Why does a vacuum flask keep hot drinks?

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What is a reusable vacuum flask?

The vacuum flask (also called a reusable bottle) keeps drinks hot and cold. Recently it is becoming very popular: colorful, practical and above all ecological. You think it’s the latest trend? No, it’s a real change. Be ready to say goodbye to disposable bottles of water for the joy of your wallet and the planet! A reusable water bottle, in fact, allows you to save money on the purchase of disposable plastic bottles and helps to decrease the amount of plastic produced (and not discarded) in the world. By the end of 2021, the European Parliament banned all disposable plastic products to reduce the environmental damage caused by plastic pollution.

What material is a reusable bottle made of?

Nowadays there are lots of alternatives to disposable plastic bottles that encourage the use of reusable and less polluting solutions. Follow our guide to learn about the differences and the materials of reusable bottles:


Reusable plastic bottles are appreciated for their light weight and practicality. They are the cheapest alternative to disposable plastic and must comply with European Laws. There are various shapes, models and they can be rigid or foldable.

Pros: They are lightweight, cheap, durable and often can be put in a dishwasher.

Cons: The plastic could alter the taste of the water. It does not maintain the temperature steady and some chemical components contained in the material could be dangerous to health.


Another alternative to disposable plastic is the glass bottles. The material does not release chemicals harmful to health and does not retain traces of the liquid previously contained. They can be used with all types of drinks but the glass is fragile and therefore uneasy to carry.

Pros: Safe for health, they keep the taste of drinks unchanged and they are dishwasher safe.

Cons: Glass bottles are heavy and fragile, that’s the reason why they are often used at home and are not recommended for outdoor activities.


Aluminum water bottles are lightweight and cheap but they are not stainless. This means that the inside of the wall needs a ceramic coatingto isolate this metal from the liquid. This makes the material more resistant and able to not alter the smell and taste of the drinks. If the aluminum bottle falls, the inner lining could be damaged, become uneven so mold and bacteria could form among the ripples. Aluminum bottles are not suitable for containing hot drinks, fatty liquids (milk, oils) and acidic drinks (fruit juices, energy drinks, lemonade).

Pros: Aluminum water bottles are lightweight and cheap.

Cons: Dangerous for health, they could alter the taste and smell of drinks and are not suitable for all liquids.


Stainless steel is common in products in contact with food. It is resistant and also does not corrode in contact with acidic drinks. As this alloy is stainless, the inner walls do not need any coating so, in case of impact, it remains intact.

Once washed, the stainless steel bottles do not retain the flavors and odors of the liquids contained previously. The European Law requires the use of certified and safe materials for health.

Pros: They are resistant, safe for health and do not alter the flavor. The stainless steel bottles guarantee excellent insulation for any kind of drink and keep the temperature steady for long.

Cons: Stainless steel water bottles are heavy, slightly more expensive than the other alternatives (especially if there is a double layer of insulation) and are not dishwasher safe.


Hydration bladders are not real alternatives to disposable bottles but are practical to carry during outdoor activities such as hiking, running and cycling. These are plastic containers to place into the appropriate pocket of the backpack and through an external tube it is possible to stay hydrated at any time.

Pros: Practical to put in the backpack to drink especially on the go.

Cons: Hydration bladders require washing and particular care to remove germs and bacteria.

Why does the vacuum flask keep drinks hot and cold?

The vacuum flask has this name because it keeps the temperature of the liquid inside unchanged. The walls of plastic, glass and aluminum bottles by nature cannot isolate the external temperature from the inner one.

Have you ever left a bottle of water inside the car during the summer? The plastic of the disposable bottle does not isolate the temperature of the liquid and in a couple of hours, the water becomes hot.

The vacuum flask, on the other hand, allows you to have the drink at the same temperature as when you filled it.

When we talk about a bottle with a double layer of insulation, we mean two insulation walls, an inner and an external one, that are separated by a layer of air. This structure slows down the heat transfer of the liquid to the outside and vice versa.

The vacuum flask does not know the difference between hot and cold, it does the same in both cases.

This is the reason why if you put hot coffee inside the vacuum flask, even if the outside temperature is particularly low (think of the freezing cold winter days), the temperature of the coffee will remain stable for several hours. The heat inside will be blocked by the first steel wall and then by the layer of air.

It works also on the opposite way. If we fill a vacuum flask with cold water and put it under the scorching sun, the temperature inside the bottle does not change and the water stays cold for a long time.

It is clear that a stainless steel vacuum flask with a double wall of insulation keeps the temperature steady for longer compared to a model with only one wall. So the price will be slightly more expensive.

What about the cap?

The double-wall contributes to the insulation but also the cap plays a fundamental role. In the vacuum flasks, there is often a screw cap with a small silicone part inside that allows it to adhere perfectly to the neck of the bottle: it prevents water from leaking and helps to isolate the liquid from the outside.

In fact, the hermetic closure seals the upper part effectively and prevents air from entering and altering the temperature of the liquid contained.

Now you may wonder: how long does the vacuum flask keep hot and cold?

In the bottle description it’s common to read Hot 12 hours – Cold 24 hours. These values are indicative and can change according to the real situation. The closure is the most vulnerable part and if you don’t screw the cap tightly or open the flask frequently, the temperature of the drink will change fast.

You can find other useful information in the guide “How to choose a reusable bottle“.

What are you waiting to choose yours? Our vacuum flask double layer 304 stainless steel keeps drinks hot up to 12 hours and cold up to 24 hours. Discover the 350 ml and 500 ml models and enjoy the colorful pleasure of life with Royal Eagle Road!

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