Editor’s Note: The Isosteel Vacuum Flask has been removed from our side-by-side comparison of the best thermoses. While this product might still be available through select vendors, the manufacturer has discontinued it in the United States.
The Isosteel Vacuum Flask is our pick for the best overall thermos. It’s an excellent value because of how well it insulates, how portable it is and the amount of liquid it holds.
This Isosteel thermos retained 96 percent of the heat of the hot water used during our testing. Even after 24 hours the water cooled by only 64 degrees, which is impressive since other thermoses lost this much after only 6 to 12 hours. And during our cold retention tests, the Isosteel kept our cold water chilled for 48 hours with only a 27-degree warm-up.
There are two caps on this thermos. The first is a pop top that lets you drink a small amount of liquid. The second cap is a screw top that covers the first top and provides a second layer of protection. It also can be used as a cup. We shook and tipped the Isosteel with both caps on, and then only one cap on, and were impressed that there weren’t any leaks or spills. Since this thermos doesn’t have a handle for taking along more easily, it’s important to know that placing it in a backpack won’t cause spills.
The Isosteel Vacuum Flask is well insulated, so even with 188-degree water inside we couldn’t feel any heat on the outside of the thermos. And the outside of this stainless steel tumbler has a smooth coat finish that prevents condensation from collecting on it and gives you a firmer grip on the thermos.
This thermos isn’t dishwasher safe, which is one of its drawbacks. Its other is how heavy it is. The Isosteel Vacuum Flask weighs 18 ounces when empty, and it holds 30 ounces of liquid. That means when it’s full you will be carrying around nearly 4 pounds of thermos and drink. This can be burdensome on a hike or walk, but since the Isosteel fits into car cup holders, it works well as a thermos for your commute. Because it is over a foot long this thermos won’t fit in a lunchbox very well but will slip easily into a backpack. If you’re looking for something a bit smaller we suggest looking at the Thermos Sipp.
Considering the amount of liquid it holds and its low price, the Isosteel Vacuum Flask is a very good value. Plus, you get a five-year warranty with this thermos.
The Isosteel Vacuum Flask thermos is the best at keeping your drinks hot or cold for a very long time, and it’s constructed well so you can’t feel the temperature of your drink on the outside. Its two caps seal this thermos tight so there aren’t any leaks or spills. It’s a heavy thermos so it isn’t easy to lug around on hikes or walks, but it does fit nicely into your car’s cup holder. It holds 30 ounces of liquid and comes with a five-year warranty, which makes it an overall good value, too.