What is Passivhaus ?

PassivHaus is an entirely voluntary building performance standard to build a low-energy home that guarantees high comfort level with very low energy consumption. PassivHaus is an entirely voluntary building performance standard to build a low-energy home that guarantees high comfort level with very low energy consumption. The standard has been named "Passive House" (or Passivhaus in german) because the passive heat inputs, delivered from externally solar heat gains through the windows and ambient energy sources (occupants, appliances,…)can minimized heat demand of the house. The Passive House can be applied to new build and retrofit buildings of all types, it allow for heating/cooling related energy savings of up to 90% compared with typical building stock and over 75% compared with average new builds. Passivhaus standard sets targets for energy demand and overall energy use of buildings and require less than 15 kWh/m² yr for heating and cooling equivalent of 1.5 litres of oil or 1.5 m³ of natural gas to heat each square metre of living space. Passive House is not a brand name or a specific construction method but a construction concept with meticulous attention to detail and rigorous design according to principles developed by the Passive House Institute (PHI) in Germany.

 

 

Building envelope

The building envelope is like a "human third skin", a protective membrane between inner and outer environment directly responsible to thermal comfort. The building envelope component of Passive House sets targets for performance requirements according to principles passivhaus standard to ensure optimal indoor temperatures that provide acceptable thermal environmental conditions.
The first step towards providing a Passive House is to minimise heat losses from the building envelope through high levels of insulation and building airtight. Increasing temperatures on the inner surfaces by keeping average surface temperatures above 17 °C prevents condensation and mould growth as well as local thermal discomfort caused by warm and cool surfaces are minimise. Well insulated building can prevent overheating in summer.Energy efficient envelope is characterized by a continuous thermal insulate layer run all around the building like a warm coat; passivhaus buildings is used high grade insulation which providing a U value ≤ 0,15 W/m²K

 

In terms of fabric heat loss, windows are generally the weakest element; Passive House windows is extremely important for the energy balances of the building. They have to meet strict requirements to increasing comfort indoor conditions: in winter, it must provide to minimise heat losses and to maximise solar heat gains and allows solar radiation to enter the building, in summer minimise unwanted heat gains which can lead to overheating internal temperature and cause indoor environment uncomfortably.

 

Orienting glazed openings in the south side of a building are necessary to reduce heating needs and make a positive contribution energy balance while windows facing north should be avoid o reducing the proportion to minimize heat losses. In summer of northern hemisphere the sun rises in the north-east and sets in the north-west while in winter it rises in the south-east and sets in the southwest. In summer the sun angle is higher than winter and south facades receive the least amount of solar radiation (incidence radiation angle is much higher). Windows facing east/west need careful planning to avoid the risk of overheating especially in summer the glazed openings have to be protected from the low morning and evening sun with appropriate types of shading devices.
High performance of windows is guarantee by correct installation minimising thermal bridges based connection wall-windows.

Heat losses can minimise also through appropriate envelope airtight will therefore have a positive effect on the indoor comfort. Well insulated and airtight building envelope will retain heat within the house and reducing energy losses from air infiltration through gaps and will prevent hot air entering the building during hot seasons. On the other hand, airtight building envelope helps to prevent damage to the interior and structural elements of building. In winter, gaps in the building structure allow air to pass through and when moist air comes into contact with a surface which is at a lower temperature can cool down and causing the moisture in the air to condense. Cold air infiltration from high permeability envelope are the cause of uncomfortably indoor space (local thermal discomfort, radiant temperature asymmetry, thermal stratification, reducing acoustic insulation, indoor air pollution ...)
Infiltration levels can be accurately verified by a blower door test. The PassivHaus standard requires  n₅₀ ≤0.6 h ^{-1 _.} Airtightness building envelope needs meticulous planning as well as careful installation.

Passive house equipment

The high thermal performance of a passive house envelope allow to use systems equipments more efficient and affordable than conventional building. The energy heating demand is usually covered by mechanical ventilation system with a heat recovery unit with high efficiency (η ≥75%); it consists in a double system of ducts:

• a duct for estract fresh outdoor air
• fresh outdoor air is supplied to the living rooms through system ducts after absorb the heat in the exchanger...
• air ducts which removed constantly out exhaust air from kitchen, bathrooms, …
• ventilation systems must have heat recovery rates of 75% to 95%.

To increase the efficiency of Passive House ventilation systems of is often used a earth buried ducts made of polyethylene with corrugated external surface to increase the heat exchange and smooth on the inner surface (to facilitate any inspections). The ground during winter has relatively constant temperature and a higher than outdoor air, and during the summer a lower temperature than outdoor air. So the ventilation air is pulled through buried ducts, in winter, heat from the surrounding soil warms up the air while in summer it is possible to cool down outdoor air before it enters the house. The ducts are placed about 100-150 cm deep in the earth under the house.
Outdoor air after running underground pipes before it enters house passes through fine filter (class F7 or more) and then enter indoor volume at 5-8 ° C. Supply heating is integrated by using a system with a limited power (0.5-1.5 kW as small heat pump (compact heat pump unit) or small heat generator based on biomass fuel. By positioning the air inlets close to the ceiling it is possible to exploit the coanda effect (where by the aerodynamics of moving air means that it temporarily clings to the ceiling before gradually falling and mixing with the other air within the room).
The indoor air quality (IAQ) is a priority of a passivhaus standard requirements where healthy indoor climate is achieved by:

• all the comfort criteria have been ideally fulfilled
• ventilation system provides clean, pollen free, dust free air while eliminating excess moisture. Without active ventilation, the concentration of CO₂ continues to rise and make living space uncomfortable.

High levels of indoor air quality are guaranteed through adequate air exchange depending on the number of people. The ventilation system is designed to provide 30 m ³ of fresh air per person per hour in order to achieve an air change rate of 0.33 ach (air change per hour). For example, in a house of 120 m² of living area to provide a requirement to 30 m³/h  for a family of 4 persons needs 120 m³ /h of fresh air. An air volume of 324 m³ (with h=2.7 m) we get an air change rate of 0.37 h^-1 .

 

Airtightness

As show the graph thermal insulation and ventilation parameters significantly influence energy balance of a conventional building.
Both are directly connected to the airtightness of the building envelope. Airtightness is an important requirement for improve energy efficient building and avoid moist room air penetrate into the construction, condense and cause damage: air tight design is important to keep pleasantly warm the inner surfaces, maintain a comfortable and healthy interior environment.
Low level of airtightness can determine:

• increased heat losses and greater heating/cooling demand
• risk of condensation
• reduced levels of comfort indoor. Draughts resulting from gaps in the facade are responsible of cool inner surfaces (local discomfort) and uncontrolled humidity inside the building.
• reduced acoustics insulation
• reduced efficiency of the heat recovery ventilation systems because warm air pass through gaps of building can not recovery

The key to reducing air infiltration through building is design continuous uninterrupted airtight building envelope. The airtightness it consists of one uninterrupted airtight envelope layers enclosing the whole heated space able to prevent air flows pass from inside to outside. Outer layer can be used to limit the passage of wind and protect the structure on the outer side against the entrance of cold drafts in winter or hot summer.
Airtightness layer is generally placed on the inner side of the insulating layer and can be used as vapour barrier.

The level of building airtightness is measured through the Blower Door Test which allows to evaluate the permeability of the envelope by calculation of infiltration rate at 50 Pa, a pressure difference between inside and outside (the pressure test generated corresponds approximately to a dynamic pressure generated perpendicular on a wall subject from the wind at 9 m/s speed.
Test is performed by using a variable-speed fan mounts into the frame of an exterior door or window (all other openings are closed) which pulls air out of the house, lowering the air pressure inside in order to create difference of 50 Pa pressure. Pressure gauge connected to the fan measure the airflow rate per minute required to keep heated space at a constant pressure. According to EN13829 standard the test does consist of a multi point reading are taken at different pressure points of underpressure or overpressure of no more than 10 Pa differences and the information gathering compares with estimated values based on the volume of the house. The presence of leaks can be identify by anemometer or a smoke pencil.
Passivhaus standard requires both directions to be tested, with the result being an average of the two. To meet the passive House criteria, the tested building must have an air change rate of 0,6 h-1 (or ach) at 50 Pa:

n₅₀ = V₅₀ / V [h^-1]

n₅₀ = is the air leakage rate at a pressure difference of 50 Pa

V₅₀ = is air flow rate measured [m³ /h]

V = is the volume of air inside the building under test, for all the zones incorporated within the test zone [m³]

Italian building legislation does not impose any requirements about air permeability of building envelope in new dwellings or refurbishment. However, there are various municipalities and provinces have adopted in their local legislation a certification process which requiring limit values of air exchange n₅₀ certified through the Blower Door Test.
The certification process Casaclima Agency of Bolzano fixed for all residential buildings of new construction a limit value n ₅₀ according to the energy class.
House built according the standard passivhaus the infiltration rate at 50 Pa overpressure is set to 0.6 ≤ h^-1_.

 

 

The EN13829 standard lists two principal test preparation methods, which are given as Method A and Method B:

Metodo A(test of a building in use): the condition of the building envelope should represent its condition during the season in which heating or cooling systems are used. This method is used to determine the value n_{50 .}

Metodo B (test of the building envelope): applies for measurement of the airtightness of the building envelope. In this case, any intentional opening in the building envelope is closed or sealed.