When I was learning about lightweight and Leave No Trace backpacking and hiking back in the late 70's and early 80's the idea of carrying a mattress with you was a bit radical. After all, if the point was to lighten the load you were carrying, why would you haul a mattress along?? What we were learning was that the sleeping pad wasn't just for comfort. It also has the much more important job of insulating you from the ground. It doesn't matter how warm your sleeping bag is, the part between you and the ground is getting squished which takes away a lot of the insulation value that it has. That is where the camp mattress comes in, to provide that insulation between you and the ground. The added comfort is really just a bonus, although one that I definitely appreciate.
Foam Sleeping Pads
The least expensive option is a simple foam sleeping pad. They are usually about 1/2" thick and the most common colour still seems to be blue, although there is nothing special about the colour. What you are looking for is a fairly dense "closed cell" foam pad. The "closed cell" part is important because that means it won't act like a sponge and soak up water.
You can usually find this type of camp mattress for $20 or less at Canadian Tire. For the price, they work quite well. They roll up easily for packing and are sturdy enough to carry on the outside of a pack. The only downside is that these blue foam sleeping pads are a bit bulky, but it's not the end of the world.
I used to see more folks bringing air mattresses than I do now. I think air mattresses are still fairly common for car camping, and in fact they seem to have gotten bigger over the years. If you are just looking for comfort and padding, then an air mattress can work quite well. However an air mattress is not a good choice for insulation. Good insulation does create small air pockets that will hold heat but a simple air mattress is usually just a big balloon that lets a fairly large volume of air circulate inside which doesn't provide much insulation at all.
Hybrid Sleeping Pads
A hybrid sleeping pad takes the best features of an insulating foam pad and a comfy air mattress and combines them into a single self inflating sleeping pad. The foam pad inside the hybrid sleeping pad provides the insulation you need to keep from losing heat to the ground while you are sleeping. The inflatable part of the hybrid sleeping pad gives you the extra cushion for comfort without adding weight or bulk to the gear you have to carry.
Probably the best known brand for hybrid sleeping pads is Thermarest. I'm pretty sure that Thermarest invented the hybrid style of camp mattress and they still produce top quality products today. Thermarest provides a wide range of sleeping pads to balance different priorities for weight, insulation and comfort. I have a couple of Luxury Camp Thermarests that give 2" of padding and loads of insulation for those spring fishing trips. I also have a ProLite Thermarest that still gives 1" of padding but which packs up to less than 1/4 of the size of the big Luxury Camp versions. If you are particularly weight conscious, then Thermarest also offers 3/4 length mattresses.
Thermarest isn't the only good brand for "compound" sleeping pads these days. Other good brands to look at include Exped who make a hybrid sleeping mat using down for the insulation, and Big Agnes. I've heard really good things about both the Exped and Big Agness camp mattresses but I haven't had a chance to try them myself yet.
Sleeping pads like Thermarests are designed to be folded over and rolled up when you are travelling, but just like sleeping bags, it's best to store them spread out and flat. Storing your camp mattress flat minimizes the creases and pressure points, especially along the seams. The first compound mattresses we had were World Famous brand ones that worked well for a number of years, but when the mattresses did get leaks, they were little pin prick holes along the side seams that were impossible to patch. Since we started storing our Thermarests flat behind the basement love seat we haven't had any problems.
©Loon Island Outdoors 2014