How to Cover Your Container Floors?
Plywood boards are manufactured from thin layers of wood (veneers) glued together in a structure in which wood grain of adjacent veneers is rotated by 90 degrees.
Such composition, results in increased strength and stiffness of the board compared to equivalent plain wood boards. Plywood is a traditional material used in residential industry. Due to low cost, and sturdiness, it is also used for floors of cargo shipping containers, although due to atmospheric exposure and insects it must be chemically treated.
Plywood for cargo shipping containers floor (Source: Tianjin Jinbin Huaxing Mechanical Fitting Co. Ltd)
Regular plywood is a widely available, familiar to everybody and easy to work with material with good thermal insulation characteristics. When protected from exposure to moisture, it can be the material of choice. No wonder that some manufacturers of Customized Shipping Container houses still use it for sub-flooring. It should be noted however, that plywood has also few significant disadvantages.
– It is prone to mold and if exposed to moisture it has to be treated to prevent decaying
– In its raw form without fire retardant, it is flammable.
Unfortunately, to counter these effects more chemicals is needed and so its eco-friendliness and impact on human health makes its use is container houses questionable.
There are several versions of plywood with varying characteristics, appearance and cost: Softwood (made from cedar, spruce, pine…), Hardwood (made from oak, beech, mahogany), Tropical (made from various species of tropical timber) and probably the most important here – Marine Plywood.
Marine plywood is manufactured from durable face and core veneers of carefully selected tropical trees (Keruing and Apitong trees, Okoumé family of Meranti trees as well as and to some extent family of Fir trees). They are characterized by fewer defects, higher density and negligible voids between layers compared to veneers used for regular plywood, what significantly limits the probability of trapping water.
Also, the individual layers of tropical wood are bonded with waterproof glue. As a result, the marine-grade plywood is “toughest” of all available plywood, and what’s more important – it is “naturally” more resistant to moisture, fungus and delamination what makes it maybe not an ideal, but rather one of only few candidates for outdoor applications. Unfortunately, despite its improved weather-resistance, it also must be chemically treated because tropical woods attract all kinds of insects.
When it comes to Customized Shipping Container houses, plywood can be still good choice provided it is not treated, well insulated from weather elements (especially its bottom part) and not exposed to moisture.
Similarly as in residential buildings, in container-based houses, the plywood is mainly used as the “structural” subfloor. As being also prone to abrasion (what doesn’t matter in cargo applications), plywood subfloors are usually finished with more durable and elegant materials like vinyl, ceramic tiles, coated hardwood, engineered wood, composites….
Interestingly, in recent years many manufacturers of Cargo Shipping Containers replaced traditional marine-grade plywood floors with bamboo ones. Well, it has nothing to do with potential aesthetics but rather with cost. While wood is a renewable resource, it takes decades to grow trees suitable for marine-grade plywood, so its cycle of renewability is quite long!
It may be shocking to many, but bamboo is not a tree but a grass so when cut, it will quickly regrow what makes it truly renewable resource (big point for eco-oriented people).
When properly selected (not too young stems), processed (removing sugars and starch by boiling and steaming then drying) and manufactured (hundreds of strands of raw bamboo are machine-woven and pressed together), bamboo boards are harder and more durable than typical hardwoods.They also have better mildew and termite resistance.
Being comparable but less expensive than hardwood floors, due to their aesthetic value, bamboo floors are already used in residential construction. Not surprisingly, they may be also considered for use in Customized Container Houses.
Note that in practice, bamboo floors are not equal (in other words they may be excellent, good, so-so and simply bad). It all depends not only on the quality of the bamboo itself but also heavily on the whole manufacturing process. Another potential problem – like most engineered boards, they may contain some “less-identified” bonding compounds that may have adverse effects on human health.
If used as the top floor, then similarly to residential hardwood floors, they will have to be coated with abrasion and water-resistant paint.
Impact of Condensation
In contrast to traditional residential buildings, steel walls and roof of container-based houses create an impenetrable air barrier. When moisture (vapor)-laden air moves through the layer of interior insulation and reaches the cold steel wall, water condensates changing from gaseous to liquid form (more on this subject in the separate chapter dedicated to condensation protection in container houses).
The bottom line is – once the vapor condensates, the water will flow down the steel walls towards the floor. If wooden (plywood) – with time the floor will develop mold, start rotting and eventually decay to the point when replacement will be necessary.
For these reasons, manufacturers of Customized Container Houses prefer using new alternatives for plywood-based floors.
Fiber Cement Board (FCB)
For some, this may sound scary because in the past, such panels were made from a mixture of cement and asbestos fibers. The latter were used to reinforce thin and rigid cement sheets. Fortunately, since long asbestos (proved to be carcinogenic) were not only eliminated, but in fact banned from use in any commercial products.
Present-day Fiber Cement Boards (also known as Fibrolite Boards) are made from a mixture of cement (usually Portland), quartz sand and cellulose fibers. They are durable, stable, Eco-friendly, fireproof, mildew and water resistant and when exposed to fire, they do not release any hazardous to health substances. If this is not enough – they have good heat and sound insulation properties. What is important from the practical point of view – they are easy to work with (can be drilled, cut ….).
Most of them are sanded so they have decorative (and if needed colorful) appearance. While that are often used as subfloors, they can also make perfectly decorative tiled floor. In other words, they have not much common with familiar cement boards used us underlayment for tiles in baths or kitchens to add stiffness and prevent water infiltration.
Left: Fiber Cement Boards (Source: Guangxi-Peakon-Technology), Right: Fiber Cement Tiles (Source: Zhejiang-Headerboard-Building-Materials)
Shortly speaking – they are perfectly safe and widely used as luxurious decorative building materials for decks, floors, facades, roofing, cladding etc… . Most manufacturers offer FCBs with different edge’s profiles, suitable for typical applications.
Available edge profiles of FCBs (Source: Guangxi-Peakon-Technology)
Fiber Cement Boards (Source: Guangxi Peakon Technology Co. Ltd)
Wood-Strand Cement Board (WSCB)
Family of FCBs includes also Wood-Strand Cement Boards. Developed by Eltomation B.V. (Barneveld, Netherlands) and available under the trade name EltoBoards, they have similar characteristics to cellulose based FCBs. They are made from long strands of softwood (pine, spruce, poplar), Portland cement and sodium silicate. EltoBoards have higher density and are stronger than traditional Wood-Wool Cement Boards.
They are eco-friendly (free of hazardous substances), almost weather-indestructible (moisture, freeze/thaw cycles), fireproof as well as insects (termites) and fungus resistant.
EltoBoards found numerous applications in the construction industry (sidings, walls, roofing, subflooring etc…).
Sulfate-MgO panels (Source: OYADE (Shandong) Machinery Equipment Co. Ltd)
MgO (Magnesium Oxide)-based panels are similar to Fiber Cement Boards. MgO (mineral commonly known as Magnesia), when grounded into powder and mixed with water then properly cured it forms strong, impact resistant yet flexible cement-like panels. In practice, the composition of most MgO-based panels is a bit more complex.
To achieve required characteristics of the final product, the mixture may also include expanded perlite (foamed structure of volcanic glass acting as water-repellent), cellulose and reinforcing fiberglass mesh.
MgO panels are fireproof, in fact they make excellent fire barriers as they do not burn, and neither release smoke nor toxic gases (MgO boards do not contain any hazardous compounds). They are waterproof with good resistance to mold and mildew, although they do not match FCBs in terms of moisture absorption. They are inedible for insects (termites) and easily workable (cutting, nailing, fastening etc..). MgO boards have relatively low R-values (around 1.2), so they are not considered as thermal insulating materials.
Unfortunately, when the basic mineral includes also Magnesium chloride (MgCl2) (known as Sorel’s cement), in contact with moisture (water) it may produce corrosive environment. The answer to this problem seems to be Sulfite-MgO boards – mixture of MgO, MgSO4 (Magnesium-sulfite), perlite and sawdust (waste product of wood industry) reinforced by fiberglass mesh. Such boards have almost identical properties as MgO boards but are not corrosive.
Note that MgO deposits are widely available in Asia, Europe and Canada but not in the US (probably that’s why MgO boards are not very popular there).
MgO Boards (Source: Zhangjiagang Leader Import and Export Co. Ltd)
Typical composition of MgO board (Source: Zhangjiagang Leader Import and Export Co. Ltd)