Outer shellThis is something that protects you from the elements such as rain, snow, wind and even the sun sometimes. It can be one of the following:
- the outside of a double wall tent
- the only wall of a single wall tent or a tarp
- a bivy bag, if that's your only shelter
- a bus stop, a bothy or some other man-made shelter
The outer shell is not really designed to keep you warm, although sometimes it may give such impression, especially in the winter. It has to be completely waterproof and well supported by some sort of internal or external structure, such as straight or arched poles (with the exception of most bivy bags) and usually some stakes. One interesting example of this is the fairly recent technology of tents with inflatable arches.
Inner shellThis is what protects you from the ground water, insects and outer shell condensation. It can be one of the following:
- the inside of a double wall tent
- certain parts of a 1.5 wall tent
- a bivy bag when used together with a dedicated outer shelter such as a tarp
- a ground sheet (does not protect from insects)
A typical double wall tent has an inner part that has a waterproof bathtub floor and mesh walls which protect you from ground water and insects respectively. The inevitable condensation happens on the inside of the outer tent and the job of the inner tent is to basically stop you from touching the wet outer walls. That's what is really meant by the many manufacturers claiming that double wall tents solve the condensation problem.
There are quite a lot of single wall integrated or 1.5 wall tents (as I call them) on the market these days. These models basically try to marry the double and single wall concept by having one outer shell, a bathtub floor that is attached directly to the outer walls and some sort of mesh wall panels.
Depending on conditions you may not need an inner shelter at all or at least have just the groundsheet, but in general such scenarios are quite rare. Therefore in most circumstances you will want to have both outer and inner shells in some form.
Ground insulationThis is what separates the body from the cold ground. Traditionally these have been foam mats, but given the superior weight, packing ability and comfort that the modern inflatables offer, the choice is clear. A somewhat exotic and niche solution is the hammock, but it comes with a lot of limitations so I personally never considered it.
Air insulationThis is what separates the body from the cold air, primarily during sleep. Simply put this is what keeps you warm. It is basically the sleeping bag that we all know, however it may come it a lot of variations. This in itself is a very very deep topic, but a very basic dimensional split of the sleeping bag universe is
Bag VS Quilt and
Down VS Synthetic.