Passive House: What is a “passive” building?

Passive House in Medesano

First of all, it is necessary to understand that capitalized “Passive House” (“Passivhaus” in German, and “Casa Passiva” in Italian) refers to the building standard that prescribes methods for designing “passive” buildings (houses, schools, anything).

Here in Italy, there has been a growing interest in the term “passive” with regard to construction and architecture. Unfortunately, however, the word is often used haphazardly or without much definition. Added to that is the general confusion surrounding vocabulary in the green building market in Italy, leaving many people misinformed.

As this is a subject to which we are quite dedicated, and one which we frequently discuss in our blog, we thought it would be appropriate to go over the correct definition of a passive building.

The necessity of this clarification became apparent to us when we took part in a day conference on the subject of Passive House, organized by Legambiente in Parma (an environmental organization) in collaboration with the Emilia-Romagna CasaClima – Klimahaus Network. During the presentation, the following was said: “A building is defined as passive when it is able to consume less energy than it produces”.

Even though the conference itself was quite interesting, this definition of passive house was incorrect and misleading. This is when we realized that there are many common misconceptions and overlaps in the the definitions of a passive building, a near zero energy building, and an active building.

The original definition of a ‘passive house’ is as follows:

A building is defined as “passive” if it is designed in such a way that is remains comfortable for the occupants, both in winter and in summer, without an active heating or cooling system.

Indicatively, a passive building would have a maximum winter heating load of 10 W/m2. In non-technical terms, it means that the small heating required can be brought to the rooms of the house via the ventilation system (which is in any case necessary for other comfort reasons). This way, the building does not need a traditional heating system, such as radiators or in-slab.

As an alternative to the winter heat load definition, a building is also considered “passive” if its energy demand for winter heating is less than 15 kWh/m2a. This means that the building may also have a small heating system, with high energy efficiency. For example a passive house that complies with this definition can guarantee energy savings higher than 90% of current average energy demand of Italian building stock. This second definition is particularly important in case of a deep energy retrofit, where sometimes structural thermal bridges cannot be entirely eliminated.

We would like to highlight how this kind of design has to be carried out by professionals with an expertise in passive buildings. The building energy balance has to be calculated with PHPP software, on the basis of the most accurate, site-specific climate data. The energy flow value of all thermal bridges (calculated according to EN ISO 10211) must also be added to the balance.

In addition to the two definitions that we mentioned in this article, which describe the energy performance of a passive house, other criteria have to be met for the overall passive design to be successful:

• thermal comfort must be consistent throughout the building;
• the structure has to be airtight;
• and the building has to respect the maximum limits of cooling energy demand and overall primary energy demand.

We are going to post more articles about these aspects of passive design over the next weeks:

• What’s the “time constant” of a building?
• Meet the Mechanical Ventilation System
• DesignPH: the SketchUp plug-in to design Passive Houses in 3D
• Discovering Colorado, the cradle of passive houses
• Our visit to the Rocky Mountain Institut
• Our impressions from the 2015 Int’l Passivhaus Conference in Leipzig, Germany
• Energy Efficiency: the importance of site supervision
• Passive House Window Talks 2015
• The blackout-proof home: Passive House with high time constant
• Our Passive House Italy group surpasses 2,500 members