From the point of view of airtightness, traditional residential houses (especially older ones) resemble strainers full of holes. Poor craftsmanship leaving plenty of gaps in the structure of the house, far from ideal sealing around windows and doors (to name only few most typical examples) are main paths for air leaks and drafts. Most of those using natural gas for heating probably do not realize that a typical furnace’s burner needs so large amounts of oxygen that they will be suffocated if it wouldn’t be continuously leaking from outdoors. It’s an unwanted but unfortunately also uncontrolled ventilation process. It has its benefits (ventilation) but also big downsides – poor energy efficiency ($$$) if you want to keep comfortable indoor temperature.
Shipping containers are not fully airtight (exception – refrigerated ones), however typically, their vents are designed only to keep interior pressure in sort of equilibrium with outdoors. These so-called “pressure vents” are small to prevent water infiltration and just good enough to accommodate changes of interior pressure due to daily weather conditions (scorching sun and cold nights). They are designed to fit into the aperture of the corrugated wall, so their installation is quite straightforward.
Typical ABS vent covers for cargo shipping containers. Source: ACE Container & Parts Co. Ltd (China)
ABS vent cover installed on the wall of a cargo shipping. Source: Seattle Tacoma Shipping Containers (USA)
Container-based houses, due to installed windows and doors will further reduce initial airtightness, although this may still not guarantee safe living conditions. To make it clear, containers’ original pressure vents will allow them to “breathe” but it does not mean that you will have enough fresh air to breathe! Converting cargo shipping containers to habitable spaces implies new sets of requirements. Given the fact that in most countries, the ventilation is part of local building standards (especially for quasi-airtight structures) you should get familiar with relevant requirements before proceeding with the design of your ventilation system.
The ventilation system in container-based houses should address several issues including:
1. Evacuation of Vapor
All inhabitable spaces generate water vapor which in turn will condensate on impermeable steel walls leading to their structural damage but also to generation of fungi-related health-hazards. We addressed the vapor condensation process and the impact of the container’s thermal insulation on it in the previous chapter (see: Condensation in Container Houses).
However, the areas with an excessive generation of vapor (kitchen, bath, laundry room etc..) need special attention. While one can always limit the spread of vapor by cooking in pots with lids, by limiting the time of boiling by using a kettle w/automatic power-off function or with an acoustic alarm (whistle), by taking a short, moderately-warm shower instead of long and hot etc… The truth is that these areas must have a dedicated ventilation system to evacuate the vapor outdoors before it can spread throughout the house.
2. Health Hazards
The total elimination of vapor condensation cannot be 100% guaranteed. In fact, from the point of view of living comfort it is suggested to keep indoor relative humidity at the level of 40 to 50%. That’s why condensation will happen, and the ventilation must address its harmful effects like mold and mildew.
Even at lower levels of indoor humidity, some areas (especially bathrooms, laundry rooms with much higher local level of humidity) but also “difficult to access” spots with stagnant air will be prone to the development of mold and mildew. Both are sort of fungus growing in warm, humid areas with stagnant air. Mildew is mainly developing on the surface, so it is visible and easy to get rid of. Mold is penetrating deeper under the surface affecting the bulk of materials, more difficult to see it at an early stage, usually, first symptoms is a characteristic odor. When spores of mold or mildew are inhaled, they can cause irritation or even illness of the respiratory tracts, allergies, asthma… Circulating air over areas prone to the development of fungi will lower the “local” level of moisture and eventually evacuate most of fungi-related spores and microorganisms into the outdoors.
3. Air Quality
Inflow of fresh air is crucial to maintain indoors a healthy level of oxygen, evacuate contaminants (odors) and create the ambiance of freshness.
The importance of bringing fresh air into container-based houses is magnified by the fact that many of them are very small. One Twenty-feet Equivalent Unit (1-TEU) has the surface of just 160 square feet and correspondingly the volume of 1,360 cu. ft. Without the inflow of fresh air, the level of oxygen can quickly drop down to dangerously low levels (note that kitchen stoves and water heaters operating on propane gas consume a lot of oxygen).
4. Combustion Appliances
While the amount of oxygen necessary for cooking on propane may be accommodated by traditional ventilation systems, big wood or propane burning furnaces may need a dedicated intake of air. In other words, they do not only need exhaust pipes but in “quasi-hermetic” spaces (as are containers) they must have direct access to large quantities of oxygen from outdoors. Dedicated intake and exhaust pipes will allow the burning system to operate in a closed-loop drawing the fresh air from the outside and expelling air mixed with exhaust gases back to outdoors.
Ecoburn Plus 5 wood burning stove. Source: Arada Stoves (UK)
During hot summers bodies of all metal structures exposed to sun will almost instantly warm up what will lead to uncomfortable indoor temperatures. The good news is that in such cases (direct exposure to sun) the proper ventilation system may still be used to cool a house’s interior.
From this point of view, the ventilation wouldn’t make any change only in hot and humid climate zones where outdoor air in its volume is equally hot and humid (the only help is the A/C system).
While ventilation cannot lower indoor temperature below the level of an ambient one (cannot cool), it can help to achieve a more comfortable, fresh environment. Such designs usually require some extra means that are beyond the scope of this writing (It will be the subject of a separate article).
(climate zone, daily temperature amplitudes….)
In moderate climate zones where neither heating or cooling periods last for long (if at all) one can assume that open windows will allow for air exchange reducing this way indoor humidity and bringing in much needed oxygen. This approach however will be very limited in hot and humid or cold climate zones.
Note that the “right” ventilation system is designed as a compromise (optimum) between the necessary amount of air to be exchanged for safe and comfortable indoor living conditions (more seems to be better) and the cost of maintaining a comfortable indoor temperature. These are two contradicting requirements – from the economical point of view, in cold or hot and humid climate zones you would not like to have any air infiltration from outdoors. Both situations (heating the cold fresh air or cooling and dehumidifying the hot and humid one) are costly!
Existing ventilation systems belong to the categories of Passive or Forced airflow systems (the latter group includes mechanical (wind-ran) and electrical systems).
Regardless of what kind of system you will use in your container house – be it Passive or Forced airflow, wall-mounted louver or roof-mounted WhirlyBird types of vents, they all must be accompanied by air intake vent(s) located on the opposite side of the container next to the floor. Efficient ventilation systems must create so-called “cross-circulation” of the air (in other words the largest possible area of the ventilated room must be “on the path” of the airflow. This is to eliminate potential zones with the stagnant air.
The extreme case of cross-circulation is created by opening two windows – each on the opposite side of the room. When it comes to ventilation, we would like to create similar conditions, although at much smaller “not perceptible” scale (no air drafts).
Passive ventilation systems use the natural convection process that moves the warmer air up and the colder one down. If the warmer air exits through the vent located in the upper part of the room (ceiling) it must be replaced by the fresh one from outdoors. That’s why such systems must include both – air intake and exit! The intake should be placed at the floor level (usually the area of lowest temperature) and the exit either in the roof or on the top of the sidewall. For best cross-ventilation effects, air exits should be located far from typical areas of unwanted air infiltration (windows or doors). The intensity of convection-generated airflow is proportional to the gradient of temperature.
Typical passive, side-mounted (louver) vents have angled slats protecting from rain infiltration and bug/rodent screens. They are the most economical, general use solution for most habitable spaces without special requirements for evacuation of vapor. In practice, “strategically” installed passive ventilation systems with adequate surface will meet basic requirements for living and sleeping areas (in other words they will provide enough oxygen for the breathing process and evacuate generated by breathing vapor).
For that to happen, most likely you will have to use multiple ABS-like Pressure Vents designed for cargo shipping containers. It may then turn out to be an unproductive and laborious effort, although due to their small dimensions and large choice of colors such vents may not spoil the visual effects of Cargotecture.
Another version of inexpensive vents are Con-Vents. They are 6” x 12” large, made from galvanized steel and are specially designed for cargo containers with geometry matching their corrugated walls.
Two types of passive Con-Vents (left) and their typical installation (right). Source: US Veterans (USA)
The main problem with the installation of general-use vents is the corrugated steel structure of container walls and roofs. That’s why most inexpensive louver vents readily available across the chain of Home Depot or Lowes stores) will need extra fixtures to accommodate the geometry of corrugated surfaces (mostly DIY effort)
Insta-Vents are designed for cargo shipping containers. They are powder-coated and bolted on two outer corrugations of the wall. For completeness, they need a matching part on the interior side of the wall (here not shown), although practically it can be just a DIY-type bug screen reinforced by rodents/birds mesh. Source: Insta Container Modification (USA)
The efficiency of passive vents can be greatly improved by adding an Intake Fan (often referred to as Supply Only Ventilation). Such a ventilator, instead of evacuating outdoors the stale/hot/humid air from home it sucks the fresh air from outdoors. It creates inside of the home slightly higher air pressure compared to outdoors what increases the airflow through passive vents. Such a system is very helpful for “peak hours” when the interior becomes very hot and you would like to speed-up the circulation of air.
Active – Forced Airflow Ventilation Systems
Wind-driven Forced Airflow ventilation systems (known as WhirlyBirds or simply Turbine Vents) are much more efficient than passive vents. For best exposure to the wind they are traditionally installed on the roofs.
Built-in cylindrical form with vertical blades (fins) around, they spin in the wind creating a zone of lower pressure inside of the cylinder. As a result, they “suck” the air from the interior of the house evacuating it at accelerated speed compared to the natural process of convection. In the absence of wind, they will act as traditional passive vents (although more “sophisticated” designs may be self-propelled by the escaping air). Obviously, the spinning of the turbine vent is proportional to the speed of the wind, so stronger the wind means more cubic feet of air is evacuated per minute (you can expect that a 12” diameter turbine vent will evacuate about 200-300 cu. ft of air per minute at a wind speed of 5 mph).
Well-designed Whirlybird vents must also prevent infiltration of rainwater and last long without any maintenance. Note that in windy locations lower-quality Whirlybird vents can wear quickly and start to squeak when rotating. That’s why it is suggested to use their more expansive versions with permanently lubricated ball bearings guaranteeing long years of silent operation!
Unfortunately, the container’s roof creates more demanding installation conditions due to its corrugated structure. Fortunately, due to the growing popularity of container-based houses you may find on the market Universal Shipping Container Adapters (USCA). Made from Fiberglass Reinforced Plastic (FRP) they tightly match the container’s corrugated profile offering much needed “transition” (coupling) between container’s geometry and much easier to work with – planar one. Slightly flexible to accommodate small dimensional discrepancies, durable, UV resistant, USCA seems to be an ideal solution for container’s roof-mounted DIY-type projects.
Universal Shipping Container Adapter. Source: Universal Shipping Containers Components (USA)
WhirlyBird installed on container’s roof with the use of USCA, Source: Universal Shipping Containers Components (USA)
Solar-assisted WhirlyBirds represent improved versions of turbine vents. They are still dependent on weather conditions, but at least their “energy” source is more diversified. Available as autonomous units with built-in small solar panels (but no energy storage) they are widely used for cooling attics in traditional houses (they certainly work when needed!). Obviously, they can do a similar job in container-based houses.
Aura Solar Fan uses both solar and wind power for more efficient ventilation. Source: Active Ventilation Products Inc. (USA)
Areas generating larger amounts of vapor may require more reliable and efficient ventilation solutions to make sure that vapor is timely evacuated before having a chance to drift to potential areas of condensation. Unfortunately, autonomous solar-powered vents may not meet a fundamental vapor-evacuation requirement – be ready to work when needed. As an example, most of us will take the shower before going to the bed and usually at that time solar panels are exposed to “darkness” (note that autonomous units do not have any energy storage to offset their working time into the night).
Solar vents: Left – Sunvent model SVT012 (Source: Solar Made (USA). Right – 10W model SW-RAF700 with adjustable panel can extract about 400 cu. ft of air per minute (Source: Solar Whiz (Australia)
The bottom line, both – kitchen (range hood) and bath need “on-demand” ventilation systems known as Extractor Fans. Activities in these areas do not closely match the daylight and wind patterns. On top of that, in these two cases, the need for ventilation is not primarily driven by discomforting hot air, but by the danger of condensation (vapor cannot wait). That’s why these two locations need electrical Vapor Extractors. They must be ready to turn on whenever it is necessary!
Well-designed ventilation systems should address most (if not all) relevant problems including prevention of vapor condensation, improvement of living comfort and quality of air. However, on the other side, the system must also meet the economical requirements (low cost) and fit into the available energy budget (energy conservation). The latter is especially important in off-grid locations.