What is building air leakage?
Air Leakage (or infiltration) is the flow of outside air into and out of a building.It is not planned by the designers and is due to imperfections in the building ‘envelope’ (or outer skin).
It will typically take place through:
- joints, gaps and cracks in the construction
- gaps created where the structure penetrates the outer skin of the building
- cracks around door and window openings
- gaps where services enter the building
Air leakage will have detrimental effects on the buildings thermal performance, comfort levels and energy efficiency.
Why do we measure the airtightness of a building?
A building that complies with recommended levels of air leakage has a guaranteed performance.
An air leakage test is carried out after the external fabric has been completed to confirm (or otherwise) that the building achieves the minimum air leakage standards set by the buildings designers.
The continuity of the air barrier is fully tested and any areas of failure can be identified. This allows any remedial work necessary to be carried out before retest.
Air leakage measurement on a new building is really the end of the process in ensuring a high performance low leakage envelope.
It follows work at the design stage to ensure continuity of the air barrier around openings and at all joints in the building fabric.
During construction, quality control must be maintained so that the design details are reproduced on site.
Finally the building is tested to confirm that the specified standards have been achieved.
Achieving a good level of airtightness
Careful design, specification, quality control during construction and performance testing the building will achieve an airtight building envelope.
The building form, position and types of opening, interaction of the structure and airtight layer all effect the air leakage performance.
An appropriate airtightness specification must be selected.
The line of the airtight barrier should be identified as soon as possible allowing examination of details at openings and junctions in the envelope.
The components and systems that make up the external envelope such as curtain walling, windows, cladding etc must achieve at least the specified level of airtightness set for the building. Responsibilities for dealing with coordination at junctions should be clearly spelled out.
All supervisory staff and subcontractors should be adequately briefed on the importance of maintaining the airtight barrier, quality control and reproduction of the designers details.
Testing the building when the external envelope is complete will confirm the performance of the building.
What standard should my building achieve?
The recommended airtightness specification for your building depends on its type and use. By good design, attention to detail and good levels of quality control on site 5 m3/(hr.m²) for commercial buildings and 7 m3/(hr.m²) for domestic buildings should be achieved with minimal or no additional expense. In other words if we do what we’re supposed to be doing anyway!
Upcoming changes to the building regulations will further increase these airtightness requirements of buildings in line with ‘nZEB’ targets.
Airtight buildings now and in the future
Increasing levels of insulation together with airtightness are bringing us closer and closer to the building which only requires a heating system for pre-heating the building during the winter season.
The relatively large heat losses associated with infiltration, which can typically be up to 30 W/m² are practically eliminated.
Internal heat gains from lighting, equipment and people are now sufficient to match the much smaller heat losses.
The traditional ‘perimeter heating’ solution that we so often see- radiators dotted under the windows- is no longer necessary.
Draughts, perimeter heat loss, and window down draughts are no longer a problem.
The capacity of heating plant is now reduced and much more closely matched to peak loads, meaning higher operating efficiencies.
Comfort within the building is increased, and together with simplification of control strategies this will result in happier building occupants.