Why is airtightness important?
Airtightness is important for many reasons, including reducing heat loss, improving comfort, and protecting the building fabric. Airtightness is achieved by sealing a building to reduce infiltration – which is sometimes called uncontrolled ventilation.
Airtightness is measured by monitoring the amount of air that escapes or enters a building at a pressure of 50 Pascals. For Passivhaus calculations, this measurement is expressed in air changes per hour (ACH) i.e. the number of times an hour that the air in the building changes when it is pressurised (either negatively or positively).
UK Building Regulations express this is a slightly different way as the volume of air that escapes per m2 of external surface area, also at 50 Pascals of pressure. This is sometimes referred to as Air Permeability.
Once the insulation levels in a building have been increased and the thermal bridges dealt with, heat losses from ventilation become significant. As shown in Figure 1, ventilation heat losses can account for a large proportion of all heat losses in a low energy building if airtightness is not considered.
Infiltration is the air leakage through the building fabric. It is uncontrolled ventilation, and can lead to drafts. When the air outside is colder than inside, this leakage can be very uncomfortable. Air velocity is one of the basic indicators of thermal comfort. Movement of air at just 0.1m/s can be felt as a draft in cold climates in the winter1. Improving the airtightness of a building is therefore likely to improve comfort.
Protecting the building fabric
All buildings should be airtight. This is not the same as being moisture closed, non-breathable or vapour impermeable. Airtightness is important as it protects all building fabrics from the moisture in the air. Movement of moisture by bulk air movement can carry far more moisture than vapour diffusion and if this air enters the building fabric, then interstitial condensation may occur.
How does Passivhaus compare to other standards?
The Passivhaus standard is typically ≤ 0.6 ACH @ 50 Pa. For retrofit the limiting threshold is the same, but a backstop of 1 ACH is permissible if the target is missed and remedial works have been undertaken. Figure 2 shows the maximum permissible size of gaps or holes in 1m2 of the external envelope for a Passivhaus and other standards.
Designing for airtightness
Fundamentally, the air barrier of a building comprises a set of elements that work together to deliver airtightness. These should be considered iteratively throughout the design process. The air barrier comprises products and installation processes that:
Form an airtightness layer in the floor(s), walls and roof (or top floor ceiling)
Seal the doors, windows and rooflights (if applicable) to the adjacent walls or roof
Link the interfaces between walls and floor and between walls and roof, including around the perimeter of any intermediate floor
Seal penetrations through the air barrier, including
Waste pipes & soil pipesx
Incoming water, gas, oil, electricity, data and district heating, as applicable
Chimneys and flues, including air supplies to wood burning stoves or similar
Connections to external services, such as entry phones, outside lights, external taps and sockets, security cameras, satellite dishes
Simplicity is key in airtightness design. The fewer junctions, balconies, dormer roofs and other features, the simpler the airtightness design will be. The images below show a simple design with straightforward airtightness junctions, and a complex design with multiple junctions. Some features can be added after the basic shell of the building has been designed – for example, balconies are less likely to affect airtightness detailing if they are externally supported.
1. Keep designs simple
2. Choose robust materials and don’t substitute onsite
3. Think about junctions in 3D, visualise how they will be built and produce clear drawings that illustrate this
4. Communicate verbally, ensure all parties have understood the design, use site meetings, workshops, phone calls
5. Put sensible site management processes in place, employ an airtightness coordinator and educate subcontractors
6. Undertake leakage tests whilst the air barrier is still accessible