Over the spring and summer months, many aspects of logistics have found themselves tested by sudden heat spikes and rising temperatures, and the cardboard boxes which form the backbone of deliveries are no exception.

Whilst custom cardboard boxes can be designed specifically to better manage the heat or protect more fragile goods from it, it is important to know how robust standard cardboard can be against the heat.

Cardboard is remarkably good at withstanding hot weather and maintaining a relatively consistent internal temperature, to the point that cats often hide in boxes during heatwaves.

Why is this the case? How much heat can cardboard boxes take? And which aspects of hot weather truly do the damage?

Why Can Cardboard Boxes Withstand Hot Weather?

For their weight and cost, cardboard boxes are remarkably good insulators, with many of the aspects that make them strong, robust and capable of withstanding the harsh conditions also making them good at handling relatively high and low temperatures.

What makes it so effective is its layers of corrugation, which create pockets of air that slow down the transfer of heat from one side of the cardboard to the next.

It is even capable of being repurposed as makeshift insulation, with some people flattening and cutting up cardboard boxes to place over windows to keep the hot weather out.

Cardboard is poor at conducting heat, which makes it remarkably good at keeping products at a relatively stable temperature in even particularly hot weather, something that is essential for services such as fresh food delivery.

How Much Heat Can Cardboard Boxes Take?

Cardboard operates best at between zero and 25 degrees Celsius, although it can go above and below this for relatively short periods.

The ignition point of cardboard, where it actively catches fire, is closer to 200 degrees Celsius, although a combination of direct sunlight, a particularly hot day and a focusing lens can cause that temperature to occur more often than you might expect.

Above a certain temperature, however, cardboard boxes can start to warp, become more brittle and cause dyes and inks on the surface to fade following exposure to direct sunlight.

What Are The Biggest Heat-Related Risks For Cardboard?

During a heatwave, it is not just the temperature that can be a cause for concern but all of the other aspects of hot weather that can have an effect in themselves.

A tropical heatwave feels different to a desert heatwave, and cardboard is more vulnerable to some hot weather effects than others.

Humidity

One of the biggest factors in how people manage heatwaves is humidity, or the relative concentration of water vapour in the air.

High humidity is typically bad for people because it makes it harder for the body to cool down by sweating, and it can be similarly destructive for cardboard boxes as well.

The wood fibres used to make corrugated cardboard naturally trap and absorb moisture, which can cause it to expand, get soggy and become far less durable.

Effectively, cardboard in humid conditions begins to act like cardboard that has been soaked in rainwater, and this can dramatically reduce the compressive strength and stacking ability of the cardboard boxes.

Lamination films and treatments can help to reduce this effect and ensure that humidity within the cardboard is kept at controllable levels.

Dry Heat

Does this mean that dry heatwaves are less damaging to cardboard boxes? Yes, but there are still ways in which dry temperatures can cause problems.

When cardboard is manufactured, it maintains a certain level of moisture content. Too much causes it to collapse, but the right amount allows it to flex, to adapt to strains and to withstand bumps and scrapes without breaking.

In extreme temperatures such as heat and cold, the moisture drains away, which can in turn create a brittle, rigid material that can easily tear and crack, further compromising its structural integrity in the process.

Direct Sunlight

Keeping cardboard out in the sun can cause not only fading of the colours and inks, particularly if you are using organic inks more prone to photodegradation, but it can also damage the box itself.

As well as this, direct sunlight and UV rays can damage the adhesives used to put certain types of boxes together, although this typically takes a considerable amount of time.

In most situations, direct sunlight only causes superficial damage, although in the case of products where the box is part of the appeal for collectors, this is a consideration to factor in when making and transporting certain products.