I love my Thermos cups and food containers. They can keep food or drink hot or cold for hours and they don't sweat like other water bottles do when you fill them with ice cold water.
But recently I had one of my insulated Thermos bottles start to sweat on me where it didn't used to. So I wanted to know why this happens and whether or not it's normal.
If most or all of the outside of your Thermos is sweating, it’s likely that the vacuum insulation is broken and it needs to be replace. However, if the sweating is only around the neck of your Thermos then this is completely normal.
This happens because this is an area of minimal insulation, and the cold water in your Thermos meets the warmer air outside, causing condensation.
I’ve had to replace a few bottles and flasks because of sweating and them no longer keeping drinks cold or hot anymore and I did some research into how and why this happens.
I also discovered how to tell if a Thermos has lost its vacuum and what to do if it has.
Reasons Why Your Thermos is Sweating
There are only a few main reasons why your Thermos might be sweating.
Your Thermos Has Lost Vaccum Insulation
A Thermos is made from a double-walled stainless steel cylinder with a vacuum between the walls. It is this vacuum that keeps your cold drinks cold and your hot drinks hot by minimizing heat transfer.
This is because heat really struggles to move through a vacuum because there are no particles the heat can be transferred through.
Regular bottles sweat because the cold drink inside the bottle makes the surface of the bottle colder than the air around it. Moisture in the air then condenses onto the bottle forming droplets and making it wet.
Below you can see this happening with my Nalgene bottle. Nalgene bottles do sweat because they aren't insulated.
Thermos bottles don't usually sweat because the outside wall of the bottle is separated from the cold water inside by the vacuum insulation. This means the outside of the bottle is usually the same temperature as the room it's in and so condensation (and sweating) doesn't happen.
However, if this vacuum is lost, then heat can move more easily between the inside and the outside of your Thermos. This can make the outside of your Thermos cold and sweating can occur.
This isn’t usually a big problem in dry, arid climates. I once went hiking in the desert with a broken Thermos vacuum without sweating issues.
However, an hour into a hot afternoon in my local area near the beach in the middle of summer, and the outside of my bottle was soaked.
If your vacuum insulation in your Thermos is broken there is unfortunately no way to fix it. The only thing you can do is contact the manufacturer for a new bottle or go out and purchase a new one yourself.
Sweating Around The Neck Is Normal
While sweating all over the bottle is not normal, sweating that occurs just around the neck of the bottle is nothing to be concerned about.
This is because the neck of the bottle is where the inner and outer walls meet and it's the area of the bottle that is the least insulated.
This means that heat (or cold) can easily pass through this section – making the outside of the bottle cold in this spot and thus allowing droplets to form.
I have noticed that sweating around the neck of the bottle only really occurs when the Thermos is completely full to the brim with something cold.
Once you drink some water and the bottle is half full the sweating around the neck usually stops.
If you are using hot items in your Thermos like tea, coffee or hot food then you shouldn't get any sweating at all.
Your Thermos Has Been in the Fridge or Freezer
Sometimes sweating happens because you have stored your bottle in the fridge or freezer.
In this case, the outside of the Thermos gets as cold as the air in the fridge. Then once you take it out, moisture in the warmer air outside the refrigerator or freezer condenses on the outer surface.
This can also happen when you move from a cold area (like being outside in the snow) and move your Thermos inside. Some sweating can occur.
This is less of an issue because if your vacuum is intact and it shouldn't continue for long. The outside of your Thermos will warm up rather quickly.
You can wipe off the outside of your Thermos, and it shouldn't continue to sweat.
How Do I Know if My Thermos Has Lost Its Vaccum?
The easiest way to tell if the vacuum on your Thermos is breached is to fill your Thermos bottle with boiling water.
Leave the Thermos for a minute or so and then feel the outside. If the outer surface is hot, the vacuum is broken, you have lost insulation, and you’re likely to get a sweaty bottle.
What Do I Do if My Thermos Is Sweating and Has Lost Insulation?
Every Thermos comes with a manufacturer's warranty.
Depending on the model, this could be either a one-year or a five-year warranty.
If your bottle is still under warranty and you haven’t caused damage to the vacuum, you can return it to Thermos for a replacement.
Alternatively if you don't want to go through that hassle or Thermos refuse to replace your bottle then maybe you need to buy a new one.
Thermos vacuum insulation technology keeps your drinks cold for 24+ hours and hot for 18+ hours. Featuring an unbreakable stainless steel exterior and interior and a locking leak proof lid with 1 hand push button operation.