When single-use plastics started getting a bad rap, S’well became the drinks bottle of choice for a host of celebrities and ‘influencers’. It is sized extraordinarily well for solo hikers who are concerned about size and weight but still wish to carry a hot brew around to warm up with. In addition, many larger flasks incorporate ‘twist and pour’ spouts, so you can pour from the flask without having to completely remove the stopper. Designed for gram-counting backpackers, this is an extremely lightweight flask with double walls that are 60% thinner than standard vacuum flasks. Stylish, eco-friendly bamboo lid / Grab handle / Bottle + packaging 100% recyclable / No plastic comes in contact with your drink (silicone seal only) / BPA-free / Hand wash only.

On the other hand, it is fairly bulky for a 750ml flask, with a small cup compared to some other designs. The lid of the Smardy is 100% leakproof and non-toxic and can act as a small cup, but isn’t the biggest on offer in this roundup. Out of sheer curiosity, we also filled it with ice over a weekend and discovered minimal melt at the end of it. These materials are either poor conductors (plastic/glass) or able to reflect heat back into the hot liquid or away from cold liquid (like stainless steel). We then refilled them with water at 90°c to simulate the temperature of hot coffee and fitted the screw tops and cups.Need to know: Dishwasher safe, twist-off lid, available in a range of colours, no cup, also available in 532ml, 596ml and 946ml sizes, lifetime warranty Jack Wolfskin Kolima 1. Other useful design features to look for include a wide mouth, which makes for easier cleaning and filling (especially if you want to add ice cubes to your flask).

Our temperature test found that heat-holding performance wasn’t quite as good as the GSI rival, and nor does it offer one-handed drinking to make life easier on the trail (though it is compatible with the Hydro Flask straw lid, sold separately). We will never sell your data and you'll only get messages from us and our partners whose products and services we think you'll enjoy. The bottle has a wide-base design, so if you’re looking for a slimline flask to fit in a rucksack’s bottle carrier we’d suggest an alternative. Hydro Flask are a fashionable US company who have made reusable flasks and water bottles immensely cool over on the West Coast of America.After four hours, our drink was still a perfect temperature, and even after our six-hour test it was still fairly hot. Nothing beats a hot brew on a chilly day in the hills, and the best way to keep your tea, coffee or soup steaming hot is by packing a good thermos flask in your rucksack. The cup arrives with a rather complex looking set of instructions of how to dismantle its lid to clean it. Equally impressive is the food-grade odourless silicone seal on the screw top lid which ensures that the cap won’t leak.

The flask itself has a press-button top, meaning you press it in to pour your drink and then press it again to lock it. Using a flask is actually also a very responsible thing to do in terms of recycling – apparently 99. If all these routes out are blocked then the heat will stay in – I remember that much from my physics degree! We hand-washed each flask with hot soapy water and let them air dry overnight, then ranked them on smell. This is one of those brand names that have become descriptive of the product itself – a bit like Hoover being synonymous with ‘vacuum cleaner’.

We opened and closed the lids and stoppers of each flask 100 times, then checked if water would leak from the flask.

c / Wide mouth for stirring coffee or adding ice cubes / Made from 18/8, food-grade stainless steel / Condensation-free exterior / BPA- and BPS-free / Hand-wash only.

It punched above its weight in our temperature test, being beaten only by the Thermos Ultimate Series 500ml and the two larger capacity flasks from Sigg and Klean Kanteen. You do need to treat this flask with care, as the stainless steel outer wall is very thin and will dent more easily than others. The supplied cap is simple in design, though it is interchangeable with other Hydro Flask lids and stoppers (sold separately). Whether you’re spending the day in the office or exploring the great outdoors, your Thermos flasks will keep food and drinks just as you like them, for as long as you need.