Design Process
Differences Between a Passive House and a Passivhaus Home
Last week, there were some conferences in Barcelona on housing projects. One of the speakers presented a project of 100 homes for seniors in the city, which she had completed 10 years ago. The first thing the architect explained was that if she had undertaken this project now, the approach would have been completely different; the initial project arguments would have focused on efficiency, net-zero consumption, the selection of local materials, and resource saving.
How could we have changed our mindset so much—and for the better—in so few years? Climate alarms have sounded, and it seems that the idea that things have been done very poorly and that we all need to change how we eat, dress, travel, build, and live to be more respectful of the planet we inhabit is beginning to resonate with everyone.
For us, it changed many years ago when we altered the way we work, the way we build homes. We realized that the place and manner in which we live have a direct relationship with a family’s ecological footprint. Emissions from the energy consumed by the operation of our homes represent 60% of the planet’s total CO2 emissions.
This needed improvement, on the one hand, by building net-zero or passive homes, and on the other, by selecting materials and construction systems in harmony with the environment, natural, and locally sourced.
This is how Slow Studio was born, an architecture firm specializing in the construction of passive and healthy homes.
Fortunately, today we are no longer alone, and as in other fields, the message of doing things well has taken hold. Many architects and construction companies now only offer passive homes because that is what the market demands.
And like everything else, when there is a lot of competition and options, organizations emerge that try to provide reliability and guarantees to companies that do things well. This is what has happened with the Passivhaus certification, a German standard that seeks to verify, according to strict construction standards, that a house operates passively and consumes no energy.
Today, we discuss the difference between a passive house and the Passivhaus standard, two distinct but complementary concepts, as a passive house can be Passivhaus certified.
Let’s begin by thoroughly defining each concept.
What is a passive house?
The concept of a passive house refers to a specific type of design that summarizes a series of bioclimatic strategies aimed at avoiding active energy consumption.
In other words, a passive house is one that, due to and thanks to its design, helps us maintain comfortable indoor conditions—in terms of temperature, humidity, etc. All these environmental conditions are more important than we realize for our health, as well as that of our planet.
When we speak of active energy, we refer to services that rely on energy to function, whether renewable or not (electricity, gas…). A large part of this energy is consumed by heating and air conditioning to generate thermal comfort levels.
It is commonly said that, in a passive house, between 80% and 90% of its operation stems from a good bioclimatic design strategy. The remaining 10% corresponds to the minimum heating input required in a house on days of extreme cold and lack of sun in mid-winter, something that, fortunately, in our privileged Mediterranean climate, only occurs for a few weeks a year.
Architectural and climatic strategies, which we will elaborate on later in the article, such as controlling openings (i.e., orienting windows to the south), using heat-collecting walls to replace heating, and concepts like thermal inertia and ventilation, allow us to achieve the goal of living in an off-grid home or even a net-zero home, meaning it relies solely on passive strategies to create habitable conditions.
What is a Passivhaus Home?
When we refer to a Passivhaus home, we are speaking of a German certification created in 1988, which aims to certify whether a building meets the technical levels to be considered a passive house.
Thus, the Passivhaus standard establishes near-zero consumption for buildings and has promoted the construction of energy-efficient and economically affordable buildings for just over 20 years. It is an internationally widespread construction concept that has been applied in over 25,000 buildings.
One of the most relevant requirements among the 5 points we will discuss later in the article is that heating consumption must be less than 15 kWh per square meter per year.
To familiarize ourselves with these energy values, we could say that it is a house that consumes less energy than a minor appliance like a hairdryer.
Both Germany and the Nordic countries have been pioneers in this sector. Recently, the Passivhaus Plus certification has emerged, an extension of this standard granted to homes that generate more energy than they consume.
Differences Between a Passive House and a Passivhaus Home
Following these two descriptions and answering the question we posed at the beginning of the article—what are the differences between a passive house and a Passivhaus home?—we see that we are discussing two complementary concepts, despite being of different natures.
Thus, a passive house can function passively without being certified, just as it can be conceived as passive and utilize many passive strategies without meeting the conditions established by the Passivhaus standard.
At Slow Studio, we design many homes that meet the Passivhaus standard without the need for certification, as there is the option to do so later, as we will explain below.
Conversely, a house certified with this standard will always fall into the category of a passive house, without exception, as the certificate serves to quantify the basic concepts of bioclimatic design.
We must understand that there are many levels of passive houses, as ultimately their efficiency will depend on the client’s budget, the local climate, and the possibilities of the site.
Bioclimatic Criteria in a Passive House
The basic principles of bioclimatic design that a passive house, certified or not, adheres to can be summarized in 5 points: insulation, exhaustive control of thermal bridges, selection of high-performance carpentry and glazing, airtightness, and mechanical ventilation with heat recovery.
1. Insulation
A large part of a home’s energy consumption is concentrated in winter heating, so good thermal insulation is a key concept for building a passive house. Basically, it is used to retain heat inside a property, which must have been generated previously in some way, either through heating or by capturing solar radiation through windows.
However, we are not interested in retaining this heat excessively in summer; rather, it should be possible to dissipate it. That is why the design of a passive house must respond according to the climate in which our building is located; it is not a recipe where more is always better.
Thus, it is necessary to calculate how much insulation we install, where we install it, and what construction solutions we use to avoid discontinuities in the building envelope. We must correctly calculate the necessary thickness based on annual temperature changes and prioritize placing insulation on the north face, which receives less radiation and is therefore colder throughout the year.
At Slow Studio, we add sustainability criteria to the materials used for such applications—beyond the evident ecological benefit of reduced energy consumption. We promote natural, non-toxic, and breathable materials, also considering the health of the occupants who will enjoy the building.
With adequate insulation, we will achieve a considerable improvement in comfort conditions, especially in winter, when the difference between indoor and outdoor temperatures is greater.
2. Exhaustive Control of Thermal Bridges
A thermal bridge is a pathway for heat escape, usually resulting from a discontinuity in the insulation, as we mentioned earlier. Following the same strategy, our goal is to store this temperature indoors during winter, so we want to avoid these exposed points where temperature flows, connecting the interior with the exterior.
In Spain, we enjoy an enviably warm climate compared to Nordic countries, which allows for many errors in conventional construction that would be scandalous for our northern neighbors. Passive architecture must also be very strict in this regard, as we must not forget that its primary objective is to minimize energy consumption.
Thus, the highest risk areas are junctions between architectural elements of different natures, such as the meeting of the facade with floors or roofs, windows or other types of openings with exterior walls, and balconies that extend outwards as an uninsulated continuation of the slab.
This control does not require material investment, and consequently, economic investment; rather, it prevents wasting money by applying thermal insulation in vain. Furthermore, a thermal bridge represents a cold spot on a warm surface, which generates condensation, leading to dampness and even mold.
3. Selection of High-Performance Carpentry and Glazing
Any opening in the envelope of our passive house represents the possibility of capturing solar radiation, but at the same time, it is a weak element in the building’s skin with the risk of generating thermal bridges. That is why the choice of the technical characteristics of our windows is key to solving problems of heat loss and dampness that we mentioned in the previous point.
Within the window, we distinguish between the glass and the frame. The glass can be double or even triple, with intermediate air chambers that can also be filled with noble gases, which improve thermal performance by limiting energy transmission. Recently, low-emissivity glass has appeared, capable of trapping heat inside.
The carpentry must ensure airtightness thanks to a thermal break system. This is not about enclosing ourselves in our house without the ability to renew the air, but about ensuring airtightness if all openings are closed. Since the windows are operable, we have the option to open them when it is necessary to dissipate heat in summer or simply to ventilate to ensure the healthiness of our indoor air.
Ventilation and air renewal, for their part, are ensured by forced renewal that allows us to control the temperature at which fresh air enters our home.
4. Airtightness
Following the previous point, the topics of airtightness and ventilation are next. Again, the concept of airtightness pursues the common goal of minimizing interior-exterior energy exchange, which must not contradict the minimum requirements for indoor air renewal. Airtightness allows us to control the interior temperature of our passive house, preventing air infiltration.
This idea is often explained using the concept of a ‘thermos house,’ which avoids any uncontrolled air exchange, such as opening a window, and leaves everything to a forced ventilation system, as we will explain in the next point.
5. Mechanical Ventilation with Heat Recovery
In a passive house, it is important to ensure both natural ventilation, through strategies like cross-ventilation, and mechanical ventilation. Among the latter, the most efficient strategy is mechanical ventilation with heat recovery, which manages to recover a large part of the energy that exits through ventilation.
Another strategy to ensure ventilation and indoor air renewal in an airtight house is to introduce air through a covered patio, a greenhouse-type gallery, or even a Canadian tube that circulates underground to naturally increase its temperature, meaning without altering its humidity conditions and thus maintaining a healthy indoor environment.
Criteria for Passivhaus Certification
Parallel to the criteria we have presented for bioclimatic design in general, we find that the specific criteria for Passivhaus certification are also summarized in 5 technical requirements. Five points whose goals consist of minimal consumption and building airtightness.
1. Heating Demand
Energy consumption for heating must not exceed 15 kWh/m2 per year. Through all the strategies summarized in the 5 points we have just highlighted, we can minimize this predominant expense during the winter months. Airtightness and insulation will be two key concepts to achieve this goal.
2. Cooling Demand
Energy consumption for cooling must not exceed 15 kWh/m2 per year. During the summer months, the strategy changes, and both ventilation and solar protection become important to prevent overheating of the building, especially in climates like ours where these costs can be similar. However, countries with colder climates allocate more resources to the first criterion, as capturing solar radiation will be important almost all year round.
3. Primary Energy Demand
Primary energy consumption must not exceed 120 kWh/m2 per year. This is the most distinctive point of Passivhaus certification, which they highlight to differentiate themselves from less demanding certifications.
Primary energy encompasses all forms of energy consumption, not just that allocated to heating and cooling. Thus, it is the only standard that considers expenditure on domestic hot water and household electricity, other forms of comfort not exclusively thermal.
Furthermore, this energy measure divides it by the ‘usable living area,’ which excludes unheated areas from the calculation. This avoids the possibility of reducing this number by accounting for unused areas, to make it seem as if we are reducing energy expenditure.
4. Airtightness
Indoor air changes must not exceed 0.6 air changes per hour at a pressure equal to 50 Pa. This point is influenced by the previous considerations of both airtightness and mechanical ventilation with heat recovery. Evidently, it is worth remembering that there are also minimum renewal rates to ensure the healthiness of the indoor air in a Passivhaus home.
5. Thermal Comfort
The indoor temperature must not exceed 25 °C for more than 10% of the hours in a year. We are again talking about thermal comfort in summer, where it is important to avoid overheating the building due to excessive airtightness and a lack of ventilation and solar filters.
What Does Certifying My Passive House as Passivhaus Imply?
Given that there are passive houses that meet the standard’s requirements but are not certified, what are the implications of certifying your passive house as a Passivhaus home?
At Slow Studio, experience leads us to identify the client profile that seeks Passivhaus certification as individuals who consider the possibility of selling their home in the future. Thus, this internationally prestigious certification facilitates access to a broader market of local and international buyers looking for net-zero homes. In fact, we have clients who tell us they want a passive house because they believe that if it’s not passive, it won’t be possible to sell it in a few years.
On the other hand, it is possible to certify our house retrospectively, provided it has been built according to passive construction criteria.
Technically, it implies a series of requirements that represent an additional cost for our future passive house. First, a Passivhaus design expert must be involved as an advisor in the design process. Once the building is constructed, another Passivhaus certifier must conduct the test.
These two technicians must be independent, and the additional cost this represents often leads clients to decide to allocate those funds to better features for their home. If they have chosen their architect well, and the architect works with an installations engineer expert in efficiency who can reliably demonstrate the building’s energy demand, the certification is a process that guarantees the chosen technicians have done their job correctly.
Passive Houses and Passivhaus in Spain
Despite being a certification model of German origin, it is gaining increasing recognition in other countries. This is the case in Spain, where the situation is gradually improving through the implementation of this standard.
This is exemplified by the small municipality of Villamediana de Iregua, located in La Rioja, which in 2013 approved a General Plan mandating compliance with the Passivhaus standard for all public institutional buildings constructed from that date onwards. It is the first municipality with this initiative, which we hope will be a pioneer for many others in the future.
Once the pros and cons of certifying a house as a Passivhaus home have been analyzed in detail, the choice rests with each individual. We hope this article has helped you review the principles that characterize passive design and understand that the issue is not so much whether our home is certified or not, but rather the pursuit of sustainability and health goals.
We want to reiterate the benefits of passive architecture for the health of our planet, ourselves, and those around us, with advantages—whether we certify the bioclimatic design or not—such as sustainability, affordability, comfort, and versatility.
At Slow Studio, we will be delighted to continue providing this advice and answer any questions you may have or show you concrete examples of ongoing passive house projects.