Passive Ventilation: A Better Idea?

As the movement towards sustainable, energy-efficient houses increases, so does the interest in passive ventilation systems. They are increasingly seen as an essential component of a sustainable home, but what is passive ventilation and how does it work?

Why Ventilate?
Ventilation of any sort only became an issue in UK homes after the mid 1940s. Until then houses were largely ventilated by default — draughts around windows and doors, movement in and out of the house and the odd airbrick provided all the ventilation needed. After the mid 1940s, air conditioning became more prevalent, construction methods improved and gas-fired central heating began to be installed. All led to the need to consider how much fresh air was required in a house and how it was to get there. But as late as the 1960s, an airbrick close to the gas boiler and the ability to open a window was still considered sufficient.

It is not until we get close to the 21st century that ventilation ‘systems’ start to become the norm. The main issue now is the realisation that uncontrolled ventilation means uncontrolled heat loss — anathema to energy-efficient house design. This has led to the development of three main passive ventilation ‘systems’: natural, passive stack and wind effect.

Natural Ventilation
Natural ventilation is more a matter of designing the house appropriately — lowlevel inlets, high-level outlets, both positioned to work with solar heat to allow natural air-flows to move through the hoause. Often extract fans will still be needed in wet rooms, but otherwise a well-designed house will allow air warmed by the sun to move through the interior, taking with it smells and excess moisture.

Natural ventilation systems can include some form of control to the vents (manual or automated) and specific windows. On more sophisticated systems, the inlet vents will alternate between north and south — in summer allowing in cooler air from the north elevation and in winter using the warmer air from the south.

Passive Stack Ventilation
Passive stack ventilation (PSV) systems are based on the ‘stack effect’ or ‘thermal stack’ principle — which basically means that hot air rises. PSV systems ventilate the whole house by siting extract fans in ‘wet’ rooms (kitchens, utility and bathrooms) and ducting vertically to terminals on the roof.

Wind will create a pressure difference around the roof terminal which helps warm, moist air to be drawn up the ducts, to be replaced by fresh air coming in through inlet vents in walls or window frames. This flow of fresh air from ‘dry’ to ‘wet’ areas creates whole-house ventilation.

Wind Effect
These systems rely on the same principle as the old middle-eastern wind towers that push air down into the building, with cowls that use the wind to draw air from inside.

These systems usually provide a fair degree of control over the amount of air moving through the building and the direction of its movement, by employing moisture- or pressure-sensitive actuation on the vents. And they use little or no energy to do it. In sophisticated systems, the incoming fresh air can be warmed by the outgoing stale air.

MAIN IMAGE: Clockwise: Four Smart Solutions. Fakro’s automatic air-inlet rooflights are perfect for ventilating loft conversions; Monodraught’s PSV unit uses a solar-powered fan for more precise control; Arup’s PSV has a special cowl and ducting system, which warms the fresh air; This electronic pyramidal rooflight by Glazing Vision tilts to provide ventilation

Futher Reading:

• Electrics: Ventilation and Built-in Vacuums 
• Passive Ventilation
• Green Technology: Heat Recovery