Design Process
Modular passive house
“Could you tell me the price of a 144 m2 house?”
This was the email we received a few weeks ago at the Slow Studio office, and it left us astonished.
How can someone need exactly 144 m2?
Normally, we are asked for information regarding a 3 or 4-bedroom house, or perhaps a house of about 140 or 150 m2, but the price for exactly 144 m2 is somewhat peculiar.
Usually, when a client writes to request such a specific surface area, it is for one of two reasons. Either they have drawn the plans themselves and measured every single room to calculate the total, or—as is more common—they are comparing prices with one of the thousands of prefabricated house companies found online today, and the quote sent by the competition is for a model home named “Serena,” “Milenium,” or some other fanciful name of exactly 144 m2.
In recent years, countless companies have appeared online featuring a couple of 3D images of “modern houses,” promising reduced costs and construction times. Furthermore, as it is currently in vogue and all clients request it, they also market them as efficient or passive, much like someone selling a car with various optional extras.
The concept of a modular or prefabricated house is imported from the United States, where extreme consumerism has invaded all sectors, including construction, and where it is possible to buy a house from a catalog that arrives on a truck.
However, here in Spain, construction regulations are somewhat more traditional, and the concept of a “ready-made” house is not realistic. In our country, before building a house, we must commission a project drafted by an architect who signs and takes responsibility for the structural integrity, a College of Architects that supervises it, and a town hall with specific regulations and a municipal architect who reviews it before granting a building permit.
In this scenario, prefabricated house companies face more complications than in the United States, as they cannot offer their houses directly from a catalog; they require an architect to sign off on every new project.
Because of this, construction companies appear with low-cost architects who draw a layout at the client’s request and strictly adhere to the construction system set by the company, without further investigation or genuine interest in the family building the home.
The client is sold a “star” construction system—whether wood, concrete, or steel—as the solution to all their problems. Since they usually only have one factory or specialize in a single system, they will sell the wonders of that specific system.
Other firms like ours still survive with that romanticism, coupled with high technical rigor, which involves working hand-in-hand with the client to decide on the design—beyond just a room layout—and evaluating the best construction system based on the use of the house and the local microclimate.
This does not mean that at the end of the process we cannot choose to build with a modular house company; at Slow Studio, we work extensively with companies that produce prefabricated wood structures. However, we are the ones who, together with the client, evaluate the options and instruct them on how to carry them out, compare prices from different suppliers, and define the installations according to bespoke passive house performance.
During this process, perhaps the most important aspect—and something we have emphasized in numerous articles—is the distinction between the role of the architect and that of the builder.
If I go to a prefabrication company to request a modular passive house and the same company offers me the architect, I can be certain that this architect—whose salary is paid by the builder—will be beholden to their boss’s directives, which likely prioritize minimum construction costs and maximum profit for the company.
If they can save on a few centimeters of insulation, use a cheap toxic plastic paint, or install an air ventilation system for heating and cooling simply because it is the cheapest option, they will do so without any regard for bioclimatic performance or the interior environment generated within the home.
If, on the other hand, I go to an architect whom I hire independently and together we evaluate different builders—whether for a modular passive house or a conventional one—I have the guarantee of the experienced opinion of an independent professional who is on my side to assess costs and systems.
So, it is not all bad news in the world of prefabrication; yes, it is possible to achieve a modular passive house with certain guarantees if there is a rigorous architect with high technical knowledge behind the project and the selection of the builder.
But within the modular passive house market, we have many options, from the classic Finnish wooden house—with a high CO2 footprint from timber transport—to local timber construction companies with highly specialized structures and factories, to concrete prefabrication companies repurposed as house sellers, or the ones we like least: steel prefabrication, a material poorly suited for a home environment and with a high ecological footprint.
Let us then look step-by-step at how to discern between the good and the bad.
What is a modular passive house
First of all, we will attempt to go into detail about what a modular passive house is—a type of construction that, as its name suggests, consists of a set of modules that can be made of wood or concrete and are usually produced in a workshop or factory.
The term modular house also applies to rapid or low-cost construction systems, such as steel systems whose studs could be considered prefabricated, or synthetic insulation wall systems covered in concrete or cladding whose pieces are also considered “modular.”
Regarding concrete, there are houses today whose modules are based on standardized measurements, or houses manufactured in large “molds” that pass through an assembly line where the interiors and installations are completed.
When we add the term “passive” to the modular house, we are referring to the fact that these houses supposedly feature a design that maximizes solar heat gain, increases insulation, and reduces energy loss at joints and openings, thereby reducing energy demand. This can be achieved with any construction system provided there is rigor in the construction, but we do not always ensure an interior environment with suitable conditions for living.
If we also speak of an ecological house, then we must take into account that we will discard most materials. Ecological design places great emphasis on the materials used, prioritizing natural, minimally processed, and toxin-free materials that are also locally sourced to minimize their ecological footprint.
In fact, this last point—the CO2 footprint due to transport from the factory—is a factor to consider when evaluating one system over another. It is not very coherent to build a modular passive house that comes from Finland if we have wood in the Pyrenees in Spain, which, although smaller in dimension, is equally suitable for construction. Here are some ways to build a wooden house in Catalonia, which represent a new trend.
What types of modular passive houses can be found on the market
We have taken the trouble to analyze a series of providers—some we already knew or have worked with hand-in-hand in the construction of some of our houses, and others we have researched to see what solutions they offer and if they convince us.
Let us look at the different modular passive house options available on the market today based on their modular construction systems. As we will see, each construction system is based on the material used, which determines the comfort, efficiency, and health of the project. Furthermore, the same construction system can be carried out with very different qualities depending on the type of company executing it.
Wooden modular passive house
Probably the most widespread construction system and the material that sounds most ecological is wood. It is true that wood is a highly energy-efficient, natural, renewable, and breathable material with a negative ecological footprint—meaning it absorbs more CO2 than it emits throughout its useful life. Additionally, it is a material that is easy to work with in a factory, allowing for rigorous control of details in the workshop and ensuring high airtightness of the building envelope.
However, we must consider the ecological footprint of its transport from the forest where it is felled to the sawmill and subsequently to the production site. Another parameter to evaluate is the source of the timber, which must always be PEFC or FSC certified, guaranteeing proper forest management.
In our country, we have modular passive house companies that have their own sawmill and therefore control the quality of the wood from start to finish. Other companies simply buy the wood from third parties but have large production facilities, often coming from families with several generations of experience in carpentry and woodwork, whose expertise is vital for the successful execution of the project.
Within wooden modular passive houses, there is a subtype called Finnish houses. Including “Finland” in the name of the house seems to provide a guarantee of quality and experience, and for good reason; Nordic countries have a long tradition of wooden construction and forests with forest management for decades that guarantee slender, large-dimension timber worked with highly technical machinery. However, beyond the good quality of this type of modular passive house, it is worth questioning the climate cost in relation to the CO2 emitted by transporting such a modular passive house to Spain.
Steel modular passive house
The steel construction system, known as steel frame, is currently in fashion. They are marketed as next-generation prefabricated houses—highly efficient, extremely cheap, and easy to build. It consists of a steel structure similar to that used for drywall partitions, usually covered with polystyrene-type insulation—known in construction as EPS—and a finish of sprayed concrete or any other low-cost paneling.
As we often say at Slow Studio, these types of houses are literally “steel cages covered in plastic,” a place very poorly suited to house a family’s life.
It is a solution that lacks thermal inertia, which is very useful in our modular passive house for storing the day’s heat and utilizing it at night. It often generates many energy efficiency problems and thermal bridges due to the high thermal conductivity of steel.
Other disadvantages of steel include its low availability as a material with limited reserves, in addition to its high transformation cost, making it one of the raw materials with the largest ecological footprint. A house with this type of solution becomes a metal cage that stores all kinds of electromagnetic waves inside (3G, Wi-Fi, wireless devices, alarms), also interfering with the Earth’s magnetic field.
Furthermore, these types of homes are usually heated with an aerothermal system, typically connected to a hot air diffuser that significantly reduces ambient humidity. A house with humidity below 30% is not suitable for living as it dries out mucous membranes and reduces our defenses; additionally, dust rises along with microorganisms and ends up in our respiratory tracts. It is important to remember that when we speak of an ecological home, we refer not only to environmental awareness but also to the health of the users who will live in it.
EPS block modular passive house
In this case, the structure consists of EPS blocks or similar, which are prefabricated polystyrene-type panels that, once assembled, form the walls of the dwelling. The companies that market it claim it solves insulation, cladding, and space for installations in a single product.
The finish of this type of modular passive house is usually executed with sprayed concrete, in addition to pouring concrete into the block cavity to provide stability. Despite being a hybrid system, we are looking at a solution very similar to reinforced concrete lined with EPS insulation. In practice, we achieve an airtight enclosure based on synthetic blocks with a high ecological footprint—since they come from petroleum—that do not breathe, thus accumulating moisture and condensation inside. Concrete is also a material with a high ecological footprint that includes a large amount of toxins in its composition, and we must not forget that it also incorporates steel, the disadvantages of which were listed in the previous section.
Concrete modular passive house
The reinforced concrete construction system is well known. In this case, we are talking about its prefabricated variant, which is usually offered by industrial warehouse prefabrication companies repurposed as builders of supposedly efficient houses.
The system for this type of modular passive house is based on a series of precast steel-reinforced concrete panels, which are significantly thinner than those used in on-site construction. These panels are installed and subsequently insulated and clad. The advantages include a fixed price and a 7-month construction period.
Also in concrete, there is a new construction system that many clients ask about at the office. It is called Ytong and consists of blocks of so-called autoclaved aerated concrete, a type of lightweight concrete marketed as an ecological material. With it, they aim to solve all technical issues in a single wall: structural strength, thermal and acoustic insulation, climatic comfort, and fire resistance.
While it is true that we save on steel in our construction thanks to the load-bearing capacity of these blocks—one of the most polluting materials—it is difficult to defend cement as an ecological material. Despite coming from limestone, the transformation process requires blast furnaces and accounts for 5% of total CO2 emissions worldwide. Ecological cement alternatives have already been invented that replace limestone with solid waste from thermal power plants, saving the calcination process in blast furnaces.
Some final reflections
Greenwashing
As a consequence of the lack of purchasing options in the ecological market, the phenomenon of greenwashing appears, where products are sold to us as ecological when they are not. We enter into contradictions such as talking about a ‘modular passive house’ with a steel structure and lined with plastic.
The trick lies in diverting attention away from ecological materials and bioclimatic strategies to concentrate on technicalities and certificates. This is the case with energy certifications, which simply take into account the energy consumption of the future home and overlook the origin, transformation, and transport of the materials we use.
Fixed price
A fixed price much lower than that of a conventional home is always a fantastic advertising hook. However, it is necessary to confirm the total investment, as the prices given by these types of companies often do not include items such as foundations, the building permit, or VAT and other taxes.
Furthermore, a fixed price usually means there is no possibility of applying modifications to the design of our future modular passive house, which prevents any adaptation to our needs. Often, prefabricated house companies work with in-house architects who stick to drawing a layout; we save on the design process but lose quality and rigor in the most important part of building our future home. By operating with an architect subject to a series of standard solutions, the cost is greatly reduced, but so is the architectural quality of the result and the options available.
At our sustainable architecture studio, we never tire of reminding people that an ecological house is a lifelong investment that we must be more than sure of, dedicating all the necessary effort to it.
Indoor environmental quality
Another point we have touched upon is the distinction between efficiency and health. Today, great importance is given to the term efficiency because it represents financial savings or the so-called energy mortgage. However, as human factors seem to sell less, little emphasis is usually placed on the indoor environmental quality of our future home. Saving on health does not seem like the best investment, and often health is not a factor taken into account when deciding on our future house.
Which type of modular passive house to choose
In conclusion, of all the types of modular passive houses we have analyzed, the only one that holds up is the local wooden modular passive house model, always monitoring its origin and extraction. There is a certification called FSC that verifies forest management meets a series of internationally agreed ecological standards. However, this certification does not ensure that the material is locally sourced and therefore does not quantify the CO2 footprint it includes as a result of its transport process.
On the other hand, the importance of direct contact with the architect is clearly reflected, particularly for the personalized design phase within the parameters of modular standardization. Simultaneously, this design must integrate bioclimatic strategies that make the project a passive home.