The container originally developed by Sir James Dewar is officially known as a Dewar flask, but most of us call it by a trademarked product name "thermos". These containers slow down temperature change using the principles of convection, conduction, and radiation. Learn more about finding a good thermos.
Thermoses all use the same insulating method to limit heat transfer and keep the contents hot or cold.
The interior of a thermos is typically made of glass and plastic or stainless steel. Both do the job, but:
The glass interior is cheaper and is better at reducing the conduction of heat, but can break easily if dropped. Look for one with a thicker inside wall.
Stainless is more durable, but more expensive.
A thermos with a large opening is better for soups and stews. A narrow mouth is better for beverages, and holds heat in better since there’s less room for it to escape.
The cap can make a big difference to how well the thermos works. Look for a deep, thick cap that reaches into the flask to prevent heat or cold from escaping. The deeper the threads on the screw cap, the better the seal.
Pre-heat and pre-cool your thermos before you use it. It will work better. Fill it with boiling or ice water for 5 minutes first.
Never put carbonated beverages in thermoses due to pressure build-up
Never put a thermos in the dishwasher or microwave, which can both wreck the seal.
Don’t clean it with a scouring pad.
Don’t submerge a thermos in water.
We tested our thermoses in the extreme by filling with boiling 100°C water, burying them deep in the snow, and measuring the temperature after time. We also took another set to the slopes for some après ski, hot chocolate refreshment. We tested:
- Aladdin (glass with snap-up spout): $9
- Thermos Brand (glass with a stash top for condiments and a twist and pour spout): $11
- Nissan by Thermos Brand (stainless steel): $30
- Starbucks (stainless steel with push-button cap): $42
Buried in Snow Test
After 3 hours we dug up our thermoses to measure the water temperature:
- Aladdin: just below 59°C
- Thermos: 60°C
- Nissan: 70°C
- Starbucks: 70°C
All were easy to pour.
The ones with screw nozzles and push buttons were our favourites.
We also liked to drink from the larger cups, even though the smaller ones were more convenient to pack.
The stainless steel thermoses kept our hot chocolate hottest.
OUR TOP PICK
The Nissan by Thermos has a deep screw cap (for better heat retention), is durable stainless steel, is not the most expensive, and kept the liquid hottest.