Designing a Passivhaus home

Designing a Passivhaus home is a concept that, although it has existed for more than 25 years, has only started to gain relevance today, as most people have begun to develop both environmental and financial awareness of the energy costs involved in maintaining a home.

Energy production and consumption are the main causes of climate change, and before replacing all the energy we need with renewable energy, it is essential to reduce demand. This starts in our own homes, and that is precisely what Passivhaus home design is based on: reducing energy consumption and meeting the small remaining need through renewable energy.

With this logical and straightforward objective, the Passivhaus label was created in Germany in 1988, granting an internationally recognised certification to homes that meet specific requirements in terms of heating/cooling demand and primary energy consumption.

The design of a Passivhaus home must meet standards based on passive architecture strategies, which greatly reduce the need for heating or air conditioning to achieve comfortable, pleasant indoor conditions.

Passive architectural design allows the home to heat up and cool down through natural processes; in other words, it captures heat from the sun and dissipates heat through air currents.

Once our home has reduced its energy needs and stored enough heat, the remaining input is minimal and can be achieved with very little energy.

The Passivhaus label

As we have said, the Passivhaus label is a certificate that ensures our home has lower heating and air-conditioning demand than a conventional home; in other words, we live in a nearly zero-energy home.

To obtain the Passivhaus label, we must hire an expert architecture studio that collaborates with a professional Passivhaus consultant, who provides support throughout the Passivhaus home design process and, at the end, certifies that the home meets the energy requirements of the Passivhaus standard.

The consultant oversees the entire Passivhaus home design process as well as the construction phase. At the end of construction, they carry out a series of tests and measurements that make it possible to verify and demonstrate that all the requirements established by this label are met.

Designing a Passivhaus home

Designing a Passivhaus home has very specific requirements regarding the home’s energy consumption. Specifically, it is established that:

As shown in the requirements, the criteria for the certificate refer to lower energy demand, but also to avoiding excessive air exchange. This in itself helps reduce heating and cooling consumption, reinforcing the other passive strategies implemented in the design of a Passivhaus home.

How do we manage to meet all these premises?

Thermal Insulation

The thickness of thermal insulation is always greater in the design of a Passivhaus home and in energy-efficient homes. Of course, this thickness is determined by the climate zone, but compared to the 8 cm that may be installed in a conventional home, efficient homes usually do not go below 15 cm in thickness.

Thermal insulation allows a home’s indoor environment to be independent of the outdoor temperature, making it much easier to control its temperature.
If we prevent cold from entering in winter or heat in summer, we can maintain a more stable indoor temperature.

This, combined with solar gain and natural ventilation strategies, allows us to maintain an optimal temperature for most of the year without energy consumption.

Absence of Thermal Bridges

But no matter how much thermal insulation is installed, we must prevent cold from entering through thermal bridges.

A thermal bridge is a discontinuity in the insulating material that occurs when construction details are not properly resolved and an area is left without insulating material.

In conventional homes, thermal bridges are often accepted and left unresolved if, in exchange, the rest of the building is well insulated. It may seem that a small gap in the insulation has little effect on overall thermal performance, but energy simulation models have shown that this is not the case, and a small discontinuity can have a major impact on the overall indoor environment.

In addition, a thermal bridge is a source of damp, as it is a cold point on a warm surface. This causes indoor humidity to condense in that area, creating hazardous damp and mould that is often not visible to the user.

It is also important to prevent the same element or material from being in contact with both the exterior and the interior of the home. This would undermine much of the effort devoted to insulating the home.

Window quality

Just as a thermal bridge can be highly detrimental, the same applies to the thermal characteristics of a home’s windows.

In the design of a Passivhaus home, it is recommended to use double-glazed windows with a low-emissivity cavity, or triple-glazed windows. These types of windows already provide the necessary insulation, although we can always improve thermal performance to achieve even more insulation.

We must look not only at the glazing but also at the window frames. Without a doubt, frames with a thermal break must be installed, but it is also necessary to check the frame’s own insulating capacity, not just that of the glass.

Thermal Mass

Thermal mass is a material’s ability to store heat and release it when the outdoor environment is colder. This characteristic makes it possible to capture heat during the day—through solar radiation—and release it at night when the environment is colder and there is a need to heat the home.

These are usually thick walls without a layer of thermal insulation, allowing heat to be stored and released as the environment cools.

Materials with thermal mass are usually heavy materials such as brick, concrete, or stone.

Airtightness

When airtightness is discussed in the design of a Passivhaus home, the emphasis is mainly on the air changes per hour indicated by the Passivhaus standard. This refers to outdoor air that manages to enter the home. It is good for a house to breathe, but at the same time it must be relatively airtight to prevent air leakage that could alter the indoor temperature of the rooms.

Contrary to what is often thought, it is not true that you cannot open the windows in a Passivhaus home; rather, the constant air renewal makes it possible to achieve optimal indoor air quality whether or not the windows are open.

It is important to remember that the design of a Passivhaus home is completely airtight, which is why they are known as thermos houses. So, if we forget to open the windows or we are not at home, indoor air quality could be very poor. For this reason, air renewal is ensured through a heat-recovery ventilator.

This means that the unit not only ensures air renewal, but also takes advantage of the heat from indoor air to warm the cold air entering from outside in winter.

The design of a Passivhaus home has certain specific requirements, but it does not imply the use of any particular material or construction system.

While Passivhaus homes are commonly designed in timber—a material that allows us to control details to the millimetre in the workshop and reduces errors or insulation discontinuities, which are so important in the design of a Passivhaus home.

In any case, to carry out the design of a Passivhaus home, it is enough to implement passive architecture strategies to reduce energy demand and cover the remaining demand through extremely low-consumption systems such as a heat exchanger.

With this, the home can be assessed for the Passivhaus label, with the energy and financial benefits that entails.

It should be noted, however, that the Passivhaus label does not guarantee the use of healthy, natural materials, and in this respect there is still a way to go before reaching an international certification that guarantees the construction of ecological, efficient, but also healthy homes.