Ventilation of a house extension
In the UK, domestic buildings have traditionally relied on air infiltration through the
building fabric to provide background ventilation. This is supplemented by extract ventilation fans or by opening
windows when additional ventilation is required.
Modern construction methods and regulations deliver a higher standard of airtightness, and it is no longer
acceptable to rely on infiltration to provide background ventilation. The maxim is ‘build tight, ventilate
In extensions, the provision of appropriate, controlled ventilation is therefore essential, in order to ensure
good air quality and avoid the risk of surface condensation. However, excessive ventilation results in unnecessary
heat loss, and consequently increased fuel use, fuel costs and carbon dioxide emissions.
Ventilation falls into the following three types.
• Background ventilation - provided by air bricks, trickle ventilators in window heads, or facilities to secure
windows slightly open in a ‘slot ventilation’ position.
• Rapid or ‘purge’ ventilation - provided by opening windows, when there is a need to expel pollutants or admit
• Extract ventilation - provided to expel moist stale air from ‘wet areas’ (i.e. kitchens, bathrooms and utility
rooms) in order to reduce the risk of surface condensation.
Minimum requirements for each type of ventilation are set out in the building regulations for each part of the
Energy efficient ventilation is achieved by providing ventilation only when and where it is needed. Wet areas
must be provided with extract ventilation, in the form of electric fans or ‘passive stack ventilation’.
• Extract ventilation fans should be controlled by humidistats, or wired to operate with light switches (with
• Energy efficient, low power fans incorporating DC motors are now available. Fans of this type reduce the fuel
use, fuel costs and carbon dioxide emissions associated with providing ventilation.
Heat recovery room ventilators (HRRVs) combine supply and extract fans in a single ‘through the wall’ unit.
Extracted warm stale air is passed over a plastic cross-flow heat exchanger where heat in the air is transferred to
the cool, fresh external air that is supplied to the room. HRRVs reduce the heat loss penalty associated with
electric ventilation fans.
Passive stack ventilation
If the extension has two storeys, or a single storey with a pitched roof, it is often appropriate to provide
extract ventilation by means of passive stack ventilators. These consist of vertical plastic ducts that connect
ventilation grilles at ceiling level in ‘wet spaces’ to terminals on the roof of the building. Warm moist air rises
up these ducts because of its natural buoyancy (assisted by wind blowing across the roof) and is replaced by fresh
air that enters via trickle ventilators in window heads throughout the house. Passive stack ventilation works best
when the terminals are located at or near the highest point of the roof (usually the ridge). It is especially good
for ensuite bathrooms, because it is silent.