Selecting a sleeping pad for a backpacking trip depends on a variety of factors including personal preference, outside temperature, terrain, and weight. It’s important to understand the tradeoffs between these when selecting the right sleeping pad for a backcountry trip.
First off, sleeping pads provide you with two primary benefits: insulation and comfort. When I first started getting into ultralight hiking I didn’t realize the importance of pad insulation and I spent many a cold night cursing the inadequacy of my sleeping bag. I subsequently learned that one of the most important attributes of your sleep system, particularly in the early spring and late fall, is the thickness and R-value of the padding below your sleeping bag.
For example, I now use a full-length Downmat 7 from Exped in early spring and autumn. The Downmat is an inflatable pad, lined with goose down, that is 2.75 inches high and weighs 28 oz. It is very comfortable and an excellent insulator with an R-value of 5.9. In the summer when the ground is warmer and I’m camping below 3,000 ft, I often use a minimal 3/4 length pad from Gossamer Gear that weighs 3.2 oz, is 3/4 of an inch in height, and has an R-value of 2.27. If I am camping above 3,000 feet and therefore often sleeping on a rockier surface, I will use a 3/4 length Therm-a-rest Z-lite which provides a bit more padding and weights about 10oz. The Z-lite is 3/4 of an inch in height and has an R value of 2.2, so not too different from the much lighter Gossamer Gear pad. So as you can see, the sleeping pad you select for a trip is highly dependent of the weather and terrain conditions you expect to encounter.