Types of Pads Self-Inflating Pads
Most backpackers use a self-inflating, open cell pad. Selecting the length (long, regular or 2/3 length) and thickness is a trade off between insulation and comfort versus weight.
Foam Pads (Non-Inflating)
Non-inflating foam pads are a cheap and lightweight way to get the minimum needed amount of insulation and padding. Their biggest disadvantage is that they aren't compressible like an open cell foam pad so they are extremely bulky. So, if right now you're thinking "Hey, why can't I just use the open cell foam pads like the ones at Walmart for beds and stuff, as a sleeping pad?" then you've just asked a good question... keep on reading.
Open vs. Closed Foam
Open foam, like the kind found in pads for beds, is great for inflatable pads, but not so good for use by itself for a couple reasons:
1) It soaks up water. Since each of the cavities of a closed foam pad are linked together, water moves through it via capillary action, and it thus acts like a sponge (for all intents and purposes, it really is a big sponge). Closed foam pads don't do this, and more closely resemble Styrofoam.
2) It doesn't maintain loft underneath the user. A mattress pad doesn't need to keep you warm (you have a foot thick mattress underneath you after all), and it's purely for comfort. Inflatable pads compensate for this by having a valve
which allows air to come in, and can be sealed off to maintain pressure and its loft.
Inflatable / Air Pads
Several companies now offer fully inflatable ground pads that do not contain foam. This means that you need to inflate them my mouth. Their major advantages they have over other types of pads are that they are thicker, more comfortable, and pack
down the smallest. However, they do come with a price: they tend to be more expensive, they have to be blown up, and they are vulnerable to punctures.
Pads like Insul-Mat's Hyper-lite are a combination of a self-inflating foam pad, with a closed foam pad. Pads like this one gain the advantages (and disadvantages) of both open and closed foam.
Except in the coldest conditions, you do not need a full body length sleeping pad. The additional comfort between a 2/3 and full body length sleeping pad is generally minimal. Most backpackers select a 2/3 length pad because they are lighter and they pack
down smaller. Full length pads weigh more and therefore are popular for short hikes and / or car camping.
Thicker pads (on average) provide more insulation, but weigh more.
Most sleeping pads' thickness lies between one and two inches, with under one inch thickness better for summer camping and over two inch thickness better for car camping (as well as rocky terrain).
The exception to these rules are the air pads. They are usually 2.5 to 3 inches thick, but do not necessarily weigh more than other backpacking pads.
Always store self-inflating pads fully inflated with the valve
open, this allows it to return to its original loft faster.
You can identify holes in your mattress by making a diluted detergent solution (camp suds and water works great), spreading it over the mattress and then watching for bubbles to form.
Specialized patch kits are available from any outdoor store. Note that most of these kits require a stove
because they require heating a pot and using the pot to melt the patch on to the pad.
If you don't have a patch kit you might be able to use a stick of hot glue (melt with a lighter or match
and seal up the hole) or you can use seam seal and duct tape (put seam seal over the hole and put the duct tape over that).