Insulated Flask

Two walls, a vacuum, and a lid that decides the rest

An insulated flask works because there is almost nothing between its inner and outer walls. Heat needs matter to travel through, so a vacuum between two skins of stainless steel removes most of the routes it could take. What is left is the neck and the lid, and that is why two flasks of the same size and shape can behave very differently depending on how their tops are built. The range here covers cup lids, straw lids, coffee and tea formats and larger travel bodies.

How long will a flask hold heat?

Longer when it is full, shorter when it is half empty, because the air gap above the liquid carries heat to the lid. Longer again when the flask has been rinsed with hot water before filling, since a cold steel wall takes its share of the heat before anything else does. Cold drinks follow the same rules in reverse and hold longer than hot ones as a rule, which is why the same body appears with both hot and cold uses in the wider vacuum flask thermos bottle range.

Which lid suits which drink?

• Cup lid: the screw top doubles as a mug and lets a hot drink cool to a drinkable temperature outside the flask.
• Straw lid: made for cold drinks, easy to use one-handed, and not suited to anything hot.
• Flip or push top: quick access on the move, more parts to clean and more places for a seal to fail.
• Mouth width: a wide mouth takes ice cubes and a cleaning brush, a narrow one loses less heat and pours without splashing.

What should never go in a flask?

Carbonated drinks build pressure inside a sealed steel body and can spray on opening. Dairy, soup and anything with solids in it should not sit warm for hours, because a warm sealed container is a poor place to store food. Bleach and abrasive powders damage the inner surface, and a dishwasher can lift printed finishes and, on some models, affect the seal. Plain water for the day belongs in the lighter bodies found in hiking water bottles.

How is a flask cleaned so it stops smelling of coffee?

Rinse straight after use rather than at the end of the day, since coffee oils set into a film once dry. For a flask already tainted, a soak with bicarbonate of soda and hot water, then a long-handled brush into the shoulder where the neck narrows, deals with the ring that a sponge never reaches. Dry it open and upside down, because a lid screwed onto a damp interior is how the stale smell returns.

What else is worth carrying alongside?

A flask is a heat store, not a supply, so it usually travels beside a plain bottle rather than instead of one. A litre of hot drink weighs the same as a litre of water and gets used far more slowly, which makes the pairing sensible on cold days out. The full picture of bottles, reservoirs and flasks sits under water & hydration.