Design Process
What is a net-zero energy home?
Building a net-zero energy home is the dream of anyone planning to build their own house. Who would not want to stop paying gas, electricity, and water bills?
But is that possible nowadays? Can we obtain all the resources we need to live from the land, the sun, the wind, and rainwater?
The answer is: it depends:
– It depends on our plot and its resources and characteristics.
– It depends on our lifestyle and our budget.
Building a net-zero energy home requires a great deal of commitment on our part, from reducing consumption to maintaining each of the systems that make up the house.
Some people are surprised that this can be achieved, because until a few years ago, energy was cheap and abundant. It was not until a few decades ago, with the first oil crises and the constant rise in the price of oil and electricity, that we began to worry not only about the initial cost of the house, but also the cost of its ongoing maintenance.
In fact, this phenomenon is what some call the second mortgage, or the energy mortgage, which sometimes matches or exceeds the construction cost of the house and which we pay in the form of a monthly bill to energy and/or water utility companies.
In traditional vernacular architecture, a certain level of comfort was achieved centuries ago using only architectural strategies that were adapted to the climate and the place. It is knowledge that seems to have been forgotten by newer generations, yet it is more relevant than ever: building a net-zero energy home today is based precisely on this—making the most of the resources available on site.
What is a net-zero energy home
As we have already mentioned, a net-zero energy home is a dwelling that does not require any type of energy source or external resource input to meet its occupants’ comfort needs.
This is achieved thanks to two basic characteristics: it requires minimal energy, and the little it does consume always comes from renewable sources.
In fact, before the Industrial Revolution and the exploitation of coal and oil as abundant, cheap energy sources, all homes were net-zero, since there was no energy beyond natural resources.
Traditional homes had to be adapted to the climate and make use of the resources available to them. Solar orientation, wind, rainwater, or wells had to be maximized, and waste had to be managed by producing compost or reusing water.
At our latitude, the four seasons are clearly defined. That is why the climate strategy of our homes was to achieve high thermal mass to regulate heat exchange with the outside.
The traditional net-zero energy home
To understand what a net-zero energy home means today, we will look back a little. It is important to understand what we have learned over generations and what seems to have been forgotten due to the availability of fossil fuels.
On this journey back in time, we will look at how a typical rural house in Catalonia worked: the traditional Catalan masia.
History of the Catalan masia
The masia emerged between the 10th and 13th centuries as an isolated dwelling in rural land. It is a type of construction almost always linked to agricultural and livestock farming.
Its variety of uses and its isolated location relative to other buildings and structures were used to provide heat from the sun. In addition, the masia was also designed to take advantage of the heat produced within the house itself.
The use of local materials was also one of its basic characteristics, as transport at that time was very limited or rudimentary. In other words, net-zero was not limited to how it operated, but also to how it was built.
How a masia worked
The traditional house had to function and provide comfort at a time when there was no heat other than the sun or fire. For that reason, the house relied on a series of strategies that made it possible to achieve a pleasant indoor environment.
The first of these was thermal mass. The walls, both interior and exterior, were between 50 and 70 cm thick.
The concept of thermal mass refers to a material’s ability to store heat.
In a way, it also acts as thermal insulation, since the heat from the sun in summer would take several hours to reach the interior. It would enter at night, which is when it is coldest in both winter and summer. In this way, in summer it would dissipate through open windows, and in winter it would be stored.
The layout of the rooms was also a design consideration. The masia typically consisted of two floors plus an attic for storing food.
On the ground floor were the space for the animals—which give off heat—and the kitchen and main living room, which also generate heat. In this way, the bedrooms on the upper floor were also heated internally.
The daily schedule was also different. The day began at sunrise and ended at dusk. So they almost always had natural light and did not depend on electric lighting.
Cooking was done with firewood, water was fetched from the well, and the light they needed came from candles.
While the masia was an optimized construction for its time, I do not think anyone today would want to spend the afternoon chopping firewood when they get home just to be able to take a hot shower. Therefore, let us review in detail how to build a net-zero energy home today.
Building a net-zero energy home today
Today we cannot ignore the technology available to us and our current lifestyle. At least in our society, basic survival is more or less guaranteed and our jobs are less manual. We have adapted to a pace of life different from that of the inhabitants of traditional homes.
Therefore, building a net-zero energy home today means drawing on knowledge from traditional architecture while also incorporating today’s technology.
This perspective allows us to live more comfortably, spending less time on home maintenance.
There are examples that use different energy-efficiency strategies to achieve net-zero, from entire neighbourhoods such as BedZed in London, where residents live and work in the same home—creating a sustainable community that manages its energy, resources, and waste—to buildings that produce more energy than they consume, such as De Bonne Energie in France, which is being certified with the Passiv Positive label.
Today, however, I will refer to an example of how to build a net-zero energy home that has been widely published and awarded: the Canopea project, winner of the latest edition of the Solar Decathlon, an international competition for project ideas to build a net-zero energy home.
A net-zero energy home powered by solar energy
The Caponea project, in the French countryside, consists of building a net-zero energy home that can be stacked to form a residential building.
The project team wanted to maintain the advantages of a house in a rural setting while also adapting to France’s growing need for housing.
Passive strategies
One of the strategies prioritised when building a net-zero energy home is its orientation. In this case, since the project is designed to be stackable, it is not possible to orient all the main rooms to the south, so the east or west orientation is used to receive natural light.
France has a climate similar to ours, although somewhat colder. That is why it was very important to take advantage of as much radiation as possible in winter while also protecting ourselves from the sun in summer.
Thus, around the homes a semi-outdoor space is created that opens or closes depending on requirements, functioning like a greenhouse that captures sun and releases it to the interior when needed.
The openings are located behind this first outer skin, which acts as a solar filter. The main space has a large window, while the other rooms have natural lighting and ventilation facing this outdoor space, with more controlled light.
Due to the requirements, and as we always emphasise, the appropriate thickness of insulation is also important. Of course, all technical details have been designed to avoid thermal bridges.
The competition also required water use. In the Canopea project, a roof tank is installed to collect rainwater. Using gravity alone, part of the water is brought down to irrigate the plants, passing through a filtration area. The rest of the water is collected in 10 m³ tanks.
At the centre there is an area of urban agriculture. It is a vertical garden, that is, platforms on which the plants are stacked. They are irrigated, of course, with the water collected on the roof. This garden area is intended mainly for residents to grow vegetables for their own consumption.
Active strategies
To heat the homes, a radiant ceiling is used, a system that is not very common in our country. The concept is practically the same as underfloor heating, but in this case radiant panels are installed in the ceiling.
The ceiling has an automated system that regulates by itself the amount of energy it must provide, thus reducing consumption to a minimum.
The energy supply is achieved thanks to photovoltaic panels. In Canopea, they are installed on the roof, around the perimeter of the water tank. Photovoltaic panels generate electrical energy through solar radiation. This will later be used for functions such as lighting, cooking, or heating the homes.
In addition, all the energy generated by the photovoltaic panels is stored in batteries shared among the neighbours. Energy storage is essential to ensure constant availability, but the building is also connected to conventional electricity supply networks in case of need.
All these systems are parameterised through a home automation system that controls the different parameters according to the climatic conditions at the time. This ensures the right level of consumption to guarantee comfort.
Although Canopea is a competition entry, we can see that the reality of net-zero is not as far away as we sometimes think. We only need to draw on what we have learned over generations and apply the new technologies within our reach.
At our studio, building a net-zero energy home is the main objective every time we start a new project, and in fact, we are achieving very high efficiency standards depending on each family’s requirements.