Airtightness

In the context of residential construction, renovation, and retrofit projects in the UK, airtightness refers to how well a building’s thermal envelope is sealed against uncontrolled air movement. It is a critical component of a building's fabric-first approach to energy efficiency, working alongside insulation to reduce heat loss. A building with good airtightness minimises the unintentional flow of air through gaps, cracks, and unwanted openings in the walls, floors, and roof, which is distinct from controlled ventilation.

A building's airtightness is measured by its air permeability, which is the volume of air leakage per hour per square metre of the building’s envelope area when subjected to a pressure difference of 50 Pascals (Pa). This is expressed in cubic metres per hour per square metre (m3/(h.m2)) at 50 Pa. The lower the figure, the more airtight the building.

For new dwellings, Part L (Conservation of Fuel and Power) of the Building Regulations sets mandatory targets for airtightness. Approved Document L1A for new dwellings requires a design air permeability of no worse than 10 m3/(h.m2) at 50 Pa, with a target of 5 m3/(h.m2) at 50 Pa being a more common goal for achieving compliance and better performance. Higher standards, such as those for Passive House, aim for even lower values, often below 0.6 m3/(h.m2) at 50 Pa.

Poor airtightness leads to heat loss via convection and is one of the biggest contributors to energy waste in homes. Draughts caused by uncontrolled air leakage can also lead to cold spots, discomfort, and a higher risk of condensation within the building's fabric, which can cause mould growth and structural damage. Achieving good airtightness involves a continuous air barrier around the heated volume of the building. This can be achieved using a combination of tapes, membranes, sealants, and carefully designed junctions between different building materials and components.

• Related Articles

• Is MVHR Suitable for All Types of Property?

  While MVHR systems could technically be installed in any building, they perform most effectively in properties with high airtightness levels (≤3 m³/hr/m² @50Pa). Recent UK research shows they still reduce carbon emissions compared to natural ...

• Blower Door Test

  A blower door test is a diagnostic tool used to measure the airtightness of a building. It quantifies the rate at which uncontrolled air infiltrates or exfiltrates through gaps, cracks, and other openings in the building's envelope. The test involves ...

• Air Permeability

  Air Permeability is a measure of how airtight a building's envelope is. The building's envelope is the physical separation between the conditioned interior and the unconditioned exterior. It quantifies the rate at which air leaks in or out of a ...

• Background Ventilation

  Background ventilation refers to small, controllable openings that allow a continuous flow of fresh air into a building. These vents are designed to provide whole-building ventilation, helping to maintain good indoor air quality by diluting ...

• At what RIBA project stage should an architect seek advice from a domestic ventilation specialist in order to design and confirm the ventilation strategy?

  Optimal RIBA Stage for Engaging a Domestic Ventilation Specialist Engaging a domestic ventilation specialist at the appropriate RIBA project stage is crucial for developing an effective and compliant ventilation strategy. Early consultation ensures ...