We're guessing many of you have experienced the agony of a cold sleepless night on an inadequate camping mattress. It's probably why you're reading this right now.
The temperature gradient between your warm body and the cold ground causes conduction of heat - in the wrong direction. In other words, if you sleep directly on the ground your body will heat up Planet Earth all night long.
That's why you need a mattress that insulates you. This is measured by R-value.
Most people understand R-value as a measure of thermal resistance in building construction. The term is also used for camping mats. Unfortunately it is not standardized in the mattress industry so comparing between brands is difficult. There is discussion on standardizing the testing methods for R-value of camping mats, but we aren't there yet.
Basically, the higher the R-value of a mat the more insulating it is. You can find mats with R-values from about 1-10. Even if you are camping in warm places, buy a mat with an R-value of at least 1.5 or it will be a heat sucker. If you are a side sleeper you won't get the full R-value out of your mat since less of your body is contacting it, so consider a mat with slightly higher R-value.
Here's a tip: if you're thinking you need a warmer sleeping bag, consider upgrading your mat instead. By getting a mat with higher R-value you will add far less weight and bulk than buying a thicker, warmer sleeping bag. Most heat loss occurs through your mattress and not through your sleeping bag anyway.
If you are winter camping, make sure your mattress has an R-value of at least 4. It's a good idea to buy one of those cheap closed-cell foam pads (usually blue or yellow) to put under your camp mat - this greatly increases its insulation capability.
In closing, here's an overview of the R-values of the popular mats we carry in the store:
Thermarest Basecamp AF R-value: 1.7
Thermarest NeoAir Venture R-value: 1.8
Thermarest Evolite R-value: 2.1
Thermarest NeoAir Camper SV R-value: 2.2
Thermarest RidgeRest Classic R-value: 2.6
Thermarest NeoAir XLite R-value: 3.2 (women's: 3.9)
Thermarest Trail Scout R-value: 3.4
Thermarest RidgeRest Solar R-value: 3.5
Thermarest NeoAir All Season R-value: 4.9
Thermarest Basecamp R-value: 5.0
Thermarest NeoAir XTherm R-value: 5.7
Exped SynMat UL 7 R-value: 3.3
Exped DownMat Lite 5 R-value: 4.1
Exped SynMat 7 R-value: 4.9
Exped DownMat 9 R-value: 8.0