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What’s the best vacuum flask? Five things our testing taught us

Discover which was the worst at keeping drinks hot, those that broke during our durability tests and the best type for ease of use

What’s the best vacuum flask? Five things our testing taught us

We put 11 vacuum flasks to the test to see which would keep our drinks hottest for longest and which were most durable. Among those we tested were Instagrammers’ favourite Hydro Flask, the trendy Chilly’s bottle and three Thermos flasks. 

Maybe you need a new flask to keep your hot drink warm during a frosty lockdown walk, or for the kids to use when they finally head back to school? Or perhaps you’re investing in one for this summer’s stay-at-home camping trip. Whatever your plans, here’s what you need to know about vacuum flasks.

The best vacuum flasks: Thermos Flask, Hydro Flask and more tested 

1. Watch out for glass

The only two glass-lined vacuum flasks we tested broke very early on in our durability tests.

We dropped each flask from a 90cm height on to hard pavement 10 times from different angles.

One of the flasks shattered on the first drop and the other fell apart, causing the inside to slide out on to the floor.

Two broken vacuum flasks from Which? test
The two glass lined vacuum flasks we tested both broke during our durability drop test


See our pick of the best water bottles

2. Handle not essential

If you’re shopping for a smaller vacuum flask, say a litre or less, you might want to consider one with a textured grip rather than a handle.

We found that the textured coating makes the flask just as easy to hold on to, but as it’s less bulky it’s easier to store away.

If issues with your hands mean a texture grip might not work for you, check out a vacuum flask with a retractable handle. This will give you almost the same streamlining.

Fixed handle versus textured coating vacuum flask
A vacuum flask with a fixed handle is bulkier to store in your cupboard or rucksack

See our pick of the best reusable coffee cups

3. Don’t believe the hype

Hydro Flask is a favourite with Instagram followers, but when we put this flask to the test it failed to impress.

Despite being the most expensive flask we tested and boasting it’s own patented TempShield technology, it ranked worst at keeping our coffee hot over time.

Thermal image comparison of the Hydro Flask and a cheaper flask.
The yellow and orange spots on our thermal image indicate where heat might be escaping

4. Button-operated lids are a bonus

We found button-operated lids more efficient than twist-and-pour stoppers. They’re easy to operate if you’re less dextrous or if you’re wearing gloves on a chilly day.

The cup fastening over the push button acts as a second lid to guard against leakages.

Button operated lid and twist and pour stopper comparison.
On the left is a button-operated lid and on the right is a twist-and-pour stopper

5. Check it’s actually a vacuum flask

Not all flasks are vacuum flasks. There are plenty of cheaper drinks bottles and flasks that are simply containers for liquid, but won’t keep anything hot or cold for any period of time.

How a vacuum flask works

How a vacuum flask works graphic

Vacuum insulation is unique because it keeps heat from coming in and out of the main body using double walled insulation. Essentially, it’s a flask within a flask.

The space in between those walls creates a vacuum and this slows heat transfer. The wall of the inner flask is silvered to prevent heat transfer via radiation.

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