Roof Tech's AlphaSeal™ gets its strength from its proprietary butyl, synthetic rubber copolymer of isobutylene with isoprene that delivers superior flexibility and is completely impermeable to moisture. It is also resistant to mildew, acids, alkalis, salts, and aging from UV light. ours was proven to withstand temperature fluctuations from -40°F to 248°F, making it impervious to any kind of extreme weather.