If you’ve ever touched a hot stove, used a hair dryer, or felt the heat of a bonfire on a cold night, you have an intuitive sense of heat transfer.
Adding heat where it is needed and removing heat where it is unwanted are two of the most energy consuming processes affecting our energy use. They both deserve your attention and planning when designing and building a container structure.
The average residence uses the majority of their electricity for heating and cooling (Source)
In this article, we’re going to discuss the physics behind heat transfer and its important implications for container buildings.
While heat transfer itself is a bit technical, we believe it is crucial for you to have a sound understanding of how it works, as it directly impacts decisions that cost thousands of dollars and affect your overall livability and comfort in your container home. We’ve cut through the non-relevant details and provided clear examples to help you navigate some of these important decisions.
What exactly is “heat”?
Before you understand heat transfer, you need to understand heat. Heat is a form of energy associated with thermal or temperature changes, and it always flows from the hotter object to the cooler one, unless acted on by an outside force (like an air conditioner).
It’s important to understand that you cannot add coldness to something; you can only take away heat. Cold, therefore, is the absence of heat. An air conditioner isn’t adding coolness to your house, it is taking the heat from your house and moving it outside (even though it is hotter outside than in). It’s similar to how you cannot add darkness to a room, you can only take away light.
The three types of heat transfer
Heat energy can be transferred in three primary ways, each of which is important to understand in the context of shipping container structures.
Conduction, the simplest form of heat transfer, is how heat moves within a single solid object and also between solid objects in direct contact.
When you touch a hot stove, the heat from the burner is transferred to your hand. If you hold a metal spoon in your hand and touch the spoon to the burner, your hand will eventually get hot because the heat conducts through the spoon. However, if you wear an oven mitt (insulation), much less of that heat energy will make it to your hand.
In a container, the exterior corrugated metal is usually the hottest (in the summer) or coldest (during the winter) part of the building, as it is closest to the outside environment. Anything touching this outer layer of metal, like the insulation, wall studs, windows, door frames, etc., will also have this heat transmitted to it via conduction. Conduction is where the concept of thermal bridging comes into play.
Thermal bridging takes place when a section of a wall system has a higher conductivity (or, lower insulative capacity) than the surrounding material, creating a ‘path of least resistance’ for conductive heat flow. This, in turn, lowers the overall insulative capacity of the entire wall system.
For instance, if you use metal wall studs that attach to the inside of the container and have direct metal-to-metal contact, heat will conduct from the exterior metal into the studs. With wooden studs, the same thing occurs, except that wood is much less effective as a conductor. Therefore, less heat will migrate into the interior of your building. Another option is placing a more insulative material between the container’s corrugated metal and the metal wall studs. If you’re interested in knowing how well common building materials insulate, this is a great resource.
The key takeaway is that you want to separate the interior living space of your container from the exterior metal with as much insulation as possible. But remember that the insulation only works when it is between the inside wall and the exterior corrugated metal. Any material that thermally “bridges” this gap through or around the insulation is reducing the effect of your insulation, and you should take steps to minimize this as much as possible. Otherwise, your expensive insulation is somewhat wasted.
The studs that are touching the container’s metal exterior can “bridge” the heat right into the interior drywall by providing a path of conduction through the insulation
Convection is the second mode of heat transfer and can be generally defined as the transfer of heat between a solid and a fluid. Fluid is an engineering term that represents both liquids (like water) and gases (like air). There are actually two types of convection, with only a slight difference between them:
- Natural Convection – A hot object exposed to a fluid naturally heats it up. Due to changes in density (and therefore buoyancy) that correlate with temperature increases, the hot fluid will naturally move away from the hot object in a convective current, with colder fluid taking its place. This leads to the common saying “heat rises”. If you’ve ever held your hand high above a stove burner and noticed the air is warmer than elsewhere in the room, you’ve experience natural convection.
- Forced Convection – A hot object that has a fluid moved over it by an external force, like a fan or pump. If you’ve ever used a hairdryer, this is an example of forced convection, as an electric fan is forcing air over an electric heating element.
In the summer, the heat from your container roof increases the temperature of the air surrounding it via natural convection (We’ll explain later why the roof gets hot!). This hot air rises in a column above the container, continually being replaced by the slightly cooler ambient air in the atmosphere.
In traditional construction with peaked roofs, placing ridge vents at the top of the roof (and eave vents at the bottom) is a way to allow hot air to escape your attic via natural convection. Ideally, the temperature of your attic would be no higher than the ambient temperature outside the house, but without this ventilation, your attic turns into an oven.
Natural convection heat flow through roof vents in traditional construction (Source)
However, for container buildings, this isn’t especially practical. Unless you add a peaked roof above the container, a container building won’t have an attic or airspace above the insulation in the roof.
Radiation is the last of the three modes of heat transfer and is the most difficult to understand. The word radiation usually has a very negative connotation, so let’s first start by understanding what radiation is at a high level as well as the discussing the types of radiation that exist.
In simple terms, radiation is energy that is transmitted through matter via invisible waves. It can be broken into two types:
- Ionizing radiation is the ‘bad’ radiation you hear about, and can cause damage to living tissue. It’s the reason you wear a lead vest when getting an X-ray, and why humans can’t visit the site of the Chernobyl nuclear reactor accident in Ukraine.
- Non-ionizing radiation is all other radiation on the electromagnetic spectrum. It includes visible light, radio waves, and many others. The thermal radiation we’re discussing here is a type of non-ionizing radiation.
Electromagnetic Spectrum including thermal energy wavelengths (Source)
The majority of thermal radiation we encounter in everyday life is transmitted via the visible and infrared portions of the spectrum (The reasons why are beyond the scope of this article).
If you’ve stood around a bonfire and felt warm even though the air around you was cold, you’ve experienced thermal radiation. The burning wood was radiating thermal energy that was absorbed by your body, increasing the temperature of your skin.
Thermal radiation from the fire heats your hands to the side of the fire, even though the air is cold (Source)
Or think about a thermal camera, that detects the thermal radiation emitted by various objects.
Thermal image showing the temperature of hands compared to the surrounding air (Source)
Thermal radiation is emitted from anything that has a temperature above absolute zero (the lowest temperature theoretically possible at -273.15°C or -459.67°F). In fact, your body is radiating a tiny amount of heat into the room around you now (This is in addition to the conduction into the chair you’re sitting in, and the convection into the room’s air).
If you are in a room with a glass of cold water, both you and the water are radiating heat energy toward each other. But because your body temperature is much warmer than the water, you are sending out much more energy than it is. From your perspective, you’re sending out a lot of heat energy, but only receiving a little, so your body temperature decreases. From the water’s perspective, it is sending out only a little thermal radiation, while receiving a lot from you, so it’s temperature increases.
The most important source of thermal radiation for shipping container buildings is the sun. The sun emits radiation at many different frequencies, much of which is blocked by our atmosphere. What’s left includes thermal energy, among others.
When a container is placed outside in the sun, the ambient air heats the exterior corrugated metal to the ambient temperature via convection. However, when the sun hits this outside metal of the container, thermal radiation causes the metal’s temperature to actually increase above the ambient air temperature. This increased heat energy is then transferred via conduction into the interior of the container through the studs, insulation, and other components of the wall. You cannot eliminate this conduction, but you can reduce it by using materials with low conductivity (and high insulating capacity), as we discussed in the section on conduction.
The best way to minimize heat grain from thermal radiation is not to insulate against it like with conduction and convection, but to block it (for example, with shade) or reflect it (with reflective coatings or materials) back into the atmosphere. However, if you’re in a cold environment, you might actually want to absorb as much of this radiation as possible so as to reduce your heating bills.
It’s important to note that thermal radiation isn’t only about making your container hotter. As mentioned previously, everything with a temperature above absolute zero is radiating heat, including your container. At night when the sun has set, the container continues to radiate energy out, just as it did in the day. But now that the sun is gone, much less heat is radiating into the container.
Under certain conditions (direct line of sight from the container to the sky, with a clear sky free of clouds, etc.), a container at night can actually radiate more energy out than it is receiving in, which is called passive radiative cooling or sky cooling. If these conditions persist for more than a few minutes, the temperature of the container can actually drop below that ambient temperature.
In the past few years, researchers have been working on special materials that are great at both reflecting thermal radiation and emitting their own heat as thermal radiation. In experiments, they’ve actually been able to lower the temperature of a material below ambient temperature while in direct sunlight. With additional refinement and research, one day we may be able to use special coatings that dramatically reduce cooling loads and reduce the need for air conditioning.
Why heat transfer matters
Regardless of where you live, you want your shipping container building to be a comfortable space. That can include the way it looks, but more importantly, it means the way it feels with regard to temperature.
Unless you live in an extremely temperate climate where air conditioners and heaters are things you only see on television, you’re going to need to think about heating and cooling your container building. And anytime you try to make the inside of your container have a different temperature than the outside, you’re fighting a battle against physics and mother nature.
By understanding the principles of heat transfer, you can apply your efforts and resources to the areas with the most impact.
For instance, in a hot environment, insulating the bottom of your container will work to keep out the ambient air temperature. However, your expensive insulation is much more important for the top and sides of the container that are exposed to the thermal radiation from the sun.
Confused about any of the topics we’ve covered? Willing to share a technique you’ve used in the past to combat one of the types of heat transfer we mentioned? Let us know in the comments!