Ventilation for a loft conversion

Housing in the UK has traditionally relied on air infiltration through the building fabric to provide background ventilation.Window opening and the use of extractor fans supplement this when required.

As modern construction methods and regulations deliver a higher standard of airtightness, it is no longer acceptable to rely on infiltration for background ventilation.Today's maxim is 'build tight, ventilate right'. In roofspace conversions, appropriate, controlled ventilation is essential for good air quality and to avoid the risk of condensation. Excessive ventilation, though, results in heat loss with consequent increased fuel use, energy bills and carbon emissions.

Ventilation falls into the following three types.

• Background ventilation is provided by air bricks (with 'hit-and-miss' shutters), trickle ventilators in window heads, and the option on some windows to secure them slightly open in a 'slot ventilation' position.

• Rapid or 'purge' ventilation is achieved by opening windows, when there is a need to expel pollutants or admit fresh air quickly.

• Extract ventilation expels moist stale air from 'wet areas' (kitchens, bathrooms and utility rooms) in order to reduce the risk of surface condensation.

Minimum requirements for each type are set out in building regulations for each part of the UK.

For ventilation to be really energy efficient, it must be provided 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 fuel use, energy costs and carbon emissions.

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 and the heat in the air is transferred to cool, fresh external air coming into the room. HRRVs reduce the heat loss penalty associated with electric ventilation fans.

Passive stack ventilation

In roofspace conversions, extract ventilation can often be provided by passive stack ventilators.These are vertical plastic ducts, connecting ventilation grilles in ceilings of 'wet spaces' with terminals on or above 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 frames throughout the house. For the stack effect to work properly, the ventilation duct must be at least one metre high, and terminate at or above the highest point of the roof (usually the ridge). Passive stack ventilation is especially good for ensuite bathrooms because it is silent.