These were the original camping mats, they consist of a thin piece of foam which insulates the sleeper from the ground. They provide some comfort but not very much because they aren't very thick and therefore your body has nothing to sink into. They are very light though, and durable. They can't be punctured - you can cut them to the size and shape you want to shave further weight. They are also quite cheap compared to other mats.
Design: The simplest closed cell foam mat is a straightforward piece of foam which you lie on, often just a few millimetres thick. One disadvantage of these mats is their packsize, they can't be compressed and therefore make a large roll or block which is often too large to fit inside your pack, it must be attached to the outside.
Some closed cell foam mats have been designed with ridges and bumps in order to improve their comfort - see Thermarest Ridgerest and Z-Lite ranges. This works, but only to a degree. Also the insulation value of closed cell mats tends to be lower than other kinds. Manufacturers are currently silvering one side of the mat to add greater insulation by reflecting heat back to the camper.
Use as a pack stiffener: The classic way to use a closed cell foam mat is to unroll it inside your rucksack so that it fully occupies the shape, then pack your gear inside the roll. Not only does this minimise the space required to store it but it also gives the pack added stability, so you can save weight by taking an unframed pack and using your sleeping mat as an internal frame.
Advantages: The most important advantage of the closed cell foam mat is that it cannot be punctured or compromised in any way during normal use. This makes it the ideal choice for long unsupported treks where reliability is of critical importance. If you are buying one for a long trek we recommend you go for one with ridges etc as these are more comfortable than the standard mats.
A second key advantage is that they can be cut to the desired size and shape, you can minimise your weight this way, by cutting off excess material from the bed - perhaps making a mummy shape out of a rectangular mat.
Other Points: Insulation value can be measured and is called the 'R-Value'. There is more than one system but we've attempted to come up with a figure for each mat (based on the manufacturers specification) that can be used for comparison. A full explanation of 'R-Value' can be found HERE.