The modern lightweight bivy bags are made of breathable fabrics. Gore-Tex is a fabric, allowing vapor from body heat to let out. This makes the body sweat-free. It also does not allow sweat to go to the sleeping pads. In short, the fabric has to be breathable and vapor-resistant. Look for mesh windows or flaps on the feet area to ensure air circulation. You may look for other openings too with a rain fly or other protection to prevent bugs from biting you.