Why Container Homes Get Hot In Summer

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Living in a metal box in harsh summers can seem like a scary and highly inconvenient idea. In its original state, a shipping container can heat considerably more than any conventional non-insulated wooden envelope. 

An uninformed or improper conversion of shipping container homes can indeed lead to an uncomfortable temperature during the hot summers. Up to 144 degrees Celsius under direct sun. But the good news is, this increased temperature can be managed and made livable with a few tweaks. 

To understand the ways to do that, it is important to know two things. ‘How hot does it get?’ so that we know what we are fighting against and ‘How does it get hot?’ to pinpoint exactly what needs to be done to minimize the heating. These seem like simple questions; however, they are not. The answer varies drastically based on the location, site surroundings, exposure, and modifications made to the container. Let’s look at some different cases.

How Hot Does It Get?

Through research, we identified that an unmodified, uninsulated shipping container could:

  • heat up to 120 degrees Celsius at the peak temperature on the hottest day in Costa Rica.
  • The same shipping container in Florida can go up to 144 degrees Celsius under direct sun on a hot day. 
  • Whereas traveling in the middle of the Atlantic Ocean, it reaches 135 degrees Celsius, as observed by some engineers at Xerox. 

By analyzing various examples and cases, it has been noted that the inside temperature of the container can actually exceed 15 degrees or more than that of the exterior temperature.

In spite of these observations, shipping container homes are often advertised as compatible for all climates. This is because they can be MADE compatible for all climates due to the availability and knowledge of the active and passive strategies and products today.

However, it is important to realize that, even with all the available knowledge and techniques, the process of climate and temperature control highly depends on the location, orientation, climate zone, surroundings, and use of the container. An uninformed choice can either result in exorbitant cooling costs or, rather, unnecessary over-expenses.  

How Does It Get Hot?

Now that we know that it can get pretty hot; let’s dive into the next question. Some targeted information, research and understanding on ‘how does a shipping container get heated?’ can be efficiently used to identify the right ways to create as comfortable a shipping container home as any other.  It is all about introducing the suitable materials and techniques to the right places to make sure the heat transfer is hindered to a maximum extent. But how does the heat transfer occur, and why is it more powerful in a shipping container home compared to other homes?

Agents of heat transfer:

There are two major heating agents for any home.

  • The ambient air transfers heat to and from the container’s walls, bottom, and roof through convection. 
  •  The sun, which heats the container through its Thermal Radiation. Being a metal box, the high conductivity of the walls and roof leads to greater heating within the wall and, therefore, more heat absorption.

Below is a detailed explanation of each of these processes’ occurrence, which will aid the process of identifying the ways to create a comfortable microclimate.

Diagram

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Figure 1 – Modes of Heat Transfer- ResearchGate

Conduction 

This process highly depends on the material. We all know that ‘metal is a good conductor of heat’. A shipping container body is made of corrugated steel, which has a large amount of free moving delocalized electrons. When these are heated by the sun and ambient air, they gain more energy and vibrate more to bump into nearby particles. As this happens, they transfer some of their energy to neighbouring particles. The process is continued and the energy is passed from the hot part to the colder part of the metal, thus heating the whole wall and all the components attached to it. 

So why does the same process not heat a wooden exterior?

The ability to conduct heat is measured by thermal conductivity. The thermal conductivity of steel is 50.2 W/mK, while that of wood is between 0.12 and 0.04 W/mK.  Therefore, the steel wall of the container heats considerably more than a conventional wooden wall.

Can insulation eliminate the heat transfer through conduction?

The effectiveness of insulation depends on its R-value. But to maintain its performance, another aspect of heat transfer needs to be kept in check:

Thermal bridge

A picture containing diagram

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Figure 2 – Thermal Bridge-Discover Containers

When the heat from the outer wall is able to transfer through the insulated wall due to some connected components, the process is called thermal bridging. Thermal bridging occurs when the conductivity of one of the components is more than that of its surroundings. In this case, if the insulation is not done efficiently and fails to cover the wooden or steel studs that are in direct contact with the outer steel wall of the container, these studs act as a thermal bridge and carry the heat inside. Thus, reducing the performance of the insulation.

Thermal bridging can occur due to following reasons

  • Insulation does not cover the studs or other components in contact with the outer wall 
  • Corners or edges are improperly insulated.
  • Joints of the wall are improperly insulated.

Conduction can only occur if the molecules are physically connected. So, the heat in the wall and the potential thermal bridges is transferred to the space using the other two modes of heat transfer: Convection and Radiation.

Convection

Convection is the transfer of heat between a solid and a fluid (Liquid or gas/air). To understand the occurrence of this process in the heating of shipping containers, we need to understand a basic concept; Heat always transfers from high temperature to low temperatures. From the thermal conductivity of metal and information above it, we know that metal heats quickly and greatly and often become hotter than the ambient air. Through convection, the heat from the metal surface is then transferred into the ambient air raising the temperature of the microclimate.

Why doesn’t it release its heat outside the house?

It does, but the outside ambient air is generally hotter than the inside ambient air in summers. Therefore if the metal is in direct contact with the air inside, it needs to release more heat inside to reach the desired equilibrium. Thus, the rate of convection for the inside is greater.

Insulating the container from outside or inside or both hinders this process. Insulation materials are made of bad conductors of heat. Therefore, they do not get heated and thus discontinue this process of heat transfer through convection.

Radiation

Heat transfer when the bodies are not in direct physical contact with each other is called Heat radiation. The intensity of such energy flux depends upon the body’s temperature and the nature of its surface. Since the temperature of the steel wall is more than the ambient temperature, and other materials pertaining to its high conductivity, it radiates more heat into the surroundings.

All the objects around us radiate heat. May it be a human body or a tree, or an empty glass. All objects radiate heat towards other objects until it reaches an equilibrium. However, in this case, the major and most effective agent of heat radiation is the sun.

The roof and walls of the container absorb Radiation from the sun and often become hotter than the ambient air, thus increasing the temperature within. Then it transfers that heat into the surroundings, using Radiation as well as convection.

Unlike conduction and convection, insulation cannot hinder the absorption of radiated energy. However, Shading, Reflective paints & coverings can be used to reduce the ability of the roof and walls to absorb these radiations. 

Final Words

Even though all these processes work simultaneously, if we were to arrange them in their order of heating effect in a shipping container home; it would be 

Radiation -> Conduction -> Convection

This order indicates a way to prioritize the steps that must be taken, which can help to filter them based on the budget, availability, timeline, or any other limitations.

Note that with proper measures and modifications, the temperature within the container can be dropped by more than 30 degrees without an air conditioner. For a more detailed description of ways to keep the container home cool during summers, please visit this article on our website- “How to keep a container home cool.”

References

  1. Sciencing – https://sciencing.com/thermal-conductivity-definition-units-equation-example-13722758.html
  2. EDinformatics – https://www.edinformatics.com/math_science/why_metals_conduct.htm
  3. https://www.discovercontainers.com/essential-knowledge-about-heat-transfer-in-shipping-container-buildings/
  4. https://containerhomes.net/articles/how-hot-do-shipping-containers-get/
  5. https://a-american.com/how-hot-shipping-containers-get-florida-heat/
  6. https://epgna.com/how-hot-do-shipping-containers-get/
  7. https://www.sciencedirect.com/topics/engineering/radiation-heat-transfer