heat. Or with no loft due to compression, the fibers can't fill with warm air which is what happens when you lie on top of a lofty sleeping bag... aside over.
The next step in hammock evolution is the closed-cell foam pad. These can be had inexpensively at big box stores and while they work great, the reality is that the comfort factor can be lacking. First of all, a CC foam pad conforms to the shape of the hammock and efforts to stretch it across the diagonal are frustrating affairs. While most campers find that it works -- and works well -- the problem with using it on a single layer hammock is that it slides around and gets frustratingly difficult to find a good sleeping position.
Enter the double layer hammock. The double layer allows you to insert a pad between the top (the layer that touches your body) layer of the hammock and the bottom (insulating) layer. This results in a pad that doesn't squirm around, that can be adjusted to the proper shape and angle of the hammock user and provides a barrier of insulation and warmth. The area where there is an opening between the layers that allows the pad to be inserted is about 30 inches long, so getting a pad in there is usually a pretty simple affair.
The final evolution in hammock insulation is the underquilt which is really a marvel that puts a high priority on light weight, comfort and warmth (particularly high-quality down UQs). But underquilts can be very, very expensive, especially for better, high quality ones. Not everyone is interested in making them or making the investment in them. And one also has to consider that no one underquilt will be perfect for all conditions. A 20 degree quilt may be too warm on a balmy night. Most frequent campers find that it takes about three underquilts to find the right zones.
So let's break it down: A quality underquilt is going to cost $150 or more. It will be adequate for a range of temperatures but no one quilt will work for all temperatures, say, from in the 20s and 30s (Farenheit) to the 60s and 70s.
A double layer hammock costs, from most suppliers, between $30 and $50 more than a standard single layer. A foam pad costs about $10-$15 at a store. Additionally, it will be adequate in temperatures at most ranges, from the 20s to the 60s. It may require a little bottom insulation, or even a reflective blanket on top of it to reach low temperatures, but these options are inexpensive. You can add or subtract padding as needed to make it fit your needs. This is inexpensive. I have a car windshield reflector and a CC foam pad that I use together when it's cold. Both cost about $10. I can use one or the other when it's warmer. Or I can buy a thinner, lighter pad as well.
With a double layer hammock, you'd be good for temperatures in the summer and in the shoulder seasons, even into the upper end of winter nights. And the cost would be around $50-$75 (layer plus pads). Compared to the cost of a single underquilt, it's a bargain.
Additionally, a double layer offers additional weight support and can come in handy should a layer of your hammock get punctured. At BIAS, we state that a double layer will offer at least 150 lbs. of additional weight capacity, though some manufacturers state it will double. We still view the seams as the weakest link, so we try to keep our estimates conservative.
The problem is that double layers do weigh twice what single layers weigh. Almost exactly. Other than the bag, the cinch cord for the bag and cordlock that goes in it, a double layer is a mirror image of a single layer. But for the weight support, the comfort and the cost of getting insulated, it's really a good option for many campers and backpackers.
Many opt for a single layer with an underquilt, and this really is the luxury option. It's effective, but costly and is the thing many of us gradually work up to as we get deeper into hammock camping.