Modular concrete houses

To discuss modular concrete houses, we must first go back to the beginnings of the modular passive house, which originated nearly 100 years ago. This model includes design and construction in an “all-in-one pack” format that began in the United States, a country historically characterized by the flexibility and streamlining of bureaucratic and legal processes.

In the early 1900s, some companies began selling houses that had been previously designed and integrated into a catalog of models that consumers could choose and implement on a plot of land.

While it is true that this model was easily established in the US due to its lax building regulations, the same model in Spain, which arrived around the 1970s, was more difficult to implement as the rules of the game were somewhat more rigid.

In Spain, before building a house, we must: commission a project drafted by an architect who signs and takes responsibility for the integrity of the structure, have it supervised by a College of Architects, and obtain approval from a city council with specific regulations and a municipal architect who reviews it before granting a building permit.

Today, the business model has evolved and modular houses are no longer just houses that arrive by truck from a catalog; instead, bespoke projects can be carried out with different models adapted to each plot and assembled with great precision in a factory.

The options and levels of prefabrication are many, timelines and construction systems vary from one company to another, and competition is very high, so it is important to understand what options are available on the market and what characteristics and advantages we gain when selecting one option over another.

Let’s look at the current landscape of modular concrete houses:

Types of modular houses

Throughout this article, we will focus on detailing the different types of modular concrete houses. There are numerous types of modular houses, which we usually classify in the firm based on their materials and, within each group, according to the construction system used.

Both characteristics will determine the comfort, efficiency, and health of our home—and consequently, the energy expenditure and the quality of the indoor environment.

Modular houses can be built using different construction systems and materials. Depending on the materials used, there are modular houses made of concrete, steel, or wood; we will cover the latter two in subsequent articles.

Is a prefabricated house cheaper?

Prefabrication allows us to save time and costs. As it is a construction assembled previously in a workshop, excavation, containment, and foundation work can be prepared on-site while the volume of the house is being worked on in the factory.

Prefabricated house factories usually repeat wall systems or modules, so the process is standardized and costs are reduced.

However, perhaps the debate is not merely economic; we must keep in mind that a passive or ecological house is much more than just a functioning structure. In this case, the bioclimatic design of our home is a key factor in achieving a zero-consumption house, comfort, and indoor health. It is important to note that not all construction systems allow for design flexibility, and not all systems marketed as efficient guarantee a healthy indoor environment.

If we want to achieve an efficient and healthy home, it is important not to settle for just any house that fits our budget. Today, it is possible to achieve healthy and efficient homes at a reduced cost if the architectural team has sufficient experience to allocate the budget to those truly important items. Being swayed by the image of final finishes, large windows, or finishes sold as high-standing can lead to building a “cardboard-stone” house where the construction system and structure do not meet what are considered minimum construction standards today.

There are no bargains in construction; good construction systems, efficiency, and the health of the indoor environment are expensive. Therefore, when opting for an industrialized system, it is important to understand what it consists of and if we are actually getting a home in proper condition.

Let’s look step-by-step at the reality of building modular concrete houses, with all their advantages and disadvantages:

1. Environmental impact of concrete

Concrete is a material composed of cement mixed with water, aggregate, and additives. Cement is the key component of the mixture, as its function is that of a binder, like a glue that holds everything together. Although cement comes from limestone, a natural resource, it cannot be considered an ecological material because its transformation process requires blast furnaces, which currently account for a total of 5% of the planet’s CO₂ emissions.

If we go further, we find that the high environmental impact of cement is proven in the report ‘Most Polluting Companies in Spain: Responsibility of Large Energy and Industrial Companies in Climate Change 2006-2016’, prepared by the Sustainability Observatory in 2017.

The report highlights that the 10 most polluting companies in Spain are electricity and cement companies, including CEMEX and Lafarge-Holcim, which emit 3 million tons of CO₂ per year. Consequently, the cement industry accounts for 11% of the country’s CO₂ emissions, ranking second behind electricity companies.

In addition to all this, concrete is usually associated with the use of steel, since the combination of both materials—known as reinforced concrete—allows for greater strength in building structures, as one works in tension and the other in compression.

Steel cannot be considered an ecological material either, despite being a natural resource, as its extraction process from rock again involves a high energy cost. Furthermore, any manipulation will once again involve blast furnaces, accounting for up to 5% of the country’s CO₂ emissions.

2. Impact of concrete on health

As mentioned earlier, concrete is not only harmful to the health of our planet but also to that of the users who interact with it, especially if it involves daily contact.

The first characteristic is its lack of breathability, as concrete is a closed-pore material. Materials that do breathe are called hygroscopic, which means they have the ability to regulate humidity naturally, absorbing and releasing moisture from the environment. Materials such as wood or ceramics manage to stabilize the humidity inside our home, whether it is a lack or excess of it, through natural absorption-release regulation according to indoor conditions.

We must keep in mind that below 30% relative humidity, the indoor environment of our home is considered too dry; our mucous membranes dry out, our defenses are reduced, and we are more prone to getting sick.

On the other hand, the steel associated with reinforced concrete contains materials in its alloys such as zinc, tin, or lead, which are declared harmful to health and are in fact included in the blacklist of the Risctox platform, which catalogs substances based on their toxicity.

Types of modular concrete houses

Below, we analyze a series of providers, comparing the type of solutions they offer and what they imply in terms of construction and costs, but also in terms of efficiency and sustainability.

1 Modular houses made of prefabricated concrete panels

Prefabricated concrete panels consist of large modules acting as reinforced concrete facade fragments, previously executed in a factory, which fit together to form the building envelope. These panels are usually structural.

Depending on their composition, we find prefabricated concrete panels with traditional steel reinforcement, but there is also glass fiber reinforced concrete (GRC), which has represented a real revolution, especially in terms of reducing thickness, weight, and environmental impact.

Their behavior allows us to distinguish between load-bearing or self-supporting prefabricated concrete panels. Load-bearing panels are part of the building’s structure, transmitting forces to the ground or foundation. Since they function as both enclosure and structure, they allow for the elimination of pillars. On the other hand, self-supporting panels act as an exterior enclosure or interior partition, anchored to the structure, usually by bolting.

Depending on their section, we can find homogeneous, hollow-core, or multi-layer prefabricated concrete panels. Homogeneous panels consist of a solid reinforced concrete prism, so they will need to be insulated and clad afterward. For their part, hollow-core panels manage to reduce slab mass and weight through a corrugated shape that provides greater lightness to the whole without reducing structural inertia. Finally, the multi-layer system incorporates thermal insulation during manufacture, which reduces time and costs on-site.

Generally, when talking about construction systems for the prefabrication of modular concrete houses, we distinguish between two classifications based on the panel configuration:

Heavy panels for modular concrete houses

Heavy panels are usually structural, meaning they work vertically to support slabs. Their thickness can vary from 14 to 20 cm, with the possibility of a length of up to 10 m, thus allowing necessary installations to be embedded.

However, they are designed to be lined except for industrial uses, which implies more work on-site. Their maximum dimensions are determined by transport and are around 12 by 4 meters.

Lightweight panels for modular concrete houses

For their part, lightweight panels reinforced with GRC (Glass Fiber Reinforced Concrete) contain 3-5% glass fibers, allowing us to reach thicknesses of 8 cm with high mechanical resistance to bending and impact.

By eliminating conventional corrugated steel reinforcement, we save the minimum thickness required to ensure the steel does not rust. However, these are always non-structural panels, mere enclosure elements that support their own weight and wind loads.

Although we cannot claim that the option of building with prefabricated panels for modular concrete houses is ecological, lightweight GRC panels will be less polluting as they use less material and eliminate the presence of steel.

2 Modular houses with concrete frames

The concrete frame system is composed of both pillars and girders (beams) that are prefabricated and reinforced with steel. There are also entire prefabricated frames for industrial buildings, i.e., pillars and beams assembled at the factory.

In any case, it is usually the structure on which we will place the prefabricated concrete panels from the previous section, normally anchored by bolting.

This type of system involves the use of large cranes but manages to reduce the construction period to record times of up to 4 months. Both elements, pillars and girders, are composed to generate a three-dimensional grid, at which point the connections between these elements become crucial.

To this end, the pillars (continuous for the different floors down to the foundation) usually have protruding corbels that allow the girders to be supported or anchored. The different types of connections consist of direct support, semi-rigid, or rigid joints.

3 Concrete block houses

The debate regarding concrete block houses is essentially the same as that for modular on-site construction, since although the small block component is prefabricated, it must be assembled piece by piece on-site. Perhaps the greatest advantage of this system is the low cost of the piece and the design flexibility it provides.

Let’s look at the different systems we find on the market if we opt for the concrete block option:

Hollow blocks

Typical dimensions for a hollow concrete block are 20x20x40 cm, and they are used to form load-bearing walls that must be reinforced by passing steel bars through the hollows of the small component, which are then joined with mortar.

Their production process is similar to that of common bricks, but in this case, we do not need firing but rather a curing process.

Large-scale blocks

A novelty is large-scale blocks, which allow for faster construction times as less time is invested in placing them, since we are talking about fewer necessary pieces that consist of a tongue-and-groove system, as if it were Lego. Typical dimensions are 180x60x60 cm or 160x80x80 cm.

The main drawback of this system is, of course, the increase in weight, which requires the use of machinery for assembly on-site.

This solution is used primarily in retaining walls since, at a structural level, it works very well without the need for steel.

4 Houses with the Ytong system

One of the most innovative systems to have appeared recently is modular concrete houses built with the Ytong system.

The Ytong system is based on the use of blocks made of autoclaved aerated concrete, a very light material that, although presented to us as an ecological material, needs to be assessed in detail.

The Ytong system allows us to use only 1 cubic meter of raw material to manufacture 5 cubic meters of final product. This reduction in mass, and therefore weight, simplifies the transport and on-site installation process, which no longer requires the use of heavy machinery.

The blocks, with dimensions of 60 x 25 cm, variable thickness, and a lateral tongue-and-groove system and handles, are easily handled on-site by workers.

The supposed advantage of this system is that it promises to solve structural strength, thermal and acoustic insulation, climatic comfort, and fire resistance requirements through a single wall.

While Ytong has high structural resistance and avoids the use of steel to reinforce the wall, one of its supposed advantages is the fact that, by containing air in small cells, it should function not only as a structural system but also as thermal insulation. In practice, this statement is not true, as it depends on the climatic zone in which we are located. There are locations where, depending on the climate, it is possible to present the Ytong wall as the sole solution, while in other locations with greater thermal variation in winter, this system is not sufficient and requires additional insulation.

Furthermore, we must keep in mind that some components of Ytong, in addition to the concrete itself, are presented as toxic to health. Therefore, in our view, we believe more studies and rigor are needed before claiming that Ytong is the ideal solution in terms of sustainability, thermal inertia, or insulation.

Finally, the single-wall Ytong facade solution uses blocks 25 cm wide, a much greater thickness than what we observed in prefabricated concrete panels. In any case, we are talking about a system that challenges the lack of sustainability of concrete thanks to new technologies, and which has begun to be used in pioneering projects such as Life Reusing Posidonia, a development of 14 social housing units in Formentera that is a pioneer in the use of natural materials like the posidonia seagrass traditionally used as insulation.

5 Prefabricated concrete modules

Finally, we find the solution of prefabricated concrete modules.

We are talking about a solution carried out in large workshops or factories that have a significant investment behind them and need to be in continuous production. These are factories that integrate small concrete plants, allowing concrete to be generated and cured under stable temperature and humidity conditions throughout the year—a great advantage considering that outdoor curing processes always depend on climatic conditions.

In these plants, freshly produced concrete is poured into large molds that form the enclosure walls of each housing unit. These modules are then joined together in the same factory or on-site—depending on the manufacturer—to form a home, substantially reducing construction time and completely controlling the rigor of the details.

If possible, installations and finishes are also usually executed in the same factory, transporting the house mold—or part of it—to the site in a custom-built format, where even the kitchen may already be installed.

An example of this system is the Student Residence in Sant Cugat designed by H-Arquitectes, manufactured by the company CompactHabit.
Its construction was carried out in incredibly short timelines: 2 months of on-site work (foundations, sanitation, and installations), 2 months of module production in the workshop, 2 weeks of transport and assembly of the modules on-site, and 3 months of general finishes.

In the case of modular concrete houses in the form of prefabricated modules, the limit is set by the road: firstly, because it is not profitable to transport modules beyond about 400-500 km, and secondly, due to the maximum legal dimensions for road transport. It is for this reason that these types of houses are made up of different modules assembled together, a fact that limits our floor plan design flexibility.