For obvious reasons, Fire Safety is one of the most important parameters characterizing buildings. Not surprisingly, it largely depends on the properties of construction materials, although the overall fire safety is also determined (especially in large apartment buildings) by interior design (accessible escape routes), the availability of early warning systems (smoke and fire alarms) etc.
In general, the Fire Safety is determined by:
1. Flame Spread Rating (FSR)
It reflects the speed at which the flame spreads across a surface of a given material in some specified test conditions. For the purpose of classification, FSR ranges from 0 (representing nonflammable materials like concrete blocks) to 100 (representing inflammable materials – in this case Red Oak is used as a reference). When it comes to building codes, the FSR is classified as: Class I (or A) with the FSR in the range from 0 – 25, Class II (or B) with FSR in the range from 26 – 75 and Class III (C) with the FSR range from 76 to 100 and up (Class D).
Concrete and bricks belong to class A materials. Acceptable construction materials represent lower end of the Class B (often divided into sub-classes with B1 representing “Hardly Flammable” (fire resistant) materials, followed by B2 representing “Very Difficult” to spread flame materials.
2. Fire Resistance
It characterizes the ability of an (exterior) wall to withstand exposure to an open fire. It is specified as time of exposure till the wall collapses. Commonly it falls into the categories of 4-hours, 3-hours, 2-hours, 1-hour and Non-Rated. The latter means less than 1-hour resistance and represents most residential houses with wooden frame and plywood/vinyl siding. Brick or stone cladding extends fire resistance above 1-hour, rock (or glass)-wool insulation adds an extra 15 minutes to fire resistance. Note that gypsum sheetrock has class-B FS rating, however it does not contribute to the structural strength of the wall, so it cannot prevent it from collapsing when exposed to fire.
Traditional residential housing – the common gypsum wall between townhouse units increases the fire resistance rating to 1-hour. Source: Richard Avelar and Associates, Oakland, California.
3. Smoke Toxicity
The process of classification of toxicity of various construction materials is quite complex because the final effect (toxicity) depends not only on the burning material itself, but also on the temperature and amount of available oxygen (ventilation). For example, low amount of oxygen results in higher level of toxic carbon monoxide. To make it even worse, methodologies and test conditions necessary to properly quantify the hazard induced by smoke are still evolving.
For the purpose of this article we would like to bring to the readers’ attention the following facts:
a. There are two types of toxicants:
Asphyxiants which act on central nervous system causing the loss of consciousness and ultimately death. Most common asphyxiants in housing industry are carbon monoxide (CO), hydrogen cyanide (HCN) and carbon dioxide (CO2).
Irritants which would lead to irritation of eyes and respiratory tracts. The most common irritant in housing industry is Hydrogen Chloride (HCl) – gas formed from thermal decomposition of polyvinyl chloride (PVC). Let’s note that popular Retardants when exposed to high temperatures also generate irritants.
a. The health hazard induced by toxicants depends on their level of concentration and time of exposure.
b. Statistically, toxic smoke (combining effect of gasses and fumes) causes more deaths than the fire itself!
4. Ignition Temperature
We will mention it only shortly, because under normal circumstances both processes – Autoignition and Flash Points Ignition (and relevant building codes) apply only to places storing fuels and chemicals!
Typically, fires in houses are triggered by internal sources like excessive heat (iron), electrical installation (overheating or short-circuit), sparks (in the presence of gas leaks), cigarette, open sources (candles, wood-burning stove, matches) and eventually lightning (external source). Note that the lowest autoignition temperatures of typical construction materials are starting at a few hundred degrees (wood at about 450 oF), and so are quite irrelevant when it comes to fire safety in residential and apartment housing.
Fire Safety and hazards in Container Houses
Under special provisions, cargo shipping containers can be used for transport of hazardous materials like chemicals, fireworks, ammunition, flammable materials etc. Once converted to commercial spaces like storages, they can be also used for storing hazardous materials, although in such cases they must meet relevant requirements when it comes to location and safety.
This chapter however is dedicated solely to container homes (in general “habitable spaces”), which excludes their use as storage for hazardous materials. If for example LPG tanks must be used in out of grid locations, LPG tanks, installations and container houses must meet relevant safety requirements (certifications, alarms, proper ventilation etc..).
Typical single TEU container house is a steel box with window and doors: Source: EUROmodul – Modular Buildings
In contrast to traditional wooden-frame structures, container-based houses thanks to their steel frame and shell have significantly lower probability of fire and related to it hazards. At least, the fire will not lead to the collapse and total destruction of the structure. It does not mean however that they are fully immune to fire. Like in all habitable spaces, interior of container houses will be finished with materials friendlier & visually warmer than metal. They will also be stuffed with furniture, decorations and all sorts of more or less personal “belongings” – most of them flammable. The wood for example, due to its characteristics (elegance, warmness and long tradition) still is and will be one of the most appreciated materials used not only for furniture but also for flooring, decorations, cladding…. However, from the fire safety point of view, wood has very poor ratings and belongs to Class III (C or D).
Synthetic materials commonly known as “plastics” are also very popular “finishing” materials used by the housing industry. With few exceptions (mineral-wools), synthetic materials like PU (Polyurethane), EPS (Expanded Polystyrene), PE (Phenolic Foam) are widely used for thermal insulation of container houses. Add to that plumbing pipes (PVC), finishing layers of floors, walls, ceilings (PVC or vinyl), carpets, window shades, cushions and frankly, this list can go on and on because synthetics secured their place in our homes for good.
Unfortunately, most synthetic materials are to some degree flammable, so they require inclusion of Fire Retardants. As the name suggests, the main task of fire retardants is to delay the spread of flames giving more time to escape from the burning structure. Unfortunately, fire retardants are complex chemical substances, so they also add additional harmful side effects – at high temperatures, they release toxic gases adding to those already released by burning synthetics themselves. Well, retardants are not sort of “golden solutions” but simply lesser evils.
Regarding the choice of finishing materials and insulation, you may have very little power when it comes to modified cargo shipping container houses. In contrast, most manufacturers of customized container houses offer several options for insulation and eventually (but rarely) interior finish. Typically, you can choose between PU, EPS or Rock-Wool sandwich panels for walls, ceiling and floor.
Manufacturers offer various types of insulation materials used in sandwich panels. Source: Inficreation House Technology Co. Ltd (Quanzhou, China)
Most floors in modern customized container houses are made from MgO or fiber-cement panels. The former can be made highly decorative so may not need any extra finishing, the latter will be usually covered by vinyl of “engineered” (synthetic) wood-like panels. Both materials – MgO and fiber-cement are practically NOT flammable compared to plywood and wooden-finished floors.
Decorative, wooden-grain fiber-cement panels
Now, just to keep all these in right proportions – leaving aside the metal structure of container houses, their habitable interior represents very similar fire risk and related health-hazards as any traditional house. Actually, even this is an understatement, because due to “all-metal” structure, container’s electrical installations must meet special requirements to prevent shock-hazards and fire. So, for example electric wires must use pipes, all passes through walls must have protective casing etc… what considerably minimizes risks of short-circuit triggered fires (more on electrical installations in a dedicated chapter).
Few technical details:
- Usually, building codes will require to replace industrial doors in houses made from cargo shipping containers. Such doors can be latched from the outside, closing the escape route in the case of fire. They can be kept only if there are alternative escape routes (standard doors, suitable windows etc…
- Cargo shipping containers are usually built as watertight. When converted into habitable spaces, they must be equipped with an adequate and effective ventilation system. Under normal circumstances, ventilation should be able to provide constant air exchange (note, that traditional houses are quite “leaky”, so regardless if you want or not, fresh air is constantly infiltrating their interiors). In case of the fire, ventilation should be able to limit (at least to some extent) the concentration of toxic gases.
- For the same reasons (ventilation), container houses should not use any open-fire stoves (probably it will not be allowed by most local building codes). But frankly, if we want to be (instead of only pretend to be) eco-friendly, we should simply exclude wood-burning stove(s) as a valid heating alternative.
The bottom line is that container-based houses, when properly designed and equipped, are less prone to catch fire and generally safer than traditional houses. Please note that an empty container house is hardly inflammable while any empty residential house will burn like a torch.
Fire-rated gypsum panels are designed for use in interior drywall applications for walls and ceilings. Their core is reinforced with glass fibers, increasing strength and fire resistance. They have FS rating of 15 (Class A) and do not generate smoke when exposed to fire. Source: Georgia-Pacific Building Products
Protection from wildfires
Talking about torch – current building codes are mostly addressing issues related to fires originating inside of the house (cigarette, gas-burning stove, fireplace, iron, short-circuit, candles, etc.
Given interior location of potential fire sources and the nature of traditional residential housing, current fire-safety requirements aim at:
- Delaying the speed at which the fire spreads to allow inhabitants to escape
- Limiting the toxicity of construction materials (by forbidding the use of harmful ones) to minimize the level of health- hazards
At the end, it is assumed that the fire brigade will do a good job (to say it shortly).
It seems however, that in the wake of frequent wildfires stimulated by human activities and climatic changes, current fire-safety requirements may not be up to the task (surely daunting one).
Typically, wildfires helped by strong winds will viciously attack the home from exterior, but the fury will last very shortly and abate once a “surrounding” fuel is gone what most likely will happen in the matter of minutes if not seconds. The induced secondary spot fires (trees, bush and any other inflammable objects like wooden deck….) usually will not have cataclysmic dimensions, and (with the exception of mentioned deck), will not be in too close proximity of the house. So, if the house structure can withstand the first brutal attack, it may actually survive the wildfire.
In such case the scenario is quite different compared to interior fires. Here it is assumed that:
- Inhabitants left the house prior to the wildfire attack (anyhow, they may not be able to survive the attack due to potentially high temperatures, and lack of oxygen)
- Fire brigade will rather concentrate defensive efforts at strategic locations than on individual houses
- House is left to its own destiny that will be determined not only by the intensity of the wildfire but also by its un-inflammable metal structure and level of combustibility of close surroundings.
These facts prompt to conclusion, that container houses (provided they have no inflammable exterior cladding) are preferred housing choices in zones susceptible to wildfires.
Note that steel will start losing its structural strength from about 500 degC (1,000 degF) and melts at around 1370 degrees C (2500°F) it. For that to happen, the container structure has to be exposed to open flames for quite some time, certainly much longer than the typical wildfire attack.
If cargo containers houses use exterior cladding (either for aesthetic reasons or to protect externally applied insulation), in most cases it will be based on modern and rather hardly flammable materials like cement-fiber or MgO panels. Similarly, eventual interior cladding (sheetrock (gypsum)) belongs to hardly flammable materials.
EPS Cement panels are covered with 6mm thick layer of calcium-silicon. It could withstand the temperature of 1000 degC for 4 hours. Source: Kimton Building Materials Co. Ltd (Beijing, China)
In customized container houses, walls and roofs are made from sandwich panels. The metal sheets are quite thin (typically 0.5mm/20 mils) so they will quickly get hot, even at the touch of flames. However, if the Rock-wool is used as the thermal insulation, it will act as a natural, mineral fire retardant, significantly delaying the spread of heat into interiors. And in such a case these extra 10-15 minutes may be enough to prevent the death sentence for the house.
In reality however, container houses are not anymore simple steel boxes. In fact, the windows and doors (if not from metal) will be their weakest points in the line of defense against wildfire. The radiating (or direct) heat can quickly shatter windows exposing mostly inflammable interior to flames. Needless to say, that here the story (and the house) will end. Such black scenario is almost sure to happen in container houses with single-glazed windows, but its probability will be significantly lower when double glazed ones are used instead. However, for better protection, special fire-resistant window glass and frames should be used.
Traditional residential homes have little chances to survive the wildfires. Source: The Guardian
The topography of terrain has its own impact on the spread of the wildfire. For every 10˚ slope increase, the fire will double its speed and intensity. Source: CFA Home (Australia)
- The fire ratings of cargo shipping container houses must be provided by companies converting them to habitable spaces. In the case of DIY modifications, the final words belong to the local authority. So before starting any “adaptation” works you should first get familiar with relevant to this class of buildings safety codes.
- In contrast, customized container houses usually come from manufacturers with appropriate fire and safety certificates. The good news is that typically, they belong to the Class II (B1) of fire safety, meaning that they are hardly flammable.
- If correctly designed and properly equipped (ventilation, alarms, fire extinguisher…), container-based houses are very safe places to live in, offering higher overall security than traditional houses. They also have a much higher chance to survive wildfires compared to traditional residential houses.