Do Thermoses Sweat? Why or Why Not?

Thermoses are designed to hold hot and cold beverages for up to 24 hours and aren't supposed to sweat. However, I've noticed some water droplets on mine and I wasn't sure if this was normal or not.

So I wanted to find out if it's normal for thermoses to sweat and what I should do to stop this.

No, thermoses shouldn't sweat much. Minor sweating may occur near the top of your thermos because this area has the least insulation. However, if the entire thermos is sweating, there is a problem and it's likely that the vacuum insulation is compromised.

Your thermos will not sweat when it contains hot drinks. It will only have some sweating if you have a cold drink in your Thermos and it causes the moisture in the air to condense onto the bottle.

Why Don't Thermoses Usually Sweat?

“Sweating” occurs when the outside of your bottle is colder than the surrounding air. When the warm moisture in the air comes into contact with the cold exterior of your thermos bottle the moisture condenses into a liquid and forms droplets on your bottle.

Over time these build up until your bottle becomes wet to touch and even starts dripping onto the table or making everything in your bag a bit wet.

Regular non-insulated bottles like Nalgene sweat very easily but Thermos bottles shouldn't actually sweat much at all.

This is because Thermos bottles have a vacuum insulating layer that stops the cold drink inside your bottle from escaping.

Because of this the outside of your Thermos shouldn't get very cold. Because the outside is then the same temperature as whatever room you're in, no condensation or sweating occurs.

It is normal to see some sweating towards the top of the bottle near the lid. This is because that is the thinnest part of the bottle and it can become cold because the vacuum insulation here isn't as effective.

Also, when you have hot drinks in your Thermos it won't sweat. This is because the bottle needs to be colder than the outside air for the bottle to sweat. If you have a hot drink in your Thermos then it'll make the bottle slightly warmer than the outside air and no condensation will occur.

Reasons Why Your Thermos Is Sweating

If you're finding that yes, your Thermos is actually sweating quite a lot and it's happening all over the body of your bottle then there could be a few reasons for this.

The following are the reasons why Thermoses sweat, plus what you should do about them.

1. Loss of Vacuum Insulation

The most common reason for thermoses sweating is a loss of vacuum-sealed insulation. Thermoses are made of double-walled stainless steel with a vacuum in between. 

The vacuum gives the cup its extraordinary insulating property and keeps the thermoses from sweating.

When filled with cold icy water this vacuum insulation stops the cold from escaping and stops the outside of your bottle from being cold to touch (which is what causes the sweat to happen).

Sometimes, the vacuum seal can break, causing the vacuum to fill with air. 

Heat travels far more quickly in air than it does in a vacuum, therefore heat (or cold) may migrate from the inside to the outside of the bottle, making it cold or hot to the touch.

Sweating happens as a result of the moisture in the air condensing into water droplets.

So, if your thermos flask is sweating, not just on the rim towards the top of the bottle, the insulation is most likely damaged.

Unfortunately there is no way to fix this issue. You will need to get in contact with whoever made your Thermos and make a warranty claim. Hopefully you'll be able to get a replacement free of charge.

2. The Neck of Your Thermos Is Sweating (This is Normal)

If just the top of your thermos is sweating (around the neck near the lid) and not the whole body, this is normal and should not be a reason for worry.

There is also usually not enough sweating to create any major problems.

The top of your thermos flask is where the inner and outer walls meet. This is the region with little or no vacuum insulation.

Cold from within your thermos might travel from inside to outside at this point. Resulting in the formation of water droplets on the neck your Thermos. But this is completely normal and isn't something you should worry about.

It also only really tends to happen when your Thermos is completely full of cold water. Once you drink some of the water and the cold water isn't near the top of the bottle anymore then the sweating around the neck tends to stop.

3. Moving Your Thermos From The Fridge 

Sweating may also occur for a short period of time when you move your thermos from a cold environment into a significantly warmer environment.

This is more likely to occur in the winter than in the summer when you take your thermos from the cold outside and bring it inside.

Another example is, If you put your thermos in the refrigerator or freezer, the exterior will turn cold. When you remove it from the refrigerator, the stainless steel may condense the warmer outside air, causing sweating all over your bottle.

Note: You should avoid putting your Thermos in the freezer as it can actually damage the vacuum insulation due to the pressure from the expanding water.

This, however, is different from sweating produced by poor insulation. This kind of sweating will be very temporary and it usually doesn't sweat enough to cause large droplets to form or puddled to form under your Thermos bottle. 

When you take it out, the exterior of your bottle will rapidly warm up to room temperature, and it will cease.

What Should You Do If Your Thermos Is Sweating?

If your Thermos is sweating a lot then it likely has an issue with the vacuum insulation being compromised. You may also notice that your Thermos is not keeping drinks hot or cold for as long as it should.

Contact their company if your thermos is sweating and you've confirmed that the insulation is damaged. 

You can confirm the insulation is damaged by filling up your Thermos with boiling water and then waiting 30-60 seconds before touching the outside. If the outside is hot to touch then it is most likely that the insulation is broken and your Thermos needs to be replaced.

Some thermos flasks come with a limited lifetime warranty that covers insulation degradation. They can find a replacement.

If you don't want to go through that hassle, or if thermos refuses to replace your bottle, you need to purchase a new one.