The parameters that significantly affect the energy balance in a building, as shown by the chart below, are the insulation of the casing and the ventilation.
Both are closely related to the air-tightness of the building casing or better, to the degree of tightness of the casing system made up of various constructive elements interconnected in various ways. These connections represent preferred routes for heat transfer, resulting in an increased thermal dispersion through the casing. A typical example are air flows occurring at the point contact of windows to the wall structures. Such air infiltration is directly responsible for the increase in the air permeability of the building, defined by the amount of air crossing the building casing due to the pressure difference between indoor and outdoor environments.
Such low air-tightness of the casing causes:
– increased thermal dispersion in winter by convection resulting in increased heating requirements
– risk of condensation formation inside strata due to the passage of steam from the inside outwards (winter) and from the outside to the interior (summer) resulting in worsening of the insulation properties of the structures
– Reduced comfort of indoor environments due to the presence of small air flows which cause a local decrease in surface temperature (local discomfort) or of entry into the room of cold/hot air flows (winter/summer) with consequent decrease/increase of relative humidity rate.
– acoustic bridges: air entering through air flows is a good vehicle of external noises
– reduction in the efficiency of mechanical ventilation systems with heat recovery. The heat contained in the hot air when dispersed outside through slits can not be recovered.
In order to avoid such phenomena, it is necessary to design building solutions ensuring good air-tightness for the entire housing. Air tightness is ensured adopting an internal air-tight layer preventing the passage of airflows from the inside to the outside. To minimize the thermal dispersion by convection, an outer wind-proof sealing layer is added.
The air-tight layer is usually made on the inner side before the insulating layer and must contain the entire heated volume without any interruption and discontinuity. It also acts as a steam brake.
The air-tightness of a building is measured using the Blower Door Test, which allows to evaluate the permeability of the casing by calculating the air intake by infiltration, creating a pressure difference between inside and outside of 50 Pa (1 Pascal represents the force of 1 N exerted on a surface of 1m2: the generated pressure in the 50 Pascal test corresponds to about a dynamic pressure generated on a wall subjected to perpendicular wind action at a speed of 9 m s.
The test is carried out using a fan which, suitably installed on an external door or window (all other openings are closed), draws air out of the building to create a pressure difference of 50 Pa between inside and outside. Suitable instruments are used to measure the air flows induced by the fan inside the building and the possible air exchange through leaks in the casing.
The Blower Door Test includes a pressurization phase where the fan blows air inside the building in 10 Pa pressure increments and a depressurization stage where pressure decreases. The number of hourly exchanges obtained in these conditions is indicated by the symbol n50 expressed in h-1.
Summarizing, n value is the amount of air exchanged every hour inside the building and can be calculated using the formula: n50 = V50 / V [h-1]. Where V50 = air flow rate measured with ΔP = [M3 / h] and V = air volume in the building [m3 / h].
Existing national regulations do not impose any air permeability requirements for the enclosure in newly constructed buildings or subject to energy-refurbishment.
However, today there are various municipalities and provinces in the country that adopt in their building regulations energy certification protocols that impose air tightness requirements on buildings. They introducing n50 air emission limit values certified using the Blower Door Test .
The Casaclima Certification Protocol of the Bolzano Agency fixes for all new residential buildings a limit n50 value according to the energy class.
A house built according to passivhaus standard is characterized by a high casing tightness with a value of n50 ≤ 0.6 h-1
The test for air-tightness using the Blower Door Test provides for two methodologies as required by UNI EN 13829:
Method B: Used to control the quality of work and to detect leaks in the enclosure under construction. The search for any leakage of air is carried out through appropriate instruments such as thermo-anemometers, thermo-cameras, smoke generators.
Method A: The test is performed when the casing is in the operating conditions suitable for a fully operational heating or cooling systems. All external openings are closed and all internal left open. This method is used to determine the n50 value.