The cold-weather problem
Most single-wall plastic bottles become brittle below -5 °C and can crack if dropped on rock or ice. A steel bottle like the Thermo Bottle 750, at 268 g, does not become brittle in cold and survives a 1.5 m drop onto concrete, a test Fjell runs on every production batch.
Plastic also offers no real insulation. A single-wall plastic bottle filled with hot water typically drops below 40 °C within 2 to 3 hours at room temperature, versus 12 hours above 55 °C for a double-wall vacuum steel bottle.
The taste and hygiene issue
Plastic bottles absorb flavors and odors over repeated use, and many require line-drying to avoid trapped moisture that breeds bacteria. Stainless steel does not absorb taste and dries faster because there is no porous surface.
Hand washing a steel bottle with warm water and soap after each multi-day trip is enough; there is no need for specialized cleaning tablets that plastic bottle owners often rely on to clear built-up odor.
What you give up
Steel is heavier than plastic. A 750 ml steel bottle weighs 268 g versus roughly 150 g for an empty plastic equivalent. For most day hikes and multi-day trips, that 118 g difference is a reasonable trade for not needing to replace a cracked or odor-tainted bottle every season.
If you are counting every gram for an ultralight thru-hike, plastic still has a place. For anyone doing cold-weather or winter trips, the durability and insulation gap is large enough that it is not a close call.
