Air-tightnessHaving high levels of insulation is only one aspect of minimising heat loss from buildings. However there is another aspect, equally as important, which needs careful consideration. This is 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.

It's important to stress however that ventilation is not a problem. In fact it can be a very good thing. Ventilation can help to control 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 Ecological 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.

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 signs of draughts


Air-tightness tape applied to floor junctions.


A blower door test in action.