Self-inflating Mats & Foam
The generic term for these seems to be ""Therm-a-Rest""®, but that is actually a registered brand name owned by Cascade Designs. There are quite a few other brands available, all looking fairly similar. The cheaper ones may just be a bit heavier, and don'
Self-inflating air mattresses
The generic term for these seems to be ""Therm-a-Rest""®, but that is actually a registered brand name owned by Cascade Designs. There are quite a few other brands available, all looking fairly similar. The cheaper ones may just be a bit heavier, and don't seem to last as long. Some cheap brands have been reported to die within days. The construction consists of a slab of open cell foam with a layer of airtight fabric glued to the top and bottom surfaces and sealed around the edges, plus a small valve in one corner. Starting with a tightly rolled up mattress, you undo the valve and let the foam swell up, sucking air into the mattress. After about five minutes you can add a puff or two and do up the valve: the mattress is inflated and ready for use. This beats the hell out of the ""blow up the Lilo"" ritual which leaves your head spinning from hyperventilation. In addition and very importantly, the foam stops the air inside the mattress from moving around, just as down traps the air. So these air beds are very warm underneath, and they work very well on snow and ice too.
You can get these mattresses in full length and 3/4 length: the 3/4 length is usually quite enough. Put a bit of spare clothing at the end for your feet, and especially under your heels. Heels can put a lot of pressure on the ground and get quite cold. In the snow a bit of thin (3-4 mm) closed-cell foam may be desirable under your feet as well - and you can sit on the foam at lunch time. You can get these air mattresses in various thicknesses from 1.5 cm to over 5 cm: that's your choice. The author prefers the Deluxe LE (thickest) version, 5 cm thick. It is about the same weight as the thinner ones, but a whole lot more comfortable.
It is easy to blow one of these mats up too much. You may not do it any real harm, but the mattress will feel hard and uncomfortable. Your weight should be spread right across the mattress, so you don't need a high pressure to start. It only takes a couple of puffs to get the right pressure, which lets your hips almost reach the ground (but not quite!). Don't use any more. On the snow you will see a bit of a depression under your hips in the morning where the snow has melted, but you won't feel the cold. Actually, snow is quite comfortable to sleep on with a good mattress for that reason: the snow quickly melts to match your body shape, leaving a nice uniform thickness of foam between you and the snow. Sand on the other hand just gets harder and colder. If the mattress doesn't feel all that comfortable in the middle of the night, try letting a wee bit of air out. Seriously.
You should never store these mattress at home in their compressed state: it takes the life out of the foam. Open the valve and let the mattress swell up. Leave the valve wide open during storage: it will let any water vapour from your breath dry out (slowly) over time. I usually allow our mats to sit in the sun or vaguely near a fire for half a day after a trip (with the valve open) to warm the insides up just enough to help with the drying. And above all else, never cut on your mat or poke a hole in the fabric! The mattress will collapse, just like a balloon. Needless to say, hot things are sudden death as well. Repairs are possible for small holes - a bit of PU ""goo"" such as Seam Grip™ should work. Some vendors supply patch kits, but the patches may require heat or the small tube of glue may expire.
The foam in the self-inflating mattresses is ""open cell"", like a lattice. If the foam is ""closed cell"" then each little pocket of air is trapped and you cannot really compress the foam very much. Closed cell foam is sold in similar sizes in walking shops to act as a (thin) mattress, sometimes flat and sometimes in fancy zig-zag patterns. Obviously they are not as comfortable as a 50 mm thick air mattress, but they do keep you just as warm, and they are a whole lot cheaper and more robust. Even a pair of crampons has little effect on them. This makes foam mats popular with youth groups and mountaineers on Everest.
On very rough campsites there is always the risk that your groundsheet could be damaged. If it is a sewn-in one the repairs can be expensive. It makes sense to consider putting your foam mat on the ground under the groundsheet in these cases. I wouldn't do it with a Therm-a-Rest type of mat though.
Our only objection to these foam mats is that novices often carry them on the outside of their pack, and the scrub scrapes little bits off as they go. The result is a track littered with little bits of coloured foam. If you must carry such a mat on the outside of your pack, put it in a nylon bag to protect it and the bush, or be labelled a stupid novice.