The most common misconception about an 'airtight' building is that it will be suffocating or stale internally. Whilst a constant supply of fresh air is essential in a passivhaus (and is achieved through the use of a carefully designed MVHR system), it is important to remember that in the context of construction 'airtightness' is not meant literally.

A typical semi-detached house that achieves the baseline passivhaus standard of 0.6 air changes per hour (at 50Pa) still has the equivalent amount of 'gaps' or air leakage as having a single hole in the wall of approximately 0.01m2, i.e. a 10cm x 10cm hole! This may sound unlikely but if this area is considered as a crack around a 10 x 10m floor area this would only be 0.25mm wide, which illustrates the attention to detail that is required to achieve this level of airtightness.

The airtight layer

The thermal envelope in any airtight building should be enclosed by one continuous airtightness layer, which is usually on the internal face of the insulation layer. The purpose of this layer is to make sure that as little heat is lost as possible due to the uncontrolled passage of warm air from inside the building to the outside via cracks and gaps in the construction. This can be in the form of a membrane, or a board material, with taped joints. It is important that the correct products are selected for the particular application: often it is possible to use a board material that can also double as a structural layer, and/or a membrane and that can also be a vapour control layer, and so reduce the costs of this part of the construction. 

In relative terms the airtight layer is not a particularly expensive element of the construction, however it plays a vital role in ensuring the designed performance becomes a reality. Critical to the success of the airtight layer is the detailing and then the installation. All junctions between parts of the airtightness layer must be considered, drawn in detail, and then the execution monitored closely on site. 

The airtightness test

It is mandatory to measure the airtightness of the constructed building before completion can be achieved, however in a passivhaus building where a stringent target is being sought we would also recommend testing prior to any wall linings being fitted. This will give the best possible chance of achieving the desired result upon completion, and minimise the disruption required to remedy any defects that may be discovered when the building is tested.

The testing is undertaken using a door-blower fan, which is basically a large fan (connected to a computer) that sits within an adjustable frame and seals off one opening to the building, typically a doorway. The fan is then used to both pressurise (blow in air) and de-pressurise (suck out air) the building with the aim of measuring the fan speed required to maintain a 50Pa air pressure difference between the air pressure within the building and the external air pressure. Multiple readings are then collated and an average value is produced, which is the stated airtightness level of the building, quoted in 'air changes per hour at 50Pa pressure difference'.

Significantly in the testing required for a passivhaus the building is both pressurised and de-pressurised which produces a more stringent result as seals are more rigorously tested, however in the normal testing undertaken for a Building Warrant the building is only required to be pressurised.

Windtightness

It is also important for a building to be 'windtight' on the external face of the insulation envelope. Thermographic imaging of airtight buildings taken during windy weather has shown that a significant amount of heat loss can be experienced if there is no 'windtight' layer on the outside of the construction. This layer can be achieved in much the same way as the airtight layer using membranes, board materials or sometimes insulation materials themselves, and is normally positioned on the external face of the insulation layer.