Grubs Ladies Frostline 5.0 Boots Berry. Grubs Womens Frostline 5.0 Boots in Shale. 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.