Questions about Air-Tightness
What is airtightness in buildings?
Airtightness is the uncontrolled flow of air through gaps and cracks in the building fabric. This is not to be confused with the controlled flow of air into or out of the building through purpose-built MVHR units.
Gaps and cracks which create drafts can account for up to 25% of all heat losses through the external envelope of a building, and especially a Passivhaus building. There are caused be caused by poor build design, poor workmanship or the use of wrong or inappropriate materials.
Why is it important to build airtight dwellings?
The energy that we use to heat our homes is primarily created by burning fossil fuels that produce carbon dioxide. By reducing air leakage of a building, we also reduce the amount of energy required to maintain comfort levels and in turn reduce carbon dioxide emissions.
Along with a properly designed ventilation system, airtightness will eliminate damp and mould growth in the building fabric and vastly improve indoor air quality. Good airtightness also requires a higher focus on build quality and quality workmanship.
How to achieve airtightness?
There are three key steps to achieving airtightness in any building type.
Design for Airtightness: To maximise the Airtightness of any building the Architect plays a pivotal role. Airtight details for common leakage areas which are difficult for the builder to seal on site can be greatly simplified leading to vast improvements in workmanship. Consequently, superior levels of Airtightness can be achieved, and of course, significant savings can also be made as a result of time savings on-site, at the installation stage. The Architect can also specify materials and propriety Airtight Sealing Products which simplify the Airtight sealing process for the installer and guarantee a clean, durable and lifelong seal.
Build for airtightness: When the building is at the construction stage communication between trades is absolutely essential. It is also essential that the airtight installer maintains a high level of consistency and consciousness to continually seal the airtightness layer at a premium level.
Test for airtightness: After the building has been sealed for Airtightness the construction must be tested to identify the level of airtightness achieved and also to identify leakages in the building envelope.
What are the common causes of air leakage?
When there is major air leakage into a floor void, every joint, edge, socket, switch and any other penetration becomes an air path into the heated envelope. Porous block work can allow air to enter a cavity behind plasterboard on dabs - which will communicate to the rest of the dwelling. Even when the cavity is filled with insulation, air will leak through it.
Common air leakage areas are:
extractor fans/cooker hood
eaves, cracks, holes in the inner walls/lining
ceiling roses/fused spurs/sockets/switches
room stats/heating controls
ends of floor joists/hangers - especially joists that penetrate walls
chimneys, particularly where flue dampers are not fitted
recessed ceiling spots
windows and hollow frames
beneath inner window sills
letter boxes, key holes
under and around door frames - especially double doors
How do you measure air leakage?
The pressure differential is measured by means of a large fan installed temporarily and a range of static pressures and environmental readings are taken. The fan is switched on and the air pressure in the property is gradually increased or decreased and the differential pressure is recorded at each step.
The result of the air leakage test is expressed as a quantity of air leakage (m3 per hour) per square metre of building envelope. The maximum allowable air permeability rate for Building Regulations compliance is 10 m3/h.m2, however it is now very common for a dwelling to have a lower design air permeability rate in order to meet the target CO2 emission rate (see SAP calculations). For example, most new-build houses have a design air permeability rate of around 5 m3/h.m2.
Where mechanical ventilation with heat recovery (MVHR) has been installed the air permeability rate should be no greater than 3 m3/h.m2 for the system to operate effectively.
Passive house by comparison should be no greater than 0.6 m3/h.m2 to achieve the standard.