The importance of considering air permeability at the early design stage
The air permeability of the building envelope underlies the potential energy efficiency of a building, as poor airtightness can be responsible for up to 40% of heat loss from buildings. The level of air permeability is determined by the standards of design, materials, and workmanship. Well-managed, it enables lower running costs; prevents uncomfortable drafts and minimises condensation risks. If overlooked at the early design stage, it can lead to underperforming buildings that, at worst, fail building compliance – leading to delays and the cost of additional, remedial measures.
Air permeability testing measures the uncontrolled air leakage of a building. This is distinct from the designed controlled ventilation strategy (i.e. mechanical ventilation) which ensures a sufficient movement of air through the building to create comfortable, liveable spaces. Air permeability is measured as the air leakage rate per hour per square metre of envelope area, or the quantity of air in m3 that leaks into or out of the building per hour measured by pressurising or depressurising the envelope to 50 Pascals (or m3/h.m2 at 50 Pascals).
Building regulations Part L sets a limiting air permeability of 10 (metres cubed, per hour, per metre squared of external surface envelope area). A well-designed, well-built new build development could realistically achieve 3m3/hr.m2@50PA (if mechanically ventilated) or 5 m3/hr.m2@50PA (if naturally ventilated). The regulations require that the Building Emission Rate (BER), or Dwelling Emission Rate (DER) does not exceed the Target Emission Rate (TER). SAP calculations are undertaken to determine the energy performance of a dwelling, in which a design air permeability target is set to ensure compliance.
It is worth noting that the air permeability rate for the TER has been set at 7m3/hr.m2@50PA, therefore design air permeability targets worse than this benchmark will need to offset emissions through other means to ensure compliance.
Infiltration occurs when air leaks through cracks and gaps in the dwelling’s fabric. The amount of infiltration is affected by design and quality of construction and by wind speed/direction: Windows and doors lose a lot of heat, so particular attention should be paid to the U-values of openings. Thermal bridging, concerning heat loss through non-repeating linear junctions, should be assessed and minimised too. Minimum U- values should be treated as just that – minimums. If the fabric of the building is well insulated, you will not need renewable technologies or further measures to comply or exceed Part L standards.
All new builds require Air Permeability Testing (APT) on completion and the resulting figure goes into the SAP calculations. It is vital that the envelope is sealed before testing, and a pre-test check should be carried out. Testing is required on all residential developments, although this may be a sample of units, and non-dwellings. In the case of only a sample of units being tested, the assessed air permeability is the average test result obtained from other dwellings of the same type, increased by 2.0 m3/h.m2. Where testing is not carried out, an assumed air permeability of 15 m3/h.m2 must be used in calculations. This default ‘15’ air permeability assessment alone highlights why air tightness testing, done early, is a worthwhile investment.
|Design air permeability targets should be considered at the early design stage but this is often overlooked. The key to success is summarised in the below steps:
1. Accurate early design construction detailing (through design review meetings, to inform the tendering process).
2. Design workshops with contractors (on site, covering the correct procedure to create a tight building envelope).
3. First and second fix intermediary air tightness testing (to save delays and costs later).
4. Consultations with M+E consultants to determine the best ventilation strategy to fit the scheme’s design air permeability target.
5. Final air tightness testing to confirm as built building compliance.
Eight Associates offers air tightness testing services, which span all of these key areas. Please get in touch to find out more.