These booties are super warm, durable and weatherproof. They’re a great change from heavy boots when you want something more lightweight but still warm. ThermoBall This insulation uses small round clusters of Primaloft synthetic fiber that mimic down. It does a better job of trapping heat than traditional continuous-filament synthetic insulations, and has been shown to be as warm as 600 fill goose down. It is lightweight, compressible and water repellent.