What Causes Condensation Inside A Water Bottle? Is It Safe?

When we think of condensation, we usually think of it forming on the outside of our water bottles — which makes sense because it’s much more obvious and a lot more annoying to deal with.

But if you’ve noticed that condensation sometimes forms on the inside of your bottle as well, you may be left wondering why and if it’s anything you should be worried about. 

Condensation on the inside of a water bottle happens when the water inside the bottle is hotter than the air outside — causing water in the bottle to convert to water vapor, saturate the air inside, and eventually deposit as droplets on the interior walls of your bottle. 

This is most likely to happen when there’s warm water inside your bottle, when the water temperature in your bottle lags behind the colder atmospheric temperature outside it, or when your bottle is left to sit in direct sunlight.

Condensation inside a bottle isn’t anything to worry about, at least not on its own. After all, it’s only pure water that’s converted from liquid to gas and back.

Below I’ll teach you exactly what causes condensation inside water bottles and a few situations where it may actually be concerning.

What Causes Condensation Inside A Water Bottle?

Put simply, condensation is water vapor converting back into liquid water.

When something cold (like the wall of your water bottle) is introduced to warm humid air, then the air around the cold thing cools down and is unable to hold as much vapor as before.

Excess vapor turns back into liquid water and clings to the cold surface. 

Usually this happens on the outside of the water bottle when you have a cold drink inside (but it can also happen the other way around).

When condensation forms on the outside of our water bottles, it’s essentially because the cold water inside our bottle cools the exterior of the bottle, which in turn cools down the air surrounding it, dropping it below its dew point. 

This is usually known as “sweating”.

For a more detailed explanation of condensation and why it happens on water bottles, check out my deep dive on condensation on water bottles.  

When it comes to condensation inside a water bottle, a lot of the same principles apply, just in reverse — the water and air inside your bottle are typically warmer than the air outside, causing condensation to form on the inside of the bottle instead of the outside. 

This can happen for a few reasons. 

Putting Warm Or Hot Water In A Bottle

In higher temperatures, water converts to vapor at a much faster rate. 

So when you put warm or hot water inside a small enclosed space, like a sealed bottle, the air inside quickly becomes saturated with vapor.

Once the air inside is unable to hold any more vapor, excess vapor converts back into liquid water, which ultimately clings to the interior walls of your bottle. 

You’ll notice this on the inside of the lids of your coffee cups when you put a hot drink in them or inside a tupperware container after you’ve placed a hot meal inside and put the lid on it.

The walls or lid of your container is colder than the warm saturated air inside and so the water in the air condenses and sticks to the side or lid of your container.

Your Bottle Was In Direct Sunlight

If you leave your bottle sitting in sunlight, no matter the initial temperature of the water or the temperature outside the bottle, the heat from the sun will cause more water to convert to vapor.

When energy, or heat, is added to water, it causes the water molecules to move faster and faster, until some eventually escape and convert to gas.

And again, once the air inside your bottle saturates, excess vapor will turn back to liquid water, and in turn cling to the walls of your bottle.

The Temperature In Your Bottle Is Lagging Behind The Colder Temperature Outside It

If you’ve ever left a water bottle sitting on your desk for a few days, you may have noticed that the inside of the bottle will at times be covered in a layer of droplets and at other times be completely clear, seemingly at random. 

Since the temperature of the water is staying consistent at room temperature, what exactly is causing condensation?

Keep in mind two things, water has a much higher heat capacity than air, meaning that water changes temperature much more slowly. And second, it doesn’t take nearly as big a difference in temperature for condensation to form as you might think. 

Because the temperature outside and even in our homes is constantly changing, the temperature of the water in our bottles is often left lagging behind.

When it lags in the direction of being warmer than the surrounding air, you often end up with condensation inside the bottle. 

Is It Safe To Drink From A Bottle With Condensation Inside?

There’s a bit of a misconception that condensation in a water bottle may be a sign that it’s unsafe to drink from. But in reality, condensation on its own is perfectly normal and perfectly safe. 

After all, it’s nothing more than pure water that has gone from liquid to gas and back. 

But that doesn’t necessarily tell the whole story. 

Certain plastic bottles contain harmful (and potentially harmful) chemicals that can leach from the plastic into your water, especially when heated.

Also, condensation may be happening because you’ve left your water bottle full of water for days without replacing the water or cleaning the bottle. In this case it’s best to empty out the old water and clean to bottle before reusing it.

To keep yourself safe:

• Don’t drink from disposable plastic water bottles that have been opened and left in the heat. 
• Avoid drinking hot or boiling water from plastic water bottles, even reusable plastic bottles that can handle it. 
• Make sure your reusable plastic bottle is BPA-free, as well as free from other faux-estrogen compounds, like BPS, BPF, and phthalates. 
• Consider swapping to a stainless steel water bottle, which is recognized as one of the safest materials for food and drink.