To keep body warm dissolve during sleeping in outside, you need thermal sleeping bag liner. Normal bag is still capable to hold strong air but not enough to protect full body warm. However, it is not plastic or steel where no air circulation can penetrate this bag. The main objective is to control temperature to suit metabolism. Thermal still has probability to release but in slow circulation. It is like room with air vent. Air goes inside and outside smoothly. This kind of sleeping bag is suitable where you go to high place such as mountain or travelling in cold weather.