Stretch-fit topline binding snugs let to keep warmth in and cold out. Fleece-lined, 5mm CR flex-foam bootie with 4-way stretch nylon. 100% waterproof, lightweight and flexible. Comfort range of -40 degree to 60 F (degree Fahrenheit). Double reinforcements on the instep, heel and achilles areas. 2 mm (millimeter) thermal foam underlay added to the instep area for additional warmth.