A Therma-Rest pad is a backpacking self inflating mattress. The mattress is stored or carried compressed, with the air expelled and its air vent closed. Inflation is caused by the memory of the compressed foam. When the air valve is opened expansive mechanical force generated by the compressed foam creates structure and causes a pressure difference to draw air into the cells. The vent is then closed and the mattress remains inflated even under the weight of the sleeper