Thermal pads or ground pads are thin, lightweight pads that are common among hikers, backpackers and budget travelers. In its simplest form a thermal pad consists of a foam-like material that is about half a centimetre thick (about a quarter of an inch). The dimensions of the pad are usually about the same length as a sleeping bag and a little wider. Slight variations in design can be found, such as rippling in the foam, but these do not significantly affect the pad's thermal properties.
A common misconception is that thermal pads are for creating a more comfortable sleeping or picnic spot. This is what gives rise to their other name 'ground pads'. In fact, the purpose of the thermal pad is to prevent the loss of body heat into the ground while sleeping. The foam pad, which is filled with air bubbles, slows down the conduction of heat through the pad since air is one of the best (and cheapest) insulators.
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—Henry David Thoreau (1817–1862)