2. What air permeability do I need to achieve?
The maximum air permeability allowed on any tested unit is ‘10’ but because of the trade-off of factors to achieve the overall calculated energy performance of the building/unit it is more typical to need to achieve ‘4-6’. ‘10’ is the equivalent of a 20p piece sized hole in every square metre of the envelope area. The envelope area being the ‘external’ surface area of the unit/building i.e. the total area of the floors, wall and ceiling which border the outside or a neighbouring unit for flats and attached houses. This is calculated by the air testing company. Air permeability is expressed in m3/(h.m2)@50Pa, this represents the volume of air being lost through unplanned ventilation (gaps, cracks, holes, etc) every hour through every square metre of the envelope area when there is a pressure difference of 50Pa between the inside of the building/unit and outside.