Wouldn't Thicker Insulated Bags Always Be Better? The Real Determinant Is This Invisible Parameter

Many people have a hunch when it comes to buying thermal bags: thicker ones are definitely better than thinner ones. That's half true, but the other half might cost you money.
The takeaway: It's not the thickness of the insulating bag that really determines its effectiveness, but a parameter that the naked eye can't see --heat reflectivity, or the ability of the inner silver material to "bounce" heat.
Let's look at a basic principle: How does heat radiate?
Heat transfer at high and low temperatures occurs in three ways.
The first is thermal conduction, which gradually "crawls" along the material itself. For example, if you touch an iron bar, the end you're holding heats up quickly-this is conduction.
The second is thermal convection, the pocket of air ``flow ''. As cold air sinks and warm air rises, temperatures become more uniform and insulation can be affected as a result of this cycle.
The third is thermal radiation, which is the easiest to ignore. Anything with temperature emits heat, just as the sun does not need to be touched to cool; it simply passes through the air in infrared light.
Ordinary thermal bags rely heavily on their thickness to capture heat conduction and convection, but they have little effect on radiation. The real difference is their ability to intercept radiation.
So what are "invisible parameters"?
This is the reflectivity of the inner aluminum foil layer.
This thin layer of aluminum foil reflects over 95% of heat radiation. In other words, the heat from your cake is reflected back by the silver film before it even has a chance to escape.
That's why some insulation bags, which are only 3mm thick, perform better than regular 8mm thick ones. This is not because they are thicker, but because of the high reflectivity the aluminum foil layer of their walls.
Instead, some bags are very thick and use EPE foam or non-woven fabric, but they lack a reflective lining. It is thick enough to block some conduction and convection, but radiation was still completely blocked. The upshot: It feels fine when you first touch it, but the temperature drops quickly after half an hour.
So, is aluminum foil enough? Not at all.
High reflectivity is only a requirement, and it also needs to be combined with a sealed layer of air.
If the space between the aluminum foil layer and the outer wall is open and airy, cold air will still enter, rendering even the highest reflectivity useless once the heat convection starts. So, a truly good bag has this structure: a piercing layer on the outside, an airtight layer in the middle, and highly reflective high-reflectivity aluminum foil on the inside. Each of these three layers has its own role, and the absence of one reduces its effectiveness.
How to judge the quality of a thermal bag Here's an easy method to do it:
Turn it over and check the inside wall before you buy. If it is matte silver and has a distinct metallic feel, touch, and reflectivity is generally good. If the inside is made of white nonwoven fabric or something similar to paper, the insulation capacity is limited regardless of the thickness of the bag.
Also, don't be misled by "6 hours of insulation" or "12 hours of insulation." These figures are usually measured in a laboratory environment, without the need to open the bag. In fact, every time the bag is opened, cold air enters, reducing the insulation time by at least half.
Finally: when choosing a thermal bag, don't feel the thickness, look at the inside.
Thickness determines the lower limit and reflectivity determines the upper limit. Investment in the unseen is the true consumption of wisdom.