The basic requirements for a Passive House are:
– The annual heating and cooling demands must not be more than 15kWh/m2 or, alternatively, the thermal load must not exceed 10kW/m2
– The annual primary energy requirement for domestic hot water, heating, cooling, auxiliary and household electricity must not exceed 120kWh/m2
– The building ́s airtightness must be verified with an air pressure test that confirms an n50 value of less than 0.6 air changes per hour
In order to meet these requirements, the following principles must be taken into consideration:
– Excelent thermal insulation: Walls, roofs, floor slabs… need to have a very low heat transfer coefficient (U-value) in Passive Houses.
A good thermal insulation also increases the feeling of comfort inside, because no big differences of temperature take place among the interior surfaces, thereby avoiding the “cold wall” sensation. Good insulation levels can be achieved no matter what structural and constructional method is used.
In compact buildings it is often easier to achieve the energy demand objective, because they have less outter surfaces in contact with the external environment.
– Thermal bridge-free construction: Thermal bridges are areas of the envelope where there are weakenings of the thermal insulation capacity, due to changes in geometry, materials jointure, changes of material, etc. Thermal bridges require special attention because:
- The temperatures of the interior surfaces are generally lower than in standard construction components. This can lead to mold, condensation, and in the worst of cases, construction component moisture penetration.
- Heat losses generally increase.
The negative effects of thermal bridges can be avoided in most cases paying special attention during the desing phase. The insulation should be laid to cover the entire surface of the building without gaps.
– High quality windows: Specially insulated window frames are used. Three-pane thermopane glazing or a comparable glass combination is offen needed, plus “warm edge” spacers, in order to reduce the thermal bridge in the encounter between glass and window casement. At the same time, the glass must have a solar factor (g) as high as possible.
– Airtightness and mechanical ventilation with heat recovery: The exterior shell of the building should be as airtight as posible. This way, we avoid damage from water vapor in air currents that would condense as it penetrates joints from the inside to the outside. Drafty rooms resulting from leaking construction are no longer acceptable, nor conventional window ventilation which has proven insufficent, especially in new airtight buildings.
The extremely low heating energy requirements in passive houses can only be achieved with heat recovery from exhaust air. Modern ventilation technology allows for a degree of available heat recovery ranging from 75 to over 95%.
– Exploitation of solar gains in winter and limitation in summer, taking advantage of the benefits of fixed or movable sunscreens and the thermal inertia of the building.
– The temperature setpoint inside the building must be set to 20ºC in winter and to 26ºC in summer. No overheating is allowed in summer for more than 10% of the time over the previous temperature.
The relative humidity should be kept between 30-70%, and preferably 40-60%.