The methodology for measuring air tightness in a building is to either pressurise or depressurise the whole building. Air leakage can then be measured to give an indication of the leakage rate or air permeability. 

Depressurisation is created by removing air from the building, and pressurisation is the introduction of air to the building. 

To carry out this process, a temporary door frame is installed within one of the external doors, and a fan is fitted within the temporary frame. 

A pressure tube is placed in the middle of the building, and another to the outside of the building. These 2 tubes measure the external pressure and the internal pressure, known as the Bias pressure. These measurements are taken before and after the test as a bench mark. Pressure tubes are connected to the fan. All pressure tubes and a fan controller lead are connected to a pressure gauge. 

Weather conditions affect the air tightness test, as well as barometric pressure, temperatures inside and outside the building, and wind speed. 

The leakage rate is measured against the total metres squared (m2) of the envelope's surface area. The buildings envelope is the total surface area of the floor, external walls and roof. This measurement is calculated so that a result can be determined. 

The air tightness test of a building is guided by a set target within the energy calculation (SAP or SBEM). The overall calculation determines the target figure. The better the air test result, the less leaky or draughty your property will be. As a minimum, the target figure must be met or bettered to pass the air tightness test. 

Temporary sealing for the air tightness test is only permitted on extraction fans, ventilation systems or unconnected waste pipes. 

All windows and doors should be fully fitted and working. Make sure they are well sealed prior to any test. Access doors, including internal garage doors, cannot be temporarily sealed in advance of the test and should be air tight. Trickle vents closed. 

The fan is run, and the pressure of the building is changed within 10 pascal increments. As the pressure is changed, the flow rate measured through the tubes connected to the fan is noted. This is the flow of air leaving or entering the building, depending on whether the building is being depressurised or pressurised. The flow rates and pressures are run through a software program, and a result is calculated. 

The maximum leakage rate allowed within the energy calculation is 10m3/(hr*m2) @50 Pa, but this number could be lower, as it is common for the energy assessor to lower the target score to enable the energy calculation to pass. The formula is 10 metres cubed per hour per metres squared at 50 pascals of pressure, which in layman’s terms means the building can leak no more than 10 cubic metres (10m3) of air per hour (hr) when the building pressure is at 50 pascals (@50 Pa). 

The leakage rate is measured against the total metres squared (m2) of the envelope's surface area. The buildings envelope is the total surface area of the floor, external walls and roof. 

By building to air tightness standards and being aware of building a tighter property, building standards will improve, giving a better end product for the client, and the overall running costs and efficiency of the property will improve. 

Air tightness testing is a mandatory requirement of Part L of the building regulations for a new build property, whether residential or commercial. All new build buildings must be tested.