# Does food stay hotter if you keep air in the bag?

If I am taking takeout home in a plastic bag, is it better to remove all air from the bag when sealing it so no heat is lost to the surrounding air or keep air in the bag acting effectively like insulation? Let's assume the air is room temp/colder when I seal the bag.

• By removing air you mean that the plastic is in direct contact with the food? Commented Oct 2, 2023 at 15:58
• Not sure but I'd imagine wrapping the bag tightly around the hot item minimizes the surface area that can exchange heat with the environment. But you'd likely need a more in-depth analysis here, maybe someone is actually willing to crunch those numbers. Commented Oct 2, 2023 at 19:58
• @JonCuster yeah the plastic is in contact or very close (like the layer of air is a lot smaller) Commented Oct 2, 2023 at 20:51
• Might Seasoned Advice be better suited for this type of question? They have quite a few food-transport questions that are very much like this one Commented Oct 6, 2023 at 2:00

Air can act as an insulator, but it can conduct heat through convection,so to make it or use it as insulator you have to stop the convection. That can be done through making layers with added insulator in partitions.

But the air as it is in plastic bag surely, going to convect the heat from plastic bag to colder or from hotter thing to colder plastic bag, now the thing that is insulating here is just your plastic bag.

So to make insulator out of air, trap it so it cannot flow.

Air is a fluid, thus having it in a bag has some properties to act as expansion in a confined volume. Laws of thermodynamics the food heats the air as it escapes from it's source. If you have a well insulated bag, the quantity of air in it is irrelevant. Ironically air can be insulation (Bubble wrap)

This is the formula for the rate of thermal conductivity between all materials:

$$\frac{Q}{t} = \frac{kAΔT}{d}$$

where $$\frac{Q}{t}$$ is the number of joules of heat transferred through the material every second, k is the thermal conductivity constant for the material, ΔT is the temperature difference between the two materials and d is distance in which the heat travels.

We know that the k value of air at atmospheric pressure is 0.025 W/(m·K) and that the thermal conductivity of polyethylene, the most common material used in plastic bags, is 0.33 W/(m·K) at 23°C. Of course, these are all estimations.

If you assume that total A, ΔT and d are all constant for both calculations, you will probably find that leaving the air inside will be more conducive in stopping the transfer of heat, as both air and plastic will act as insulators for your food.

If you want to find the specific calculations for this problem, you can plug in the specific measurements of the factors into the above equation.

However, one thing I would consider is that the rate of heat transfer for plastic and air is different, as shown by their k values. Therefore, when calculating the joules transferred when air is kept in, you would probably need to calculate the number of joules transferred twice, once between the food and the air and a second time between the air and the plastic.

Hope this helps :)

• Of course, using air as an insulator isn't the most effective way to retain heat in the grand scheme of things. If you really need to keep something warm, I would recommend using some sort of foam and lining the insides of it with a reflective material, such as silver or aluminum, to reflect heat radiation back towards the food. Commented Oct 8, 2023 at 22:12
• Perhaps the Dow voter could cite rationale for the downvote Commented Oct 12, 2023 at 6:46