Wilderness tests three vacuum-insulated flasks that promise to keep drinks hot on cold winter days.

How we tested

For both hot water tests, we boiled water and filled each bottle to capacity. Before sealing the flask, we took a temperature reading of the water. That is our ‘start temperature’.

For the daytime test, we imitated a day-trip where you might stop for a drink every so often and checked the water temperature at semi-regular intervals: 1hr, 3hr and 6.5hr (We also tested the Stanley at 24hr because it is the only flask that promises to keep liquids hot this long). 

For the 12hr test, the flasks were only opened at the completion of the test. The flasks were kept outside in maximum daytime temperatures of 13℃.

For the cold water test, we put ice accounting for 10 per cent of the volume of each flask inside and then filled them with cold tap water. The flasks were kept indoors where the air temperature was a steady 20℃. 

The Fifty/Fifty has a comfortable three-finger grip.

Fifty/Fifty 40oz Insulated bottle
$69.99 / 1.1l / 499g

A double-wall vacuum-insulated stainless steel flask with a screw-on three-finger grip lid that allows comfortable carrying and the torque needed to screw the lid on tightly and back off again. It’s the largest volume bottle tested which makes its low weight surprising. But the extra volume means it’s not so stable when carried in a pack’s side drink bottle pocket – it’s best placed inside the pack. A lip at the rim rests on a cup to help avoid spillage when decanting.

Daytime water test: Start: 99.7℃. 1hr: 93.7℃ / 3hr: 84.9℃ / 6.5hr: 72.1℃.

12hr hot water test: Start: 99.1℃. 12hr: 56.7℃. 

Iced water test: Start, 5.8℃. 7hr: 5.1℃.

Verdict: With slightly more volume, this is the flask for those wanting to supply drinks for a group. It’s best at keeping water hot for less than 6hr.

The Camelbak Chute Mag is the most packable flask.

Camelbak Chute Mag
$74.99 / 1l / 493g

A compact double-wall vacuum-insulated stainless steel flask that, despite holding 1l, looks half the size of the Stanley and about two-thirds the size of the Fifty/Fifty. This makes it the lightest flask and the easiest to stow in a pack’s side drink pocket for on-the-go access.

It has a wide opening that makes filling it a breeze and when you need a drink, just twist off the magnetic cap which clings to the handle for obstruction-free drinking or pouring. The spout makes accurate, spill-free pouring a breeze.

Hot water test: Start: 100.5℃. 1hr: 91.8℃ / 3hr: 81.1℃ / 6.5hr: 67.1℃.

12hr hot water test: Start: 100.2℃. 12hr: 53.1℃.

Iced water test: Start: 5.9℃. 7hr: 4.4℃.

Verdict: If it’s all about weight and packability, this is the flask for you. It’ll reliably keep drinks hot for 3hr and cold for longer.

The Stanley Classic Flask is a top performer and able to keep drinks hot for 24 hours.

Stanley Classic Bottle
$79.90 / 1l / 860g

It’s the heaviest stainless steel double wall vacuum insulated flask by quite a margin, but it’s also the best performer by an equally large margin – especially when it comes to keeping liquids hot.

It has an insulated lid that doubles as a cup and helps slow conductive heat loss – unlike the lids on the other flasks, this lid didn’t warm up. An inner plastic lid can be partially unscrewed to allow pouring – a feature that prevents too much heat escaping when decanting. The side handle improves pouring accuracy and is comfortable to carry. It’s the tallest flask and for that reason needs to be stowed inside a pack.

Hot water test: Start: 99.1℃ / 1hr: 95.8℃ / 3hr: 91.1℃ / 6.5hr: 85.3℃ / 24hr: 62.5℃.

12hr hot water test: Start: 100.9℃ / 12hr: 79.5℃.

Iced water test: Start: 5.5℃. After 7hr: 4.5℃.

Verdict: Easily the best at keeping drinks hot or cold. It comes with a cup and provides easy heat-retaining and spill-free pouring.