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Question: I put hot food in 1st & 2nd container and cold salad in top container My son said salad was a little warm? Is this only for all hot or all cold?
Document: I purchased the Zojirushi Classic Bento Vacuum Lunch Jar so that I could pack a hot lunch for my son that just started 5th grade. He's not too fond of the school lunches and the paper bag lunches are always room temperature by the time he has lunch in school. He LOVES the Bento. I pack a hot soup in the soup bowl, a hot meal in the hot container with the insulated lid and usually cold cut up fruits or yogurt in the top cold container. He said that the hot food stays HOT and the cold remains cold by the time he has lunch 5 hours later.
(it is not designed to) A salad or veggie sticks in the top container will not stay cold, it'll be lukewarm. (no big deal)If you pre-heat the vacuum jar with hot tap water and heat your food until it is too hot to eat - it will still be steaming hot after three to four hours when kept in the insulating bag.I gave it four out five stars because I feel the sealed soup container should be larger. (enough to hold an entire can of soup)Otherwise, it is great and even though the price of admission is a little steep, it has saved me a lot of money and I'm eating better too!
I'm so happy to have warm lunch meal without using Microwave during busy lunch time.It comes with three layers, soup bowl (bottom, sauce of pasta), main bowl (middle, pasta), side bowl (top, salad).All three containers are able to use microwave to warm up and soup bowl & main bowl are able to keep warm, and we can still put salad in side bowl (top). Amazing! But I feel side bowl (top container) gets warm up slightly by the heat of main bowl, so I'd prefer not to mixed up warm & cold meal in one lunch box.I normally heat up soup on kitchen stove and put only liquid in soup bowl and stuff in main bowl, because if you put whole soup in main bowl which is the biggest container among three, it will leak.
I am disappointed with this product for 2 main reason, which are the main reason for purchasing it... first, it doesn't keep the food hot/warm enough. It won't make you food go cold but it doesn't maintain your food near it original temperature when you prepared it. And 2nd, the middle container is only designed to hold dry foods like like. I tried to put in some soup or food with sauce, and it leaked and spilled all over. The only container that you can put in soup or any food that has sauce is the bottom one which is very small.
When my husbands gets home he always tell me that how his friends were interested in his lunch  and wanted to taste some. This item is good to keep food warm about 5hours and after 5hours you might have to use microwave. ( but food is not cold just needs to be heated little) containers are easy to wash and dry fast with towel. I like the bag came with it. There is small room on the top so you can put your snack on it. I put apple or little bag of nuts.
You can put soup in the center container, but it leaks a bit, although it still kept said soup very hot for several hours. Keeps cold things cold as well, although I have not tried to keep a hot container together with a cold one all morning.Still quite a bit to wash, but very handy and well designed.Recommended.
My 5 year old son recently started going to a child care facility that does not provide lunches and also does not heat up food. We struggled with looking for a thermos in our nearby stores, and thankfully we were able to find the Zojirushi Classic Bento on Amazon. It works perfectly in keeping his food warm.We heat up his food (pizza hut personal pan, rice, chicken, soup) in the  morning before school, and it's definately still warm during lunch. Some days, not all 3 containers are packed with food, so then my wife will put hot water in the bottom container.
I purchased the Zojirushi SL-XB20HG Classic Stainless-Steel Lunch Jar along with the Zojirushi Classic Bento Vacuum Lunch Jar and I am satisfied with both. As with other Zojirushi products expect top level Japanese quality. The parts are very well made and appear to be made to last a while. The jar keeps food warm from early morning until lunch very well, especially if you heat the inside of the jar with hot water before you put the containers in. It is good to note the top container is to be used for room temp foods as it is not insulated.
My son just started using this Classic Bento this past month, but we've already tested it with a hot meal (Stir Fry with an eggroll) and a cold meal (sushi with pickled cucumbers). He's looking forward to using it in the fall for his senior year of high school when he'll be able to take leftovers rather than purchase marginal food at the cafeteria or rely on a multitude of pb&j and ham sandwiches which can become boring week to week. He loves that the bag is easy to carry and allows an apple or other portable fruit to be carried in the top and the fact that it comes with those fabulous chopsticks, I love that it is totally reusable (less sandwich bags) and that it is saving our family money and providing my son with healthy vegetarian food at lunch.
That's really my only complaint.Not having to use the microwave to have warm food is a great convenience for those that are on the go or maybe work in construction. Also, you can have a large variety of meals instead of always having a form of soup from a thermos. For example, rice in the soup container, roasted veggies topped with some grilled chicken in the middle container, a salad in the top container. That's a balanced meal.
Answer: Insulation is primarily done by the outermost container.  You can mix hot and cold in the smaller containers, but they are not well insulated from each-other.