Home » How Does Vacuum Insulation Work? (in coffee thermoses and travel mugs)

How Does Vacuum Insulation in a Thermos Work?

Vacuum insulation in a thermos is one of the best ways to keep your coffee hot for a long time. On the surface, the principle seems relatively simple. The vacuum is there to prevent the heat from escaping and cold from coming in. But there is some serious physics going on under the lid.

So, how does vacuum insulation work? This article takes a closer look at the way heat travels, the reasons coffee gets cold, and the inner workings of vacuum insulation.

How Does Vacuum Insulation Work? – A Heat Travel Guide

There are three basic ways heat travels – conduction, convection, and radiation. Here is a quick overview of all three principles.


Conduction happens when two objects touch. To be precise, heat moves from a hot object or a liquid onto a cold one when they come in contact.


For convection to occur, the two objects don’t need to be in direct contact. It is best to use a fan heater as an example to describe how this works. As the fan blows, the hot air pushes the cold air and rises up. In other words, the hot air sits higher in your room and cold air gets nearer to the floor. (1)

If you use a convection microwave, you can use this principle to reheat your coffee.


Because their atoms get additional energy from heat, hot objects emit light before quickly reverting to their usual state. Sometimes you cannot see the light but can still feel the heat. This is called infrared radiation. For example, you can feel the radiation from an incandescent lamp. (2)

Smoking hot coffee might emit heat from a cup, but you shouldn’t feel any heat escaping with a vacuum insulation thermos.

hot coffee for hours - how does vacuum insulation work in coffee thermoses like this

How Does Vacuum Insulation Work in Your Thermos?

To understand the inner workings of a vacuum thermos, it is best to take a look at its components. On the outside, a thermos features a screw-on lid and an outer case made of stainless steel or plastic. There are two layers of glass or stainless steel under the casing.

The vacuum sits between the first (outer) glass layer which features a reflective coating and the second (inner) layer made of glass or stainless steel. These layers are fused at the top to ensure a tight seal. Plus, there are supports at the bottom to prevent wiggling. There might also be extra insulation between the outer case and the glass layers to protect the inner glass from fractures and damage in case you drop the thermos.

The key thing you need to understand is that vacuum flasks keep hot things hot and cold things cold.

When all is said and done, each design element of a vacuum-insulated thermos is there to prevent any kind of heat transfer. To be precise, there shouldn’t be any conduction because of the vacuum. Convection is prevented by the screw-on lid on top, and the reflective coating keeps the radiation inside within the inner chambers.

As indicated in the quote, the same principle works like a charm if you want to keep the coffee cold. This time around, the thermos elements prevent the heat from seeping in and retain the cold within the inner vacuum flask. (3)

Currently, Zojirushi Vacuum Bottle Co. Ltd. holds the patent for the vacuum thermos. However, it was originally designed and patented in 1908 by Gustav Robert Paalen.

Why Is Vacuum Insulation Important for Travel Mugs?

Simply put, vacuum insulation is important because it is the best way to keep your coffee hot for hours on end. But why does coffee get cold in the first place?

The answer lies in the difference between the temperature of your coffee and room temperature. The ideal coffee brewing temperature is 195°F to 205°F. In fact, the best brews are very close to 205°F. However, the temperature of your room is usually around 70°F. Due to the great difference between the two, the heat quickly moves from the pot or cup into the room.

But if you pour the coffee into a vacuum-insulated travel mug, you prevent the rapid heat transfer via convection, conduction, or radiation. This way, your coffee should still be smoking when you open the mug after a few hours. Not sure which travel mug to pick? See our pick for the best travel coffee mug here.

Final Thoughts

The way vacuum insulation works boils down to basic physics and clever design. That said, some vacuum-insulated mugs retain heat better than others. As such, the best coffee thermoses are usually from brands that have veritable track records.


Yes, heat can transfer through a vacuum. Does this make vacuum-insulated mugs less effective? Of course it doesn’t. As long as you pick a well-built mug, you don’t need to worry about your coffee getting cold.

Vacuum absorbs heat and increases its temperature. Soon, thermal equilibrium is reached and some of the heat atoms evaporate. But remember, vacuum mugs have an inner reflective coating to prevent the heat from getting in touch with the vacuum.

Vacuum insulation means that your thermos features an insulating layer between the two inner stainless steel or glass sections. Together, these design elements keep your coffee cold or hot.

  1. https://www.cnet.com/home/kitchen-and-household/what-is-a-convection-oven/ Retrieved from https://www.cnet.com/how-to/what-is-a-convection-oven
  2. Radiation Heat Transfer. (n.d.). Retrieved July 3, 2019, from https://www.sciencedirect.com/topics/engineering/radiation-heat-transfer
  3. US4427123A – Stainless steel thermos bottle. (n.d.). Retrieved July 3, 2019, from https://patents.google.com/patent/US4427123A/en
Giada Nizzoli
I’m the resident, Italian espresso expert, sharpening my extraction skills from the rainy UK. I love the Oxford comma, and I have ink and coffee in my veins (not literally, or I’d be dead by now).

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