What is the best thermal insulator for a lunch box?

Insulation plays a crucial role in selecting a lunch box, ensuring that food remains at the desired temperature for extended periods. Various materials serve as effective insulators, each with distinct characteristics and applications. This article explores top-quality insulating materials for lunch boxes, shedding light on their unique features.

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Foam: Commonly used due to its lightweight nature, affordability, and commendable insulation, foam is a popular choice. Crafted from materials like polystyrene and polyurethane, foam can be molded into diverse shapes and sizes to fit snugly inside a lunch box. One advantage is its flexibility, allowing easy customization to match your lunch box's contours. While foam may not be as robust as other options, it offers satisfactory performance when handled with care. Choose the TACTICISM lunch bag for superior heat preservation, sealing, shading, and moisture resistance. It suits various settings such as school, work, picnics, hiking, beach outings, office, gym, and more!

Neoprene: Neoprene, a synthetic rubber, is another insulation material used in lunch boxes. Renowned for its durability and flexibility, it retains insulating properties even when damp. Neoprene can be sewn or glued into place, boasting tear and abrasion resistance. However, it may come at a higher price point compared to alternative materials, and its weight and ease of manipulation may not match that of foam.

Thermal fabric: Lightweight and thin thermal fabric, like Thinsulate, excels in maintaining warmth or coldness. Frequently employed in clothing, it also serves as effective insulation for lunch boxes. Thermal fabric proves malleable and easy to work with, allowing for sewing or adhesive application. Its lightweight and compact nature make it suitable for those seeking a minimized lunch box size and weight. However, thermal fabric might offer slightly less insulation than alternative materials and may not exhibit the same long-term durability.

Ultimately, the choice of the best insulation material for your lunch box hinges on personal preferences and specific requirements. If you prioritize affordability and low weight, foam or thermal fabric could be ideal. Neoprene, on the other hand, offers enhanced durability and flexibility. Opt for the insulation that aligns with your needs, enabling your lunch box to uphold the desired temperature for an extended duration.

To help find one that suits your needs, we researched top products suitable for packing up leftovers or make-ahead meals , as well as dishes to add to the kids' lunch boxes . We tested to see how long our best models would keep hot food warm and cold drinks chilled.

When you're always on the go, it's easy to get into the habit of stopping for a quick-service meal, but relying on fast food can also add up&#;both calorie- and money-wise. With the right storage containers, taking homemade food along to the office, school, or even on a hiking trail is a cinch.

Sleek and portable, the Klean Kanteen Food Canister is the perfect size for tucking into a lunch box. The swivel loop can be turned up to help open or close the thermos, then back down when you don&#;t need it. In testing, it didn&#;t leak when we filled it with water and turned it on its side. However, it didn&#;t keep food hot as well as some of the other thermoses&#;at the five-hour mark, the soup inside was lukewarm. Still, it&#;s a sturdy little thermos that only got a few scratches when purposely dropped from counter height, and the wide mouth makes it easy to fill, wash, and dry. The only thing we didn&#;t love about it was how much the stainless steel showed fingerprints, streaks, and watermarks.

This canister has the capacity to keep both hot and cold food, like ice cream, at a consistent temperature for hours, and the interior is incredibly easy to clean. Better yet, a portion of each sale is donated to the nonprofit 1% for the Planet .

If shopping sustainably is a non-negotiable value, this food canister by Klean Kanteen is an ethical consumer&#;s best buy. As a certified B Corp , the business has been vetted for meeting the &#;highest standards of verified social and environmental performance, transparency, and accountability&#; within the industry, partially thanks to all of its products being Climate Neutral Certified .

This thermos can be a little unwieldy to carry when full, so we recommend using two hands when picking it up and using the insulated bag if you&#;re traveling with it. When purposely dropped from counter height, the outer lid sustained a small dent, so you&#;ll want to be a little careful when handling it. The large opening makes the thermos easy to fill and clean, and while it&#;s not necessarily practical for bringing to and from the office, we could see it getting some good use on a campout.

The FEWOO thermos demonstrated impressive heat retention in testing. After five hours, the soup inside was still plenty hot, and the outer lid made for a convenient bowl to eat from. The inner lid has a button for releasing air pressure to make it easier to open, too. When closed, the silicone sealing ring ensures no liquids or smells leak out of the container, which is key for long travels as it gets bumped around in your backpack.

For taking your food on outdoor adventures with peace of mind, consider this highly rated thermos. The 18/8 stainless steel interior ensures no flavor transfer or lingering odors, and the 27-ounce capacity is plenty for your soups or on-the-go meals. Our favorite part is the handy insulated bag it comes with: This not only protects your container during transport, but it also extends the heat retention of your thermos and its contents.

This holds a decent amount of food without being unwieldy, and it&#;s a solid everyday option for enjoying a warm lunch on the go. For less than $20, you get a well-designed, attractive thermos that does the job, with a matte finish that doesn&#;t show fingerprints or streaks. The only limitation we found was that the depth of the thermos made it slightly difficult to eat chunkier soups or stews, as getting to the very bottom with a spoon can be tricky.

In testing, this thermos proved to be sturdy and reliable. It did not leak when we filled it with water and turned it on its side, and when intentionally dropped from counter height, it didn&#;t get a single scratch or dent. It kept Italian wedding soup hot for more than five hours, and the top handle proved useful in screwing the lid off and on and carrying it around.

Look to Simple Modern&#;s Vacuum Insulated Food Jar Thermos for a leakproof and sweatproof vessel on the cheap. Thanks to the wide opening at the top, it&#;s very easy to clean, and the rubber ring inside of the lid can be removed for more adequate scrubbing to prevent mold accumulation. The accompanying handle makes it easy to carry around, and the premium food-grade stainless steel is rust-resistant.

Cleaning is a bit tricky. Washing the interior of the lid does require a sponge and some detailing by hand.

It's imperative to strictly adhere to the instructions for use, such as warming the interior with hot water for five to 10 minutes before adding any food that will be eaten later. We found that when we did, this kept hot food warm for five hours, even if it was opened every hour to test the temperature. It also kept cold beverages cool for seven hours, with ice cubes still visible at the end of the test.

Kids will adore the array of colors and character themes the Funtainer comes in. More importantly, a wide brim makes eating less challenging for little ones getting the hang of using utensils, and the stay-cool exterior won't harm their hands. Best of all, it's lightweight and compact enough for little hands to open and carry in their lunch box .

One thing to note: When it was run through the dishwasher there was food left caught in the lid and gasket, so you're better off washing it by hand.

Our tester liked the double-wall insulation and its compact size since adults can also use this for hot sides and drinks. But our reviewer focused on one of the most important aspects of a thermos: How long will it keep your food hot? Thermos claims that this keeps food warm for up to five hours, which it fell a bit short of in tests (we don't think that's a deal-breaker, though!). Mac and cheese only stayed hot for two hours, was still warm enough to eat at four hours, and needed to be microwaved (in a separate container) by hour five. When it came to cold liquids, it did live up to its nine-hour claim, with ice cubes still present after a full workday.

Ergonomically designed with kids in mind, the BPA-free Foogo is a favorite with parents, as its cool-to-the-touch 10-ounce body is the perfect size for a kid's lunch . The base and lid are made of rubber to make the jar easier to grip for smaller hands, too. Other perks include cost efficiency and a tough body resistant to scratches.

To test durability, we intentionally dropped the thermos and noted that it wasn't damaged in any way. Also, its wide mouth is useful for eating straight from the container, and when you need to quickly hand-wash it. Each component of this model can safely be washed in the top rack of your dishwasher, allowing easy upkeep for busy professionals on the go.

Thermos King states this thermos can keep liquids hot for nine hours and cold for up to 14 hours, so we put that claim to the test in our kitchen. We filled all 16 ounces with freshly made mac and cheese and opened the thermos every hour to see if the temperature remained consistent. It wasn't until the ninth hour that the temperature dipped. Additionally, it excelled at keeping water ice-cold: We still observed ice cubes after 14 hours in the thermos.

Thanks to an uber-tight seal, the Thermos King is touted as having maximum temperature retention for hot and cold food&#;and it stood up to those claims to earn our best overall spot. That's all thanks to the insulation from two walls of stainless steel. The fact that the lid is designed to be used as a bowl in tandem with the included standard-sized stainless steel folding spoon adds to the convenience and overall value.

Final Verdict

For a decently sized thermos that will keep foods, such as soup, stew, and pasta, warm for up to an impressive nine hours, look no further than the Thermos Stainless Steel King Food Jar. Kids, on the other hand, will love the fun colors and cool touch of the wide-mouthed Thermos Foogo Vacuum Insulated Stainless Steel Food Jar.

How We Tested Hot Food Thermoses

Based on extensive online research of available models, we chose a collection of hot food thermoses to purchase and send to our experienced home testers, who filled, sealed, and even intentionally dropped each container to test its effectiveness and durability. We took the temperature of hot food and the measure of the container&#;s size and shape for packability. We also rated each thermos on design, performance, functionality, and overall value.

We Observed

• How well the thermoses sealed: Before adding any food to a thermos, we filled it to capacity with water to check for leaks. Each thermos was sealed tightly and then tilted on its side onto a kitchen towel. It sat for five minutes before we checked for wet spots on the fabric.
• How hot (and cold) food stayed in the thermoses: A thermos was filled to the brim with a variety of hot foods, from soup to stew to mac and cheese. We checked the contents&#; initial temperature using an instant read thermometer, sealed the thermos tightly, and checked hourly for five hours to see when and how much the temperature dropped. We tested in the same way with iced beverages.
• How durable the thermoses were&#;even when dropped: We intentionally let every thermos fall from counter height to the floor while empty. Any body or lid damage was noted, including scratches and dings. We looked beyond the cosmetics for bent edges that prevented a lid from being tightened or an uneven bottom that kept the thermos from sitting flat.
  

We Rated

• Design and functionality: In terms of design, we considered body and lid materials, including whether they are user- and environment-friendly. How smoothly the lid screwed on and off was noted, along with whether the overall size, shape, and mouth opening made a thermos easy or difficult to fill, pack in a lunch bag, and clean. Bonus accessories like a spoon or insulating bag were considered, too.
• Performance: Beyond testing each thermos&#;s seal and ability to hold heat, we looked at other characteristics that made it perform better or worse than its competitors. When filling a thermos, we noted how easy it was to pour in a soup or stew. We not only checked for leaks but also recorded how badly or how well a thermos held its contents when flipped on its side.
• Value: After testing was complete, we revealed the price of each product to decide whether the thermos was a good value for its price, overpriced, or underpriced based on its performance.
  

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What to Look for in a Hot Food Thermos

Design

Food thermoses tend to be shorter and wider than drink thermoses. This is so you're able to pour contents, such as soup, into the flask&#;and eat out of it as well. Most have a twist-on lid, which helps prevent spills. Some thermoses will have additional seals or interior lids for further spill protection. 

The Spruce Eats / Suzie Dundas

Material

Stainless steel is the preferred material for thermoses. It won&#;t absorb color or scent from hot food that is stored in it for a long time and cleans easily. An inner flask is surrounded by a partial vacuum that won't conduct heat away from the flask, keeping the contents warm. An outer case, usually also steel, allows you to hold the container without condensation or burning your hand. The outer case may also be treated with a coating or surrounded by plastic. 

Even inexpensive models tend to be vacuum-seal stainless steel. However, the vacuum-seal method can be used with other materials like glass and plastic. Because they are not as durable as stainless steel, they are not recommended. 

Capacity

The size of a thermos largely depends on what you want to use it for. Most thermoses for adults tend to be around 16 ounces, and ones for children are a little smaller. The food will cool down faster if there is excess air in the thermos, so if you want your food to stay hot for several hours, you'll want to use the best size thermos for the amount of food you're putting in it. If you just want the option for a small soup bowl at lunch, you can go with 10 ounces or smaller. If you want to carry a hearty meal in your flask, 16 ounces is likely the better fit. 

Price

Thermoses tend to be fairly affordable, and you can typically expect to get a long-lasting one for somewhere between $15 and $30. The more expensive models may boast more fun colors, an extra-strong seal in the lid, or certain accessories, but they all will use the same stainless steel vacuum-sealed design. 

The Spruce Eats / Suzie Dundas

Ease of Cleaning

A stainless steel thermos is relatively easy to clean up. Because it won&#;t absorb scent or color from your food, you can treat it like you would any other dirty dish and wash it with soap and water. Many thermoses are dishwasher safe but not all. This isn&#;t because of the material but the construction: Dishwashers may mess up the vacuum seal. Interior lids and other accessories, like collapsible forks, may also require additional hand-washing to get rid of built-up grime. 

Hiking and Camping Thermos Tips

Hot food thermoses are handy to have around any time, but they can be necessities on hiking and camping adventures. We talked to Kate Bast, nature and forest therapy guide and founder of Shinrin-Yoku Madison, who offered some tips for taking thermoses beyond the classroom or office.

• Hydration: On hot days, Bast recommends hikers bring at least twice as much water as they think they need, even if that means bringing two or more thermoses. She also suggests adding hydration packets to water or filling a thermos with fruits that have high water content, such as watermelon, which can be frozen the night before to maximize coolness. On chilly days, Bast carries one thermos with room-temperature water and another containing a hot or warm beverage.
• Sustenance: "Think about the sustenance you'll need to match your exertion levels and what will be most digestible," Bast says. Her go-to hot beverages include apple or pear slices steeped in cider with maple syrup, white pine tea, and spruce needle tea. For hot food, Bast suggests bone broth, veggie broth, or other soups that you can add proteins to. For something heartier, she suggests a casserole or stew.
• Snow Days: "If you happen to be in a snowy situation after you've consumed your hot thermos items, fill it with clean snow. The residual warmth will help melt it ensuring you have more water at the ready."
• Quantity: "Always have another thermos or two [in the car] of what you brought into nature with you," Bast says. "You'll be glad to have it and be more likely to consume what you need on the trail knowing there's more waiting at the trail's end." 

FAQs

Is the brand Thermos trademarked?

Thermos is synonymous with the product it produces&#;thermoses. But unlike Kleenex and Tupperware, Thermos has not been able to maintain the trademark on the term "thermos." The Thermos company, first started in Germany, was the first to figure out how to take industrial vacuum-sealing techniques and apply them for home use. Today, Thermos makes a wide range of insulated containers, from beverage flasks and mugs to coolers and lunch boxes. 

How do I choose what type of thermos to get?

Before purchasing a thermos, narrow down a preferred size and what it will most frequently be used for. This could range from beverages, like coffee and tea, to meals that need to survive a long day and transport to after-school clubs and sports practices. Perhaps durability is the top concern for long outdoor treks. Check customer reviews to read common raves and complaints about each manufacturer&#;s products.

How does a thermos work?

Using a stainless steel vacuum-sealed design, an inner flask maintains contents at a consistent temperature, since warm or cold air cannot easily penetrate or escape from the flask. The outer material, usually made of hard plastic or steel, allows the thermos to be comfortably held and transported without liquid leaking out.

How long do thermoses keep contents hot or cold?

High-quality thermal flasks can keep food and beverages warm for as long as a full 24 hours. More economical brands will typically retain the original temperature of contents for four to six hours.

Why Trust The Spruce Eats?

Rachel Werner teaches culinary writing through Hugo House and has been selecting food-based businesses for editorial coverage for over six years. Her product and restaurant reviews, food styling, and photography have appeared in a variety of regional and national publications including Fabulous Wisconsin, BRAVA, and Hobby Farms Magazine. For this roundup, she interviewed Kate Bast, nature and forest therapy guide and founder of Shinrin-Yoku Madison.

Abigail Abesamis Demarest is a freelance journalist who specializes in food and drink content. She tested three of the thermoses in this roundup to see how well they kept food warm, if they were leakproof, and how durable they were.

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