What is a Passive House

A Passive House (Casa passiva in Italian, Passivhaus in German) is a building designed and constructed under strict parameters that allow to minimize (and sometimes even eliminate) the energy it requires for warming/cooling.

Passive Houses were first built in Northern Europe, and then quickly diffused in Middle Europe (Netherlands, Germany, Austria, Switzerland) and in Italy too.

The Passive Houses are NZEB (near zero energy building) in accordance with the European Directive 2010/31/UE, implemented by the Italian Government with the law L.90/2013, which starting from 2020 requires every new construction and major renovation to reach the standard of passive building.

A passive building is not necessarily built with wood, but can also be built using concrete.

The “classic” method of constructing or renovating a building (using Exterior Insulation and Finishing System (EIFS), double-glazed windows, etc…) consists only in trying to keep cold in and warm out during the summer, and keep warm in and cold out during the winter.

Passive Houses instead, besides using methods and materials which minimize thermal dispersion, adopt appropriate solutions in order to optimize solar radiation, limiting it in the summer and exploiting it in the winter.
Thanks to Mechanical Ventilation Heat Recovery (MVHR), which employs a heat exchanger, it is possible to recover all of the thermal energy that otherwise would be dispersed (such as, for example, during the winter when windows are opened to cool the kitchen while cooking, and in the rest of the house the heaters are still consuming gas).

Entering in more detail, we have that the Passivhaus certification is based on 5 points:

  1. Thermal envelope
    A certified passive house, as already mentioned above, can be either in wood or concrete, and either in natural or artificial materials. The envelope is the building’s body, so it is very important to study its characteristics (type, thickness, etc.), according to the parameters required by the certification and the location of the property.
    Usually, when talking about low-energy houses, only winter heating costs are considered, so it is natural to think that a passive house built in Italy, compared to the Nordic countries (or a house in northern Italy, compared to southern Italy) requires more thermal insulation. The passivhaus certification, instead, includes among its criteria the calculation of the cooling needs, i.e. the energy required to obtain an optimal temperature even in case of high external temperatures.
  2. Windows
    When talking about renovation and energy saving the most common solution is the use of double-glazed windows. Passive houses generally use triple glazed windows, or certified double-glazed windows that meet certain isolation requirements.
    One thing that often is not considered enough is the choice of window frames. Installing double-glazed windows on old or low-performance frames yields poor results. The choice of window frames is therefore central in passive houses.
  3. Thermal bridges
    A simple definition of thermal bridge is that it is a discontinuity in the construction of a building. Thermal bridges are the “weaknesses” of a building.
    A thermal bridge may be due to the coupling of different materials that have different thermal conduction behavior (eg. the point of contact between a beam and a pillar, or between the window frame and the wall), or to geometric variations, eg. an edge or the corners of the building.
    Thermal bridges are present in every building, but a good design allows to cancel or limit them to a minimum.
    Even the installation of the materials has considerable significance, since unskilled workers could compromise the good work of the designer.
  4. Air tightness
    When speaking of air tightness, what naturally comes to mind are the “drafts”, from under the doors or entering through the windows, that are present in every home.
    As mentioned above, window frames are of fundamental importance and must block any air entering from the outside.
    In passive houses, however, air tightness must be absolute so, in the construction phase, all possible areas (often unimaginable or hidden) where air can infiltrate must be analyzed and sealed.
    For example, in wood buildings the so-called “taping” is performed, which consists on applying special tapes at the joints of wood panels where air could enter. Even all the pattress boxes (sockets and switches) must be installed following certain criteria, ensuring air tightness.
    During the construction, and also at the end of the work, the “blower door test” is performed, consisting of temporarily replacing doors with two large fans blowing and sucking air. This allows to check for any area where the air infiltrates and eventually certificate the absence of weak spots.
    Air tightness, together with triple-gazed windows and thermal envelope, leads to uniform temperature rooms, on the contrary of traditional houses where, for example, the temperature near the windows is different from the temperature at the center of the room or close to the perimeter walls.
  5. Air ventilation system
    For a proper use of a passive house it would be important not to open the windows.
    Not that it is forbidden, it is allowed at any time and in any room, but the ventilation system, called Mechanical ventilation heat recovery (MVHR), reaches the maximum efficiency and produces a greater comfort if the windows are kept closed. The air is constantly sucked from the rooms where it is “dirty” (i.e. bathrooms and kitchen), it reaches a heat exchanger, loses its heat (in winter, while in the summer it absorbs heat from the warm air coming from the outside) and it is ejected outside. At the same time, and from another conduit, external air is sucked, filtered (generally using F7 class filters, which purify it), passed through the heat exchanger (which heats it in the winter and cools it in the summer), dehumidified and finally blown into the rooms that need clean air (bedrooms and living room).
    This system, in addition to allowing a recovery of thermal energy and therefore a cost reduction for heating/cooling, guarantees air always clean (as if the windows were kept constantly open), with only a small percentage of CO2, smog and particulate matter, a greater climate comfort and the absence of mold and pollens (ideal for allergy sufferers). To give a practical example, the first thing that is done after waking up in the morning is to open the window to change the stale air (how much wasted energy!!). When living in a passive house this won’t be needed, because the air is constantly exchanged and cleaned.
    The continuous exchange of air also eliminates any bad smell in the house, for example in the kitchen when cooking, and reduces air humidity, for example when clothes are hung for drying.
    In addition to this, not having to open the windows allows for a greater acoustic comfort, especially in the cities or in the busy areas.
For more information, please see this brochure (in English) from the International Passive House Association, which explains all the features and benefits of Passive Houses.