The mid layer is often known as the insulation layer. There are many types of insulating fabrics: wool, fleece, pile, down, and acrylic. All of these materials are excellent insulators because they retain body warmth efficiently. Fleece and pile have recently become very popular because they are lighter and dry 3 to 4 times faster than wool. These extra pieces should have the ability of being added to or taken away from the system, easily. They should be used at the user's comfort and discretion. Synthetic fleece or polarfleece (commonly referred to as fleece) can come in many styles and weights. The weights coincide with their intended uses. Micro-denier fleece is a great mid layer on those cool spring and autumn hikes or bike rides (mid-weight PowderStretch®). Usually the manufacturers will list their fleece at a weight of 100, 200, and 300. But an easy way of looking at it is that 100 is extremely light, 200 is a mid, and 300 is a very warm piece of fleece. There is also a new fleece to the market. PCR (post-consumer recycled) fleece is made from recycled plastic bottles. This stemmed from the fact that many outdoor manufacturers are looking for ways to reduce waste and so recycled bottles was a major step for these manufacturers to stop processing new material, but rather use that material that was recycled. Synthetic material used in plastic bottles is similar to that used in polarfleece, so recycled plastic bottles is an excellent material for the outdoor industry. The result is an effective, comfortable fabric that is made of recycled material. Each year we contact Malden Mills or other manufacturers to ask if we can send them our scraps left over from making garments and each year we're told, "We're not ready yet". Pile is a heavy weight fleece used in weather systems for very frigid conditions. Down is a wonderful, lightweight, compressible, efficient and effective insulator. Its life span is unknown. Unfortunately, down will lose most of its thermal value when it is saturated. Therefore, a down jacket may prove ineffective without a waterproof shell over it. This brings us to the Outer layer (shells) section because, with the right outer system, down can be used safely.