Smaller, Hotter, and Unfit

Why shipping containers make for hot homes

Jonathan Bean, Ph.D.
7 min readJul 13, 2023

The Arizona Department of Housing is offering three Sparkbox shipping container units to nonprofits who promise that they will only use the units to house unsheltered people. The structures had previously been used as part of a $1.2m Sparkbox Park demonstration project in Phoenix funded by the same agency.

Housing people is an unquestionably good goal. But if we want the people we house to be able to survive extended periods of heat in a region that is hot and getting hotter, any stand-alone structure the size of a shipping container is a spectacularly bad answer.

A modern steel sculpture in front of Sparkbox shipping container structures
Sparkbox Park (image: Steel + Spark/fair use)

Smaller, standalone buildings — and by that I mean structures on the spectrum from tiny house to McMansion — get hot faster than larger or grouped buildings such as a warehouse or an apartment building. The proportion of exterior area exposed to hot sun and hot air compared to the volume inside that building is much, much higher in a small building than in a large building. This inconvenient reality draws directly from basic laws of physics and heat transfer, and it’s why a french fry takes less time to cook than a whole potato.

When mechanical cooling systems fail, the temperature in a small structure will rise faster than in a large one — just as how a french fry would char if cooked as long as a whole potato.

Counterintuitively, adding too much insulation can actually increase the risk and duration of overheating. This happens because insulation slows down the rate of heat transfer. Yes, a well-insulated building will take longer to get hot, but once it is hot, it will take longer to cool off.

What makes all shipping containers prone to overheating?

While any small building is inherently more prone to overheating, there are constraints specific to all shipping containers that warrant additional concern.

  1. Geometry picks a losing fight with physics. Shipping containers are long, narrow tubes, with interior dimensions of just 7'-8" wide and 7'-9" high. These proportions guarantee a a structure with most exterior building skin exposure possible for the amount of habitable volume enclosed, which also means the maximum amount of heat gain. If this geometry were swimwear, it would be a string bikini. This has ripple effects: it requires a larger and more expensive cooling system. Heating and cooling cost more. And a long, thin tube will heat up faster than a cube-shaped building enclosing the same volume and useable square footage. (Compare the skin-intensive form of the Sparkbox wing, advertised to deliver “100 beds in 5 days.”)
  2. Thin walls and low ceilings limit solutions. The dimensions also mean there’s very little space to run systems or place insulation. A ceiling fan — if it fits — won’t perform as well as it could because it will be shoved close to the ceiling.
  3. Steel conducts heat like crazy. While steel is a fantastic material for shipping containers, it is a problematic material for buildings. Steel conducts heat extremely well, which is why it’s used for frying pans, but not mittens.

How do design features of the Sparkbox amplify overheating risk?

  1. Very large areas of glass. Too many windows compared to the interior volume will collect and trap a large amount of heat from the sun, similar to a greenhouse. Note how almost one entire wall of the SB1 model is glass. That’s too much!
  2. Limited possibilities for natural ventilation. Openings on only one side limit the potential for air movement, which can provide some cooling in a power failure — hopefully enough to keep the people inside from succumbing to heat stroke or worse.
  3. Dependence on air conditioning as the only means of cooling. Retrofitting ceiling fans would help, especially if they would operate in a power outage using power from the Sparkbox battery (but see below for an important caveat.)

Ok, but aren’t the trade-offs worth it because shipping containers reduce steel waste?

Any suggestion that reusing a 12,000 pound shipping containers reduces waste is dubious; there is a robust recycling system for shipping containers, and 90–100% of all newly produced structural steel in the US is made from recycled materials. Given the robust global market for steel, pulling a shipping container out of the recycling flow simply increases the amount of new steel that must be produced, with correspondingly higher carbon emissions. In comparison, wood, the material conventionally used for most residential structures, has a much lower carbon footprint.

Despite all the problems, why do repurposed shipping containers such as the Sparkbox and tiny homes remain attractive as affordable housing solutions?

Or, put another way, doesn’t everyone think shipping containers are cool? This is the crux of the question, and paying attention to the aesthetic design of the Sparkbox units can move us towards a sensible answer — and towards a solution that increases the value of the units for the Arizona Department of Housing. The modernist design of the Sparkbox units — all Corten steel and clean lines — has social and economic value to an upper-middle-class consumer. The people touring the Sparkbox Park demonstration park — or reposting images of well-designed tiny shipping container homes on Instagram—are likely to be politically engaged and have an opinion about homelessness, houselessness, or the unhoused. The debates over what we call it are an indication of just how political this problem is, which is an important reason technical solutions — especially ones that also appeal to aesthetic sensibilities—get traction. Focus on technical innovation lets us overlook inconvenient and change-resistant aspects deeply embedded in culture. For example, the supposedly “temporary” nature of movable structures, such as tiny homes and repurposed containers, can help sidestep slow-to-change and politically loaded building and zoning codes. But this could also be seen as kicking the can down the road and delaying solutions that lead to the construction of much-needed decent (and permanent!) homes.

Conveniently, the upper-middle-class group who appreciates the aesthetics of the Sparkbox also tends to attract individuals with have both the technical inclination and resources to maintain complex systems. Members of this group are likely to live in a neighborhood where they worry little about the security of their home — or have an internet-connected monitoring system for peace of mind. But this is unlikely to be the case for a newly housed individual moving into a Sparkbox. Because the only apparent opening for ventilation is a sliding glass door, occupants may not feel that the unit is secure against intruders and keep the door shut even when ventilation could help cool the unit.

Likewise, note that to prevent overheating and provide comfort in normal operation, a Sparkbox depends on a relatively complex technical system consisting of PV panels, a battery, and a minisplit system. The nonprofits that take these systems will need to either a) find occupants that have the ability and resources to maintain these systems or b) ensure proactive maintenance, repair in case of failure, and available backup housing in case it is not possible to immediately repair the system. Tying the Sparkbox systems to the power grid would reduce some of these concerns and have the potential to provide cooling during a grid power failure, but at the cost of additional system complexity and cost. Furthermore, the individual components of these systems are expensive and have a limited lifespan. Battery lifespan, in particular, is reduced by heat.

Finally, consider the inclusion of an incinerating toilet. According to the description on the Sparkbox website, the device “saves 2,000 gallons of water per person each year.” Incinerating toilets do exactly what you think: they burn human waste. That process leaves ash behind, requiring regular cleaning. While this does eliminate the breathtakingly extravagant use of using drinkable water to flush a toilet, regular maintenance is required for an incinerating toilet [“weekly, or more often as required,” and “every 90 days, depending on usage (or if excessive noise or vibration occurs.)”] Upper-middle-class building nerds like me might find virtue in ashing a toilet, but this seems an unfair burden to an individual struggling for stability and basic needs.

What’s a better solution?

The immediate problem of the three Sparkbox units presents a unique opportunity to the housing department. Rather than burden new occupants with difficult-to-operate structures prone to overheating, the housing department could sell or auction the structures (or, if that is not possible under state law, transfer the structures to a partner who could do this). The initiative would be wise to target upper-middle-class buyers who would value both the advanced technological features of the Sparkbox units and their design. Furthermore, this group would also value the knowledge that the money they spent on their nifty new backyard ADU went to construct heat-resilient structures that will support those currently without housing.

To address the long-term problem of an affordable housing shortage, we need to pull out all of the stops while focusing on those projects that can most quickly deliver large numbers of high-quality, durable, comforable, and safe units. I put these words in italics because an approach that aims only to get the most units built at the lowest cost can — and, I would argue, will, backfire. In the long term, no one benefits from the ugly, cheaply built housing now standard issue across the market-rate and affordable sectors.

While we’re all working on that, housers might interrogate the role of aesthetics and our own aesthetic attachments. A few questions for further thought:

  • What’s the role of aesthetics in driving, dissolving, and distracting public support for affordable housing?
  • Does a modernist shipping container say “welcome home” to the unhoused? What other messages might it send?
  • Is the lack of affordable housing a problem that can be solved through technical innovation, or is the real issue to solve how to increase the income of the growing number of people who do not have the means to afford a home?



Jonathan Bean, Ph.D.

Assistant Professor of Architecture, Sustainable Built Environments, and Marketing & Chair, M.S. Arch-Sustainable Market Transformation at University of Arizona