Grubs Womens Frostline 5.0 Boots in Black. The Grubs Womens Frostline 5.0 Boots are ideal waterproof boots for most work or leisure activities. The Frostline combines Grubs unique Insu-foam ultra technology with advanced 5.0 thermal engineering providing protection in temperatures down to -10 °c. The boots offer under foot chassisâ ¢ and Nitrocellâ ¢ insulating and cushioning foot bed for support and comfort. The Frostline 5.0 Boots feature a Trax sole pattern with small lugs front and rear allowing a good push off grip and a prominent heel for a breaking grip on different terrains and a 2mm thick layer of tough rubber on the lower calf area for protection against damage against brambles and other potential damaging factors.