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 right’.

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 fresh air.

• 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 UK.

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 timed ‘run-on’).

• 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.