Sleeping Pads

While I don't necessarily consider a sleeping pad to be essential for backpacking, the do serve to enhance the backpacking experience for most people.  A sleeping pad performs two primary functions.  They make you more comfortable when you are sleeping on the ground, and they also provide added insulation between you and the cold ground to cut down on conductive heat loss.  Remember, your sleeping bag is compressed under you and doesn't really do much for you in terms of insulation when it's pressed against the ground.

Insulation
A sleeping pad provides insulation in much the same way that your sleeping bag does, through the use of dead air space.  It traps a layer of air space between your body and the cold ground.  Through the process of thermal equilibrium, your body will try to slowly warm  the dead air space while the ground is slowly trying to cool it.  In effect, it becomes an insulating barrier and slows heat exchange between you and the ground, keeping you warmer.

Choosing A Pad
Choosing a sleeping pad, as with most equipment purchases involves the evaluation of several factors. You then must decide which factors are more important to you and which you are willing to compromise on.  When evaluating a sleeping pad, keep the following in mind;

• Comfort
• Insulation
• Weight / Bulkiness
• Construction / Durability

The priority of these factors in the decision making process will be different for different people.  You might want to examine some of your backpacking habits to see how you would prioritize these to choose your sleeping pad.  Ask yourself

• Where will I use it? - Will the terrain be rocky or do you primarily camp on soft ground?
• How cushy do I want it to be?   How comfortable do you need to be to get some rest.
• How much am I willing to carry?  Thicker, more comfortable pads weigh more.
• How much space do I have?  Again, a thicker pad will usually be larger and bulkier to lug around.

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Once you sift through those questions, you have a few options available.

Air Mattress - The standard air mattress is just that.  It offers a basic inflatable mattress that you lay on.  They're often pretty comfortable and inexpensive.  However, the air inside circulates more than with other pads, so it doesn't insulate as well.  Also, they tend to be quite bulky and heavy.  Depending on the material used, they are usually not too durable and can be punctured pretty easily.  Generally, this one is left for the car campers.

Open Cell Foam  - Open cell foam is a sponge-like material that is comprised of many open air cells.  This is commonly sold as "egg-crate," but the material comes in other designs too.  These guys are usually pretty comfortable depending on the design.  They're light, cheap, and better insulators.  The foam cells create more dead air space than you find in an air mattress and limit air circulation.
Beware though, this material will readily absorb and hold moisture which can cause issues in wet conditions.  They're not very durable at all and while they insulate better than an air mattress, a closed cell foam pad still insulates 4 times better at the same thickness.  Open cell foam pads are also pretty bulky and difficult to compress.

Closed Cell Foam - These pads are made up of dense foam consisting on tiny closed air cells.  These pads are inexpensive and relatively immune to punctures because of the material used.  Because the closed cell construction eliminates air circulation, they're also much more insulating than anything listed so far.  The closed cell foam also does not absorb water.  It should be noted on that though that the pad will soak up some water between the cells, but it is minimal.  Closed cell foam is denser and so is heavier than its open cell counterpart, but the difference is pretty marginal.

Self Inflating Pad - Cascade Designs made these famous with their popular Thermarest product, and you see these most prominently among other backpackers.  They consist of an open cell foam pad that is wrapped with an airtight nylon shell.  These pads offer the comfort of an open cell foam pad with insulating properties close to that of a closed cell pad.  They are self inflating and offer adjustable firmness via a small valve.  Depending on the model and design, they can be very compact when rolled up.
The downside is that you get what you pay for.  These pads can be expensive, sometimes close to 200 dollars depending on the type you choose.  They can also be punctured somewhat easily, but field repair is not difficult.  If you go with one of these, be sure to pick up a field repair kit.

Test Rest
There are tons of sleeping pads on the market today, and all of them vary slightly in design.  They come in varying lengths, widths, thickness, and material  When all is said and done the feature lists can only do so much.  The best way to decide is to actually try them out.  Spend some time and bring your sleeping bag with you.  This will help you get a feel for what suits your needs.  Some retailers also rent various sleeping pads.  This is a good way to really try them out for a minimal amount of money.