The building envelope represents a human “third skin”, a protection diaphragm between the inner and outer environment directly responsible for the overall well-being of the individual. The components that constitute the casing of a “passive house” (designed according to passivhaus principles) must meet certain performance requirements in order to ensure optimum levels of comfort in indoor environments.
To reduce heat exchanges between inside and outside, the thermal casing must have both a good thermal insulation and an effective air-tightness. Thermal insulation, on one side, guarantees the healthiness of the environments through the absence of surface condensation responsible for the formation of efflorescences and molds on the inner surface and on the other ensures during the winter surface temperatures of the elements > 17 ° C with consequent reduction of heat exchanges by irradiation between the individual and the surrounding environment (thermo-hygrometric well-being). The heat-insulating layer finally reduces the building’s overheating.
The walls delimiting the heated volume are thus characterized by an external thermal insulation layer with thickness ranging from 20 to 35 cm depending on the climatic zone.
Glazing components in the passive house are a key element in the energy balance of the building: in winter they maximize free solar inputs which positively balance transmission losses; in summer, however, the high thermal performance of the windows reduces the danger of overheating the premises.
South orientation is the ideal choice for the proper functioning of the energy balance, since glass components receive southern solar radiation that in winter serves to cover most of the energy requirements of the passive building. In summer, the sun is high on the horizon, so the glass element receives the smallest amount of solar radiation (the rays have a greater angle than the incidence plane). In general, a south facing glazed surface is expected to be 25-30% compared to the facade. Window frames with orientation to the east and west do not guarantee large solar allowances in winter, while greatly contributing to overheating during the summer and consequently require effective shielding systems.
North is the most unfavorable energy orientation and therefore the windows must be as few as possible and small in size.
In order not to affect the high performance of the window, it is of utmost importance not only to design the wall-mounting connections correctly, but to ensure proper assembly.
The envelope of a passive house is also characterized by an effective air-tightness, which plays an important role in terms of energy saving and comfort in indoor environments.
A good tightness of the building , reduces the infiltration of outside air inside the heated space, thus reducing energy losses due to convective air flow through the enclosure. Such flows cause either a significant reduction in the insulating power of the material and an excessive exchange of air, and, in the presence of a mechanical ventilation system equipped with heat recovery, it results in a decrease in the system’s efficiency since the heat contained in the air passing through drafts is not recovered.
During the winter period the warm air inside the rooms tends to migrate, through the casing, to the outside. Hot air contains steam that in contact with cold surfaces can condense and create damaging accumulation within stratigraphies that can cause mold and mushroom formation. The bigger the drafts, the bigger the risks that such events can occur.
Cold air infiltrations caused by high air permeability, are the cause of discomfort in indoor environments due, for example, to the reduction in surface temperatures (air stratification), the reduction in acoustic insulation, poor quality of indoor air (input of harmful external pollutants such as nitrogen and sulfur oxides, powders …)
A passive house is characterized by a high shear tightness proven by the Blower Door Test, a vacuum and overpressure test that ensures the air tightness of the enclosure to fulfill requirements imposed by the passivhaus standard (n50 ≤ 0.6 h- 1)
To Achieve a good air-tightness entails a meticulous design and an equally scrupulous installation.