Advice: Airtightness

Having high levels of insulation is only one aspect we seek to address when designing to minimise heat loss from buildings. Another critical aspect, which needs careful consideration is airtightness. Airtightness in buildings is the reduction in  unwanted ventilation / draughts.


Draughts can account for over 50% of the total energy used in a building, as heating systems compensate for the heat being lost to the outside from uncontrolled draughts. Furthermore, draughts can lead to additional problems:


  • Occupant discomfort, as air moves through spaces at variable rates.


  • Unwanted draughts through a construction can lead to condensation and mould growth, which in turn might degrade materials, reducing their lifespan. 


  • Subsequent mould growth could lead to poorer internal air quality, possibly causing respiratory related health problems.

Air tightness tape used to seal key junctions and minimise draughts. Photo by Fox (2011). 

Methods of minimising un-controlled ventilation in new buildings include using high-performance air-tightness tapes and air-tightness layers or membranes, which wrap around the entire external envelope.


Air-tightness can be assessed using blower door measurements, where the building is pressurised to measure the flow of air through the fabric. Such measurements are required if seeking to achieve standards such as PassivHaus.

Thermal Image showing draughts from above the window. Photo by Fox (2018). 

It's important to stress however that ventilation itself is not a problem. In fact “controlled” ventilation is essential to a healthy indoor environment. Controlled ventilation can help to deal with internal moisture build-up and replenish stale internal air with fresh outdoor air. Ventilation problems occur when there is no control over the ventilation source.


An example of a controllable ventilation source would be open-able windows and mechanical ventilation systems. An example of un-controllable ventilation would be gaps around windows and doors.


At Fox Eco Architects, we strive for high levels of building fabric air-tightness. Minimising all sources of un-controllable ventilation. Yet we purposefully design in controllable ventilation sources, to control moisture levels. Systems such as MVHR (Mechanical Ventilation with Heat Recovery) units are required for buildings constructed to the PassivHaus standard, and are what we typically recommend for good ventilation control, which retains the heat from the internal environment through the use of a heat exchanger. 

Airtightness testing using a blower door measurement system. Photo by Fox (2022). 

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