Thermos bottle in the freezer? Why you shouldn't
On hot days the idea is tempting: thermos bottle in the freezer, freeze the drink, and it slowly thaws on the go – staying ice-cold all day. Unfortunately, that is one of the most reliable ways to damage a good insulated bottle. In this guide we explain why thermos bottles don't belong in the freezer, what happens physically and which alternatives still keep your drinks cold for a long time.
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Thermos bottle in the freezer: the short answer
No, better not. Some manufacturers state this explicitly in their instructions – Thermos, for example, with the note not to put the product in the freezer. For the IZY bottles in our shop, the product description says nothing about this. For the reasons explained below, however, we clearly advise against it here too. This applies especially to filled bottles, but an empty bottle has no business in the freezer either.
What happens in the bottle when you freeze it
Water expands
Water is a special case among liquids: when it freezes, its volume increases by about 9 %. Anyone who has ever forgotten a glass bottle of water in the freezer knows the result. In a closed stainless steel bottle, the ice cannot escape. It presses against the inner wall, bottom and lid. At best only the lid bends; at worst the bottle's inner wall deforms.
The double wall is more delicate than it looks
An insulated bottle consists of two walls with a gap between them. This construction ensures that heat passes only poorly from inside to outside. If the inner wall is deformed by pressure, in the worst case it can touch the outer wall or put strain on the connection between the two walls. This creates a thermal bridge, and the bottle insulates less well – often permanently.
Lid and seal suffer too
The closure also takes the pressure. A warped thread or a squashed seal no longer closes reliably afterwards, and the bottle is no longer tight.
Glass inserts are even more delicate
Classic vacuum jugs and some bottles use a glass insert instead of stainless steel. Glass copes neither with the pressure of freezing water nor with fast, strong temperature changes particularly well. An insert that develops a fine crack in the freezer usually cannot be saved – and nobody wants to risk glass splinters in their drink. For such containers, the rule applies all the more.
Why it doesn't work anyway
Even if nothing broke: the trick makes little sense with an insulated bottle. The insulation slows heat exchange in both directions. So it doesn't just keep the warmth of the surroundings out, but also the cold of the freezer. A drink in a well-insulated bottle therefore takes a very long time to freeze at all in the freezer – if it manages it overnight at all. And once frozen, for the same reason it thaws only very slowly on the go. You would end up with a block of ice in the bottle but nothing to drink.
On top of that: when you take a very cold bottle out of the freezer, moisture from the room air quickly forms on it. That isn't ideal for printed or decorated surfaces and makes your bag wet – exactly what a double-walled bottle is supposed to prevent.
Better alternatives for long-lasting cold drinks
The good news: you don't need the freezer at all to keep cool drinks cool for a long time. A good insulated bottle already does that. For our IZY bottles, the manufacturer states that cold drinks stay cold for up to 24 hours. With these methods you get the most out of it:
| Method | How it works | Effect |
|---|---|---|
| Pre-chill the drink | Put water or tea in a carafe in the fridge the evening before | Cold start, the bottle holds the temperature |
| Pre-chill the bottle | Fill with ice water for a few minutes, pour out, then fill | The inner wall no longer warms the drink |
| Ice cubes | Add ice cubes first, then the chilled drink | Extra cold reserve for long days |
| Frozen fruit | Freeze berries or lemon slices separately and use instead of ice | Cools and adds flavour without watering down |
| Full rather than half-full | Fill the bottle well, but not to the brim | More liquid holds the temperature longer |
| Store in the shade | Don't leave it in blazing sun or a hot car | Less heat from outside |
By the way, ice cubes are best frozen in a silicone mould whose cubes fit easily through the bottle opening. For narrow openings, small cubes or crushed ice help.
What about the fridge?
Putting an insulated bottle in the fridge generally does no harm, but for the same reason as with the freezer it achieves little: the insulation keeps the cold out. It makes more sense to pre-chill the drink in the fridge and only pour it into the bottle in the morning. If you want to save space, put a glass carafe of tap water or cold tea in the fridge door and pour it over in the morning.
What to do if it has already happened?
If you have already frozen the bottle: stay calm and don't use force.
- Let the bottle thaw slowly at room temperature; don't put it in hot water or heat it with a hairdryer.
- Only open the lid once the ice has loosened. Don't lever it with tools.
- After thawing, check the bottle and lid for deformation, dents and cracks.
- Do a leak test over the sink: fill with water, screw shut, hold upside down.
- Test the insulation: pour in hot water and feel after a few minutes whether the outer wall gets warm. If it becomes noticeably warm, the insulation is probably damaged.
If the bottle is deformed or no longer insulates, you can often still use it as a simple water bottle for drinks at room temperature, as long as it is tight.
Our thermos bottles for cold drinks
The IZY x Pip Studio "La Dolce Vita" thermos bottle and the IZY Bottles Van Gogh Flowers thermos bottle are, according to the product description, double-walled stainless steel and keep drinks cold for up to 24 hours and hot for up to 12 hours. No condensation forms on the outside, and the screw closure is leak-proof. Both hold 500 ml and, according to the manufacturer, are washed by hand, not in the dishwasher. You will find more ideas for hot days in our article Water bottle in summer.
Related reading: Preheating and pre-cooling a thermos bottle: step by step · How does a thermos bottle work? The physics simply explained · Thermos bottle broken? Test the insulation in three steps
Conclusion
A thermos bottle doesn't belong in the freezer. Freezing water expands and can deform the inner wall, lid and seal, and the insulation prevents the drink from freezing quickly or thawing again on the go anyway. Better: pre-chill the drink in the fridge, pre-chill the bottle with ice water and add ice cubes or frozen fruit. A good insulated bottle then keeps things cold for a long time on its own.
Frequently asked questions
Can an empty thermos bottle go in the freezer?
We advise against it. There is no benefit, because the insulation doesn't let the cold in, and moisture forms on the outside when you take it out. Some manufacturers rule out the freezer entirely in their instructions.
Why does water expand when it freezes?
When freezing, water molecules arrange themselves in a crystal lattice that needs more space than liquid water. The volume increases by about 9 % in the process.
Can I put ice cubes in a thermos bottle?
In most cases yes, as long as they fit through the opening and the manufacturer has no objection. Ideally add the ice first, then the drink.
How do I know freezing has damaged the bottle?
Typical signs are dents or bulges, a lid that no longer closes tightly and an outer wall that gets warm with hot contents. Then the bottle no longer insulates properly.
How long do drinks stay cold in the IZY bottles?
According to the manufacturer, up to 24 hours. Pre-chilled drinks and a pre-chilled bottle help you get as close as possible to this value in everyday use.
Personally written by Martin – founder of BeachAndSteam and sauna operator at Lake Kulkwitz for more than 18 years.