Airtightness testing is a vital step if planning to achieve an effective air permeability.
Airtightness testing is required under technical guidance document part: L and the test must be carried out by a certified tester to Test Standard IS EN:13829:2000 (Thermal performance of buildings. Determination of air permeability of buildings. Fan pressurization method).
The air permeability of the building must comply with the regulatory reasonable upper limit of 7m3/[h.m2].
Before a test begins, all openings such as; windows, doors, vents and chimney flues have to be closed. You should measure the internal floor area and ceiling height, as well as the envelope area to calculate volume.
The air permeability is measured by taking the average of the pressurisation and depressurisation test results. These are calculated by taking the Air flow at 50 Pa, v50 [m³/h] and dividing it by the Envelope Area [m²]. This gives the result in q50:m³/h/m². This result is required to be below 7m³/h/m², to conform to the reasonable upper limit outlined in TGD:L and it’s likely to be reduced to 5m³/h/m² with the introduction of nZEB.
The passive house standard uses a different method to measure the airtightness of a building. Where q50 is an expression of the amount of air leakage in relation to the total exposed surface (used for TGD:L), n50 (used for passive house standard) is how the air permeability rate is expressed in air changes per hour [ac/h]. Where q50 is calculated by dividing v50 [m³/h] by the Envelope Area [m²], n50 is calculated by dividing v50 [m³/h] by the useable volume [m3].
It’s important to note that q50 and n50 aren’t interchangeable. They can often be close but may vary greatly depending on the shape of construction.
Achieving the reasonable upper limit on an airtightness test is only half the battle. The greater incentive is to achieve high levels of airtightness to increase the BER (building energy rating). Achieving an air change rate of below 3 will allow for effective implementation of mechanical ventilation heat recovery (MVHR) and achieving greater levels will only increase the systems effectiveness.