Save Energy in Shipping Container Homes

Temperature Control Container House

How to Save Energy in Shipping Container Homes

Keeping moderate indoor temperatures throughout the whole year is essential for a comfortable living experience. It is not easy but it is possible to save energy in shipping container homes. Unfortunately, it is significantly more difficult to accomplish in container houses than in traditional ones and eventual off-grid location makes it even worse.

It’s because:

a) Metal structures warm up very quickly when exposed to the sun converting quickly into slow cookers.

b) The majority of container houses do not have attics isolating habitable spaces below from the first strikes of the sun’s radiation.

c) Off-grid locations usually do not have enough energy to run an A/C unit.

Most of us (especially in the “Western World”) cannot even think about life without air-conditioning systems. We consider them as an obvious element of everyday life and like electric lights, tools, stoves, heating systems, availability of fresh & hot water, etc…– we take it all for granted and well deserved! But nothing can be more wrong. For thousands of years, human civilizations were living without A/C units and while we may still use them in years ahead, given the disastrous devastation of Earth and its resources we may have to scale back their use and go to more natural ways to maximize our comfort and health.

Fortunately, growing eco-awareness among populations seems to be bringing some fruits. So-called Energy-Neutral designs “unearthed” many simple ideas and techniques that our ancestors were already using to make their lives more bearable. Below we discuss the most important factors helping to keep your home at comfortable temperatures.

For obvious reasons, this whole chapter is dedicated to hot and moderate climate zones where over a major part of the year (or long enough to be unsupportable), the scorching sun will make our lives miserable. However, when planning our house, we should consider more than just its geographical latitude (distance from the Equator). Important factors are also its orientation towards the sun, the shape of surrounding terrain (mountains, valleys, plains, deserts …), local microclimate conditions (breezes at shore or lake locations, windy areas), the proximity of large masses of vegetation (forests), etc…. All these factors can be used to our advantage, however when neglected – will cost us dearly.

Notes for Temperature Control of Container Houses:

a) Natural techniques of cooling take advantage of differences in local temperatures. Note that generally the ambient temperature in the shade (northern side of your house, under the trees, bush, over the water … is lower than in areas directly exposed to the sun. Usually, there are also daily gradients of temperatures between day and night. Making the most of these patterns, taking advantage of natural breezes and winds, creating the dedicated airflow path sucking the air from “less warm” areas, and facilitating its flow across the house is what we can do to improve the living conditions.

b) Another common approach is to limit the direct exposure of the house to the sun. If you remember old, village houses with thick stone walls (mostly in Europe), thanks to the high thermal mass they can keep comfortable indoor temperatures for quite a long time. In contrast, container houses (in general metal structure) when exposed to direct sun will get almost instantly “red hot”. However:

c) Do not expect miracles – not much can be done in a “natural” way in areas where temperatures are steady high 24/7. If you cannot take it, then either you are in the wrong place and/or at the wrong time. Bottom line – when you cannot choose the place and time, then most likely the use of A/C will be non-negotiable.
Well, to be correct – there are some other natural ways to lower the indoor temperature (for example water evaporation), but it is outside of the scope of this chapter.

Below, we are the details of some beneficial approaches and techniques to keep comfortable living conditions in your container house.

1. Geographical Latitude

The latitude determines the solar exposure (angles of sun rays) throughout the year. For obvious reasons those living closer to the Equator will like to limit exposure to the sun, those closer to the South or North Pole would like to extend it as much as possible while all those located somewhere in between will have to find the optimum solution. It often means – minimizing the impact of exposure to the sun in hot summers but at the same keep full sun exposure as much as possible during the winter.

Part of the strategy is purposefully located overhangs. They can mitigate the effects of summer exposure at the most critical times – starting at about noon and extending a few hours into the afternoon. While the prevention of direct infiltration of the sun through windows significantly helps, not to ignore is also the accumulation of heat by walls and roof. Traditional awnings can “partially” do the job, although it is usually more costly and maintenance requiring solution compared to fixed overhangs along the exposed section of the house (south and south-west sides)

Left: Maximum and minimum exposure (solar angles) corresponding to the location at 40 degrees northern latitude (Source: BuildingAdvisor.com)An extreme example of overhangs is the porch.

 

Part of the strategy is purposefully located overhangs. They can mitigate the effects of summer exposure at the most critical times – starting at about noon and extending a few hours into the afternoon. While the prevention of direct infiltration of the sun through windows significantly helps, not to ignore is also the accumulation of heat by walls and roof. Traditional awnings can “partially” do the job, although it is usually more costly and maintenance requiring solution compared to fixed overhangs along the exposed section of the house (south and south-west sides)

An extreme example of the overhang is the porch:

Temperature Control of Container Houses

Large porch is an important element of the natural cooling process. Source: MotherEarthNews.com – photo – Barbara Bourne

Sometimes it may be reasonable to plant deciduous trees on the southern and south-western sides of the house. During summer, they will act as a living sun barrier obstructing direct exposure to the sun, while in the winter, when all leaves are already lost, bare trees may allow (depending on the density of branches) the penetration of daylight into the house. Note that conifers will block the sun throughout the whole year!

2. Windows

The optimum design of the house is a compromise between a long list of wishes and an equally long one of limitations. The location of windows is generally high on the priority list because visual contact with nature is one of the essential ingredients of a peaceful mind, harmony, and overall happiness. After all, one of the reasons for choosing a container house is the relative easiness of its set-up process in a remote area surrounded by nature.
The bottom line – large, panoramic windows (if not entire glass walls) are usually non-negotiable wishes. It does not mean however that we are powerless when it comes to indoor greenhouse effects created by sun exposure.

Fortunately, these days you can choose dual or even triple-pane Low Emissivity (Low-E) windows minimizing the amount of transmitted infrared and UV energy.

The special ultra-thin coating on interior sides of the window’s glass is practically transparent for the visible part of the solar light spectrum so it does not affect the quality of the window (penetration of daylight and visibility of outdoors). It reflects however the reddish (infrared) fraction of solar light that is mostly responsible for the transmission of heat. Note that Low-E windows have a dual effect, they reflect the solar heat radiation coming from outdoors, but also reflect the interior heat back preventing it from escaping outdoors. In other words, it helps to save energy for cooling but also for heating.

In dual (triple) pane windows air gaps are filled with either Argon or Krypton (both gases have low heat transfer coefficient) and then are factory sealed. Their performance is indicated by the U-Factor representing the ability to transfer the heat (typically in the range from 0.25 to 1.25 [Btu/h·ft²·°F]). In a way the U-Factor (representing transfer) is opposite to the more familiar R-Factor (representing Resistance to transfer), so in this case, lower U is better.

As it could be expected, Low-E windows are more expensive than traditional ones, but the initial investment may be justified by future energy savings and priceless comfort.

Graphical representation of the performance of dual and triple-pane Low-E windows. Source: StanekWindows.com (USA)

Another “good” question is the geographical location of windows (in general of “glass”). While often it is determined by the administratively imposed orientation of the house and local landscape (scenic views), whenever possible it is suggested to avoid large amounts of glass facing the north (no benefits of winter sun), and to limit glass on the west side (impact of summer afternoons overheating). So, for practical reasons, the south seems to be the preferred orientation of windows.

Shades are another, generally less costly ($$$) option, but its real price is the deprivation of benefits coming from clear visual contact with nature. Needless to say, that panoramic views of the surrounding landscape are most likely some of the most important arguments behind your decision to move to the countryside.

Fortunately, modern shades (or rather shade-screens) have nothing to do with heavy, old-fashion window shades that were also used as protection from burglars. Modern, light designs use bamboo as a construction material and their main purpose is to disperse the direct radiation of the sun while preserving some amount of visibility across. Scattered daylight can still infiltrate the interior but the impact of the direct heat can be avoided.

The “shading efficiency” depends on the “density” shade-screens – obviously, better shading efficiency means also more obscured views across the shades.

Temperature Control of Container Houses

Passive House in France (Karawitz Architecture) proves that shade-screens can block large portions of direct light but at the same time, they can add stylishness to the architectural form of the structure. (Source: Archdaily.com)

Note that skylights due to their angled location if left unprotected, act as open gates for the incursion of solar-based heat energy!

3. Ventilation to Temperature Control of Container Houses

Moving air (even only slightly less hot than the ambiance) gives a feeling of freshness. And there is a reason for this perception – moving air helps the evaporation of moisture (sweat) from our skin. Evaporation is the endothermic process in which water absorbs heat from surroundings (that is the tool our body developed to keep its temperature within safety limits). Well, that’s why any gentle breeze is a gift from Mother Nature.

In an eco-friendly approach, we should take advantage of any such opportunity – be it windy areas, shore or lake locations with well-defined patterns of daily breezes, proximity to forests, or wild nature acting as large thermal masses, etc. Harvesting these resources may require initial investments but at the end of the day, it will pay off because Mother Nature will offer a lasting free run.
The bottom line is – to take advantage of the natural cooling process, we have to keep air moving across the house.

– Cross-Ventilation

The most familiar scheme of cross-ventilation consists of two opened windows at opposite sides of the house. Usually, it will create the draft across the house with all its benefits as discussed above. At a much smaller scale, the cross-ventilation is implemented by the installation of intake and outlet louver vents on opposite walls of the room. If the intake vent is located next to the floor, and the outlet one next to the ceiling, the cross-ventilation will work even in rare cases of outdoor stillness.

– Stack Ventilation

For many reasons, sometimes it may be not possible to keep day-long open windows to create drafts. Here comes the benefit of Stack Ventilation. It takes advantage of the natural air convection process due to differences in local air temperatures. The coolest air near the house is usually on its north side, especially if the area is overgrown by trees and bushes. Locating an air intake vent near the ground level gives you access to the “reservoir” of certain fresher and most likely “less” warm air compared to the exposed open areas. The exhaust vent (outlet) should be located across the house as high as possible. The intensity of natural convection depends on the difference in temperatures between the intake and exhaust points. To increase the speed of the airflow in the single-level container structure you can build a ventilation tower. It will create familiarity for most of us chimney effect magnifying the intensity of natural convection.

Stack Ventilation schemes. Source: Secret of Architecture – Vision & Identity

For better effectiveness of natural ventilation, we have to make sure that the air is sucked into the house ONLY from the dedicated place is the source of fresh, cooler air. Any other “parasitic” air infiltration paths will diminish the desired effect. Of great help will be an intake fan pushing the cooler air into the house (it’s so-called “supply ventilation”). It allows for better control of where the air comes from (note, that an exhaust fan is not selective, it speeds up the air evacuation process, but the uncontrolled intake air comes along “paths of least resistance” which does not guarantee to suck the air of lowest temperature).

In off-grid locations, an inexpensive solar fan will do the job (traditional WhirlyBird ventilators work only as exhaust units, so acting alone they wouldn’t be that efficient).

The idea of a natural ventilation scheme. Source: Inhabitat.com (Wikimedia)

– Windcatchers

In windy areas (this also includes breezy locations in proximity to the water) you can take advantage of predictable natural air currents. Water has a much higher thermal mass than land, so its temperature always lags behind that of the land. As a result, during sunny days the air temperature above the water is lower than that over the land which creates a continuous breeze toward the land. During the night, the situation changes, because the soil cools down faster than the water and subsequently the breeze changes direction.

It may be difficult to efficiently harvest breezes and wind by using traditional intake vents. But human ingenuity does not have limits. One of the solutions is so-called windcatchers – sort of “architectural” modifications to the structure of the house designed to “capture” natural airflows (winds, breeze) and direct them into the house.

The basic idea of windcatcher. Source: NaturalBuildingBlog.com

Note that roofs provide a good location for windcatchers because the higher the ground level, the stronger the wind. More sophisticated windcatcher systems redirect the harvested air to basements, underground tunnels, above-the-water pools, fountains, etc to cool it down for additional effect.

– Local Low-Pressure Zones

Accelerated speed of airflow develops a local area with lower pressure (it’s a so-called Venturi Effect). If the zone of lower pressure is strategically located over the house, it will suck the hot indoor air and carry it out. Such “assisted” ventilation is relatively easy to implement in container houses by erecting an extra roof with the inclination following the most frequent path of the wind. The air pushed under the angled section of the roof will gain speed creating this way mentioned above the effect of lower pressure.

The mechanism of the Venturi effect and its practical implementation. In this case, the angled section of the roof has built-in windows that allow controlling the ventilation process according to needs. Source: Benjamin Garcia Saxe Architecture (homeli.co.com).

4. Reflective Exterior Surfaces fo Temperature Control of Container Houses

Those traveling to Mediterranean countries certainly realized that traditionally, most individual houses (at least in the era when A/C did not exist) were painted in white. After all, the name Casablanca (in Spanish – White House) exactly represents this tradition. For many reasons, the white color maybe not be practical, for at least one is greatly beneficial. The point is that white (and shiny) surfaces reflect sun rays, while black (and matte) absorb them. It’s the physics, whether we like it or not, light-colored exteriors help to keep the interior cooler.

The most sensitive to the overheating part of the house is the roof because its horizontal or slightly sloped geometry increases the absorption of sun rays’ energy. It’s an especially intense process during the most critical part of the day – around noon (angles of sun rays are closest to 90 degrees). From this point of view, walls are less prone to overheating because at the time when the sun is at its highest point above the horizon, incident sun rays will hit vertical surfaces at the smallest angles and so they will be mostly reflected back to the atmosphere.

Physics: Metals are very reflective (that’s why their surfaces are shiny), but unfortunately, they are also absorptive (light’s energy is quickly absorbed by metal’s crystalline structure). If the angle of the incident light beam is small (red arrow representing the case of vertical walls), most of the light is reflected back to the atmosphere. However, when the angle of incident light striking the metal is closer to 90 degrees, a large part of the energy carried by sunrays is absorbed. (Source: modified drawing from LibreTexts -Engineering, University of California)

Obviously, dark-colored roofs and walls will intensify the absorption process of overheating. Given the fact that metal is a good heat conductor, the heat energy will quickly spread over the whole metal structure significantly affecting the indoor temperature.

An extra roof suspended over the container-based house is a relatively low-cost and effective solution to overheating. It can be the only viable solution in desertic areas where the natural shade provided by trees is unrealistic. Note that not much can help the container’s metal structure to reach the ambient temperature, but at least the detached extra roof acting as the “first line of defense” will absorb direct sun rays’ energy. The air gap between both roofs (especially if properly designed) provides an extra opportunity for cooling (see Venturi Effect).

Multiple strategies of natural cooling in desertic areas: Extra detached roof, airflow (in windy areas), and thermal mass (concrete slab) can help to achieve amazingly comfortable living conditions. (Source: FastCompany.com – “Living in the desert…”)

5. Green barriers

– Trees

High trees can create an effective barrier between sun rays and the house and even if the shade is only partial, it will have a significant effect on indoor temperature. Note that in general all shaded areas created by greenery (trees, bush…) form local zones of fresher and cooler air that can be used for the purpose of ventilation.
Among fast-growing deciduous trees, the two most popular are Hybrid Poplar and Northern Catalpa but also Quaking Aspen, Glory Red Maple (beautiful for its autumn colors), River Birch, Dawn Redwood, etc…. They can grow very tall with up to 6 to 8 ft per year quickly providing much-needed protection from the sun.

Left: Hybrid Poplar trees. Source: FastGrowingTrees.com
Right: Northern Catalpa tree. Source: University of Idaho (Arboretum and Botanical Garden)

– Climbing Plants

Other popular green heat barriers are trellises with climbing plants (vine, wisteria, clematis….). Similarly as mentioned above fast growing trees, vines, and wisteria will also quickly spread over trellises. Note that trellises have an advantage over traditionally planted vines directly climbing (overgrowing) the walls. By leaving a small air gap between the trellis and the wall you allow for the circulation of air increasing this way the efficiency of this natural cooling process.

Soon this trellis covering the roof will be overgrown by climbing plants. Note the air gap between containers’ roofs and trellis left for the free circulation of air (wind, breeze….). Source: Dwell.com (design by Indonesian company Atelier Riri).

Assuming that you plant deciduous greenery for the sun barrier, to some extent you will still enjoy benefits of sun penetration during the winter (conifers better known as evergreen trees due to their vividness may be more attractive and “warm” but will certainly deprive you or physical warmness of sun).

– Living Roofs

On top of making an efficient sun barrier, Living Roofs can also add a lot of aesthetics. We wouldn’t also exclude their practicality (imagine a small flower garden with enough room for a sunbed or a small coffee table allowing you to indulge from above the earthlier-looking ground level? Or a veggie garden enjoying protection from rodents?
Yes, it all sounds like a great and inviting idea, although let’s face it – It’s not for everybody and not for all climate zones.
In hot and dry areas, you will need a large amount of fresh water to keep it alive (what may be a mission impossible). In wet climate zones, you will need a good irrigation system to prevent the accumulation of water on the roof (with all the known disastrous consequences that it can create). On top of that you may have to be ready to deal with constant water-induced erosion of soil that in the form of mud will be carried away through gutters. And if this is already not enough – be ready to maintain the garden the way you can also enjoy it personally. Nevertheless, the roof garden can be an attractive solution to overheating in moderate climate zones.


Container’s Roof Garden as an element of protection from overheating. (Source: Poteet Architects)

A more natural, grass-living roof will be less maintenance demanding. Source: GrassRoofCompany (UK)

6. Thermal mass

Thermal mass represents the ability of certain materials to absorb, store, and release heat when exposed to changing external temperatures. For long such properties of some construction materials were used to mitigate the impact of high extreme ambient temperatures characteristic of hot climate zones.

Century-old, stone wine cellar provided exactly what was needed – stabilized indoor temperature for the future benefits of wine and the pleasure of its consumers)! (Serbia)

Probably most of us had a chance to experience the “blessing” effect of a big thermal mass when vacationing in one of such stone-built houses you can still find in centuries-old Mediterranean villages. Thanks to their walls built of large blocks of stone they can efficiently flatten out daily variations of outdoor temperatures making the interior a much more comfortable area for living.

Impact of the thermal mass on variations of indoor temperature. As can be seen, the indoor temperature follows the outdoor one but it lags and dampens down exterior variations. Source: GreenSpec (UK)

To make it clear – a similar effect happens when outdoor temperatures substantially drop down at night. In this case, the thermal mass of the house’s structure will start releasing the heat accumulated during the day flattening out short-term low outdoor temperatures. Note, that massive structures with very large thermal masses can delay the impact of sustained high or low outdoor temperatures for days.

Two main factors determining the impact of construction materials on indoor air temperature is their Heat Capacity and the Thermal Time Constant. The heat capacity defines the amount of heat energy that can be absorbed (stored) by the given material. The thermal time constant defines the time necessary for a given material to reach the ambient temperature (in other words how long it will take to stabilize its temperature).
The physical interpretation of the thermal time constant can be seen below in the case of a hypothetical concrete slab used as the thermal mass.

If we assume that the ambient temperature rises in step-function to its maximum daily value, the temperature of the Concrete Slab will keep increasing exponentially. However, if the slab’s thermal mass and constant are large, it will not reach the ambient temperature by the end of the day. At night, the opposite happens – now the slab’s temperature goes down. The difference between the maximum slab temperature and the ambient one represents the natural “damping” gain induced by the thermal mass. However, If the thermal time constant of the slab will below (for example metal structure), it will heat up very fast and reach the ambient temperature well before the end of the day with no benefits of damping gain.

It should be clear from this picture that the thermal mass can’t help much when it is exposed to heat for a long period of time (long compared to its thermal inertia). Even the massive stone house when permanently exposed to high outdoor temperatures sooner or later will reach the same indoor conditions. Fortunately, even in hot climate zones, there is substantial variation between day and night temperatures (a good example is a desert, where days can be hellish, but nights are often surprisingly cold).

As we already know, containers (in fact all metal structures) have significantly lower heat capacity than water or traditional construction materials (concrete, brick, wood, stone…). What makes it worse, is that metals also have small thermal time constants. In other words, they transfer heat very quickly, increasing their temperature almost instantly. Obviously not much can be done when it comes to the container’s structure – it is steel! However, we can apply the knowledge of thermal mass to the container’s foundation making it in a form of a deep concrete slab buried in the ground. If its thermal mass will be high enough, it will never overheat during the day, noticeably smoothing-out indoor temperature while slowly releasing the accumulated heat into the ground during the night to be ready for another daily temperature cycle. In practice, a well-designed, ground-level floor will keep its refreshing, “lower” temperature well beyond the afternoon’s heat peak helping to create a comfortable ambiance.

“Fat” deeply buried concrete slab thanks to its huge thermal mass slows down the overheating process. (Source: – “Living in the desert…” )
To improve the effect, the thermal mass (in this case the concrete slab) should be shaded from direct exposure to sun rays. Additionally, along its perimeter, it should be thermally insulated from the soil to prevent penetration of heat from the surrounding overheated surface. However, the bottom part of the slab thanks to its “earth coupling”, can use moderate temperatures of deeper layers of soil as an enormous heat sink with an almost infinite thermal mass to benefit from.

Note that typically insulations do not have any meaningful thermal mass. They cannot absorb and store large amounts of heat. Their main purpose is to limit the natural transfer of heat between hot and cold areas. To give you a better idea of the difference:

The Thermal mass is like a Rechargeable Battery – it can be charged with energy, increasing its output voltage till it is fully loaded (stones or concrete are “charged” with heat energy slowly increasing its temperature). Insulation acts like a high-value Resistor (hence the familiar R-factor) limiting the flow of the current (in this case the flow of the heat across the wall, ceiling, floor etc…).

7. Appliances

It’s obvious that during hot seasons, every interior source of heat will have a huge impact on the comfort of living. While we cannot simply turn off all vital appliances, at least some of them we can move outdoors. Fortunately, in container-based houses, it is somehow almost a natural process. Living in the countryside, surrounded by nature, favors outdoor kitchens. Be it traditional barbeque or cooktop (for obvious reasons protected by an awning or extra roof), it seems to be an important part of an eco-conscious lifestyle. Insulating a hot water tank (if any), drying dishes by air, doing laundry outdoors, using LED lights, and so on are obvious steps not only to conserve energy but also to minimize indoor overheating. All these methods are well known to all aficionados of off-grid living.

Example of container-based school in Uganda (note the extra roof and “veranda” type bamboo-protected central area). Source: Olivier Garcia: – Aleutia / ICT Classroom Project (a collaboration between Esala and Aleutia)

8. Final notes

The “right” solution for your container house highly depends on its geographical location. Note that what will be beneficial in hot climate zones (for example reflective surfaces, windcatchers, etc), may be questionable in areas with moderate climate and detrimental in cold (arctic) areas where you would like to catch every existing sunray and block the arctic wind. That’s why none of the above strategies should be adopted blindly.

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