Kids Lunches

How to Keep Food Warm in Lunch Box (Easy ways)

When packing school lunches, it’s always nice to have the option of sending a warm lunch. Let’s talk about how to keep food warm in a lunch box so you can confidently send food off to school.

Depending on what materials you have and what type of food you’re sending, you can find what works best for you and your family.

These techniques can work with picky eaters who are only eating their favorite soup lately or adults heading out to their shift looking forward to lunch. You can send air fryer fish sticks for lunch or this super easy one pot rigatoni to switch up lunchtime!

Try out these tips and see which ones work best for you (detailed explanation below).

  1. Use an insulated lunch bag
  2. Use a thermal bag
  3. Incorporate hot packs
  4. Pack a hot water bottle
  5. Pack moist foods that retain heat well/pre-heat your food
  6. Wrap food in aluminum foil
  7. Utilize vacuum-insulated stainless steel containers
  8. Pre-heat your lunch box or container
  9. DIY insulation methods

Let’s nerd out for a second. Here’s how heat works in your lunch box:

Understanding Warmth; The Basics of Heat Transfer

Heat moves in three main ways: conduction, convection, and radiation.

Conduction is when heat moves directly through materials that are touching. Think of a spoon getting hot in a pot of soup.

Convection occurs when heat circulates through liquids or gases. It’s like how warm air moves around a heated room.

Radiation is when heat travels in waves directly from a heat source. The warmth from the sun is a perfect example.

Heat Loves to Move:

First off, heat is always looking to spread out. It moves from hot things to cooler things, trying to make everything the same temperature (usually to room temperature). That’s why your warm food will start to cool down if it’s not insulated; the heat is moving out into the cooler air around it.

Insulation is Key:

To keep things warm, we need to slow down this movement of heat. Insulation is like putting up a roadblock for heat; it makes it harder for heat to get out. Materials that are good at insulating, like foam or certain plastics, have lots of tiny pockets of air. Since air is not good at moving heat around, these pockets help keep the heat in.

Using Insulation for Food:

When we pack food in an insulated lunch box or wrap it in foil, we’re using these insulation ideas to keep our food warm. The insulation slows down the heat from escaping, keeping your food warmer for longer.

The Role of Containers:

Some containers are better at keeping heat than others. Vacuum-insulated stainless steel containers are like champions of insulation. They have a space between two layers of steel where there’s almost no air (a vacuum). Without air, heat can’t move out easily, so your food stays warm much longer.

Pre-heating Helps:

Another trick is to pre-heat your lunch box or container. By filling it with hot water before you put your food in, you’re giving it a head start on staying warm. Just like pre-heating an oven for baking, pre-heating your container makes sure it’s warm before you add your food, helping the food stay warm longer.

9 ways to Keep Food Warm in Your Lunch Box

1. Use an insulated lunch bag

This is your first step in keeping your food warm in your lunchbox. These are a few things to think about when choosing an insulated lunchbox.

  1. Outer Material: Typically made from durable, water-resistant fabrics like nylon or polyester for longevity and protection against the elements.
  2. Insulation Layer: Features closed-cell foam or reflective foil linings to slow down heat transfer and maintain temperature.
  3. Interior Lining: Often lined with easy-to-clean, food-safe materials such as PEVA, aluminum foil, or food-grade plastic, ensuring a safe and clean space for food contact.
  4. Zippers and Seals: Utilizes high-quality zippers and possibly additional seals like Velcro to keep the bag airtight and enhance insulation.

Preheat your thermos with boiling hot water. Keep it in there for about 10 minutes before you pour out the water, dry it and place food.

2. Warm water bottle

This technique provides consistent warmth and is very cost-effective. It’s safe and convenient. These water bottles are portable, reusable and easy to prep. You can also use this for a few different containers so it’s pretty versatile.

  1. Fill a Hot Water Bottle: Boil water and fill a small, sealable hot water bottle with it. Make sure the bottle is durable and can safely contain the hot water.
  2. Prep Your Lunch Box: While the water is heating up, prepare your lunch box by making sure it’s clean and dry. If your lunch box is not insulated, consider lining it with a towel or cloth to improve insulation.
  3. Wrap the Hot Water Bottle: Once filled, tightly seal the hot water bottle. Then, for extra safety and to prevent direct heat from damaging the lunch box or the food containers, wrap the hot water bottle in a towel or cloth.
  4. Place Inside the Lunch Box: Put the wrapped hot water bottle in the lunch box along with your food containers. The ideal arrangement is to place the hot water bottle on one side of the lunch box and the food containers on the other side, not directly touching. If there’s enough space, you can even place the wrapped hot water bottle between two food containers to distribute the warmth evenly.
  5. Close the Lunch Box: Seal your lunch box as usual. The heat from the hot water bottle will fill the enclosed space of the lunch box, keeping the food warm until it’s time to eat.

3. Use a thermal bag

These bags are similar to insulated lunch boxes by creating a heat barrier. They’re usually lined with reflective materials that help maintain the temperature of your food, whether it’s hot or cold.

4. Incorporate hot packs

Hot packs can be heated in the microwave and then placed in your lunch box to keep things warm. They’re a portable and convenient way to add a source of heat, helping your food stay warm for hours. You can also use instant hand warmers as a heat pack.

A heating pad could also work, although not ideal for a school lunch, but maybe for an adult lunch.

5. Pack moist foods that retain heat well/pre-heat your food

Some foods that have high moisture content, like soups, stews, and saucy dishes, tend to stay warm longer. The liquid helps distribute the heat evenly and keeps the temperature up for a longer period.

Heating your food to a higher temperature before packing can help it retain heat longer. Make sure your hot food is *piping* hot before it goes into the container. This will slow down the cooling process.

6. Wrap food in aluminum foil

Aluminum foil is awesome for retaining heat. Wrap your hot food in foil before placing it in the container to help lock in the warmth. This method works well for solid foods like sandwiches, burgers, and baked goods. To take this a step further, wrap it in a small towel or tea towel.

7. Use vacuum-insulated stainless steel containers aka a thermos

This type of insulated food container is the best way at maintaining the temperature of their contents, whether hot or cold. The vacuum insulation prevents heat transfer, keeping your food warm for many hours.

An insulated food jar is a great way to include your kid as well. They can pick out what color or design they want.

Why stainless steel?

  1. Durability: Highly resistant to dents, scratches, and corrosion.
  2. Corrosion Resistance: Doesn’t rust, ensuring longevity and hygiene.
  3. Non-reactive: Won’t alter the taste of contents, maintaining flavor integrity.
  4. Excellent Insulation: The vacuum seal between steel walls prevents heat transfer, keeping contents hot or cold for hours.
  5. Safety: Free from harmful chemicals like BPA, making it safe for food and beverages.
  6. Ease of Cleaning: Its smooth surface is easy to clean and doesn’t harbor bacteria or odors.
  7. Eco-Friendly: 100% recyclable, reducing environmental impact over time.

If you’re worried that your food (like chicken nuggets) might become soggy, here’s a little trick. Place a clean, dry paper towel or a piece of bread on top before sealing the thermos. This can absorb excess moisture, preventing it from settling back on the food. The bread one would definitely throw my kids off, so I just use paper towels or a napkin.

A few types of foods that hold up well in a thermos include stews, curries, rice and warm pasta.

8. Pre-heat your lunch box or container

Warming up your lunch box or container before packing your food can really increase heat retention. You can fill it with hot water for a few minutes or place it in a warm oven if it’s oven-safe. Just be sure to empty or remove it from the heat source before packing your food.

9. DIY insulation methods

After doing a deep dive, I have learned that some people have found success wrapping their food containers in bubble wrap or fleece to help with keeping their food warm. Let me know what works best for you in the comments below!

For a long time, I always thought that lunch had to be boring, but I was wrong! Looking forward to a hot meal for lunch time has truly been a game changer. It also really helps changing what I send for lunch. Cheers!