The vests are completely lined with the highest quality phase change material .PCM are substances that can absorb and release latent heat when their physical state goes through a change. Our vests are designed to keep the core temperature of the wearer at an optimum level. Vests weigh 16-25 ounces, depending on size. We use washable materials that are water resistant or water proof. We maximize airflow for cooling. You add an insulated clothing when temperatures drop. There is a back inside waist adjuster for fit.