You want to take last night's leftovers for lunch and you want it to stay hot until you're ready to eat it. You might look at that insulated lunch bag and wonder – will it keep my food hot just like how it keeps it cold? The answer is yes! It actually will.
Insulated lunch bags can keep food hot by using a foam layer to prevent heat from dissipating. Many insulated lunch bags also have an inner lining of foil that’s heat reflective. This keeps heat trapped inside the lunch bag and helps your food stay warm for longer. To keep food hot for longer wrap in aluminum foil or add extra thermal mass like a hot water bottle.
It's important to note that on their own, insulated lunch bags can’t actively heat your food for you. There is no heating element in there that is going to keep your food hot like an over would on a low setting.
However, insulated lunch bags can keep your food warm for a long time, especially if you follow the suggestions in our article. They are actually one of the best ways to keep food warm without electricity, which is great if you're on the go and don't have access to a microwave or oven to heat things up.
How Do Insulated Lunch Bags Keep Food Hot?
When you have an item that is hot (like last night's leftovers) the heat wants to escape into the outside air to create an equilibrium of temperature. To keep your food hot you need to try to stop this transfer of heat and this is exactly what insulated lunch bags do to keep things hot.
Insulated lunch bags have multiple layers. While most don’t have vacuum insulation (which is the best insulation for keeping food hot or cold) their foam layer can prevent heat from leaving the lunch bag.
Insulated lunch bags contain foam, which is filled with thousands of tiny pockets of air. Heat struggles to move through this foam/air combination and as a result it gets stuck inside your lunch bag.
This is the same mechanism that usually keeps heat out of your lunch box and keeps things cold, but here we are using it in reverse. You can even use coolers to keep food hot as well and the better the cooler the longer your food will stay hot.
The better your lunch bag and the thicker your foam layer – the more likely you will have warm food after several hours. The Yeti DayTrip is an expensive (but absolutely amazing) lunch bag and it can keep food hot better than any other lunch bag I know of.
Cheap disposable insulated bags might use a layer of plastic, paper or cardboard instead of foam to insulate against heat loss. This works in the same way but foam tends to work better and will keep food hot for longer.
Ideally, you'll also want to ensure your lunch bag has a reflective coating to keep heat radiation.
Heat can escape through 3 main mechanisms – convection, conduction and radiation.
The foam, plastic or paper layer stops convection and conduction but the reflective layer is what stops heat radiation from escaping.
This is why people wrap food like takeout in aluminum foil to keep it hot for delivery. Aluminum is a great reflector of heat radiation – reflecting back about 97% of all heat radiation.
A combination of a foam insulator plus reflective radiation helps to stop heat escaping out of the insulated lunch bag, and if it can't escape from your food then your food will stay warm.
It isn't perfect at doing this so over time your food will begin to cool down to room temperature.
While your insulated lunch bag can’t actively heat cold food for you, it can be used effectively to slow down the cooling process and keep hot foods hot.
How Long Will Food Stay Hot In An Insulated Lunch Bag?
Like keeping foods cold, how long insulated lunch bags can keep food hot can vary. Various factors impact how long your food will stay hot:
- The outside temperature
- The quality of your lunch bag – we recommend the Yeti DayTrip as one of our favorites
- The type of food and how much thermal mass it has
- The temperature of your food when it goes in the bag
The general rule of thumb is that your lunch bag will keep your food hot for 2-3 hours without an additional heat source.
However, different foods may stay warm longer. For example, soup will stay warm a lot longer than rice which will stay warm a lot longer than bread.
How To Keep Food Toasty Warm In Your Insulated Lunch Bag
So how exactly can you keep your food warm in your lunch bag? Well there are quite a few good ideas that will keep food hot for hours at a time.
Pre-heat your food extra hot before putting it in the lunch bag
It’s important to remember that your food is going to cool down somewhat in your insulated lunch bag.
If you make it the perfect eating temperature when you put it in the bag, it’ll be lukewarm or cold when you take it out several hours later.
Instead heat your food as hot as possible (without ruining it) before putting it into your lunch bag. That way, it’ll take longer to cool down and will be more likely to be the perfect temperature when you’re ready to eat!
My daughter loves taking leftover pasta to school for lunch (it beats sandwiches) and I always remind her in the morning…make it EXTRA hot before you put it in your thermos or insulated lunch bag. To her credit this is one of the few times she actually listens to her dad.
Don’t mix hot and cold food so just use the lunch bag for hot food
This might seem like a no-brainer, but cold drinks and hot food in your lunch bag mean both will be lukewarm when you’re ready for them!
Heat will transfer from your hot food into your cold food or drinks warming them up while cooling down the hot food.
Instead of trying to take them in the same lunch bag, consider using a thermos for your drinks and keeping your hot food separate or visa versa.
Even using 2 different insulated lunch bags and keeping them from touching is a good idea if you want to keep your food hot for longer.
Minimize empty air space
The less air space you have in your insulated lunch bag, the slower your food will cool down. I don't know the complete science behind it but if you have more empty air space in your lunch bag then food will naturally cool down a fair bit faster.
You can pack your bag tightly with food or use things like tea towels, paper towels and aluminum foil to fill the gaps.
These materials are all good insulators in their own right and extra insulation will minimize air space AND it means your food will be hot for longer!
Wrapping your food containers in a towel will help keep the heat contained for longer.
You can also try squishing your lunch bag to remove the air and this can help.
Add thermal mass (like a hot water bottle)
A hot water bottle can be a great option if you don’t have a thermos to keep good hot.
Plastic bottles full of hot water, or single walled stainless steel bottles with boiling water inside can help keep food hot for longer than just the insulated lunch bag on its own.
Interestingly water has some of the highest thermal mass (or more accurately specific heat capacity) when compared to other substances. As you can see in the table below nothing easily available even comes close.
This means it takes A LOT of energy to warm up or cool down water. So by placing a hot water bottle in your insulated lunch box it adds a lot of thermal energy that can be transferred into your food over time keeping it hot.
Another good option is a heat pack, like a hand warmer or even a DIY rice sock. You can heat these and put them in with your food as well. However, they usually don’t stay warm as long as the hot water will.
Keep food in an airtight container or aluminum foil to stop steam from escaping.
Airtight Tupperware containers are a great way to keep heat contained. They trap the steam with the food, meaning no heat is getting out!
Steam actually takes with it A LOT of heat (remember the high heat capacity of water we mentioned before) and losing steam coolers your food very quickly.
If you wrap your airtight container in aluminum foil, you can stop steam from escaping but also provide an additional reflective layer of insulation.
Wrap food or lunch bag in a towel or sweater to further insulate it.
Insulation is key when it comes to keeping your food warm. Insulated lunch bags have foam layers, but you can provide extra insulation with a sweater or towel.
Consider wrapping your food containers in tea towels and wrapping your whole lunch bag in a sweater to help keep it warm. The tea towels will help fill up space in your lunch bag so the air can’t circulate!
Invest in a high-quality lunch bag
The best way to make sure your insulated lunch bag will keep food warm, of course, is to make sure you have a high-quality lunch bag. While there are plenty of insulated lunch bags, not every bag is created equal.
One of our favorites is the YETI Daytrip Lunch Bag. It has excellent insulation and is easy to use and clean.
While an insulated lunch bag won’t heat your food for you, it’ll keep your food warm if you prep it first. Make sure your food is as hot as possible, pack your food tightly, and don’t mix cold and warm food choices.