Occupants of a house traditionally combine background ventilation with the opening and shutting of casement and door to fine-tune the quality of their air. Letting ‘fresh air’ in or keeping ‘cold air’ out is so in-grained in the way that most people control their space that it’s almost visceral – which makes the prospect of turning to less intuitive forms of controlled ventilation both difficult and emotionally charged.
But the fact remains that in older houses around 20% of the energy lost from space heating is lost through ventilation; Within more modern, well-insulated houses where less heat is lost through other means, the ratio increases to above 35%. This loss of heat occurs through the opening of windows and doors, but also through the more chronic uncontrolled ventilation provided by gaps around windows, doors and services penetrations, airbricks and chimney flues etc. If this rate of heat loss is to be significantly reduced, other mechanisms of ventilation have to be introduced.
The ideal solution is one where uncontrolled ventilation is prevented by eliminating the air filtration through the building fabric (airtightness) whilst providing for necessary ventilation in a controlled manner – ‘Build tight, ventilate right’. Through controlling the quantity of air that is drawn into and expelled from a space, a tight rein can be maintained over the amount of energy that is expended in heating that air.
Air tightness standards
• The Building Regulations 2006 standard is 10 m3/hr/m2
• EST 'Good practice' air permeability is 5 m3/hr/m2 and ‘Best practice’ is 3 m3/hr/m2
• The Passivhaus Standard is 1 m3/hr/m2
Ventilation standards
• Common practice ventilation rate is recommended for an entire dwelling of between 0.5 and 1.5 air changes per hour (ac/h)
• Passivehaus recommends 0.3 ac/h
Ventilation design
A design strategy will include:
• Extract ventilation in ‘wet’ rooms such as kitchens, bathrooms, WCs and utility rooms.
• Whole building ventilation which replaces stale air with fresh air throughout the house.
• Purge ventilation throughout the house to remove built-up pollutants. This kind of ventilation is typically provided by the opening of windows.
System types
Heat lost through ventilation is a significant proportion of a building’s overall energy consumption and measures to reduce it should form an essential part of an energy efficient refurbishment.
Selecting an appropriate ventilation system can prove difficult where balances between energy efficiency, occupant education/behaviour and cost will have to be struck.
Having secured a high level of airtightness and in terms of simple energy efficiency, the obvious route will be to select an MVHR system which retrieves most of the heat from extracted air – but the successful implementation of such a system, relying as it does on a degree of occupant discipline, will have to be carefully considered, as will the capital cost involved.
The other systems are less demanding: A passive stack system is the simplest, requiring little intervention from the occupants, but this should be balanced out against its capacity to both under and over-ventilate; A mechanical system will provide more control over humidity levels but will require a higher to degree of user intervention. Both systems are incapable of retrieving heat from extracted air.
Passive Stack Ventilation (PSV)
This form of ventilation is based upon the ‘stack effect’ whereby warm air naturally rises within a space to be replaced by cooler incoming air at low level.
Vents are located to rooms that need fresh air to replace air that is moisture-laden or odorous such as in bathrooms and kitchens. The warm moist air is drawn up ducts through to an outlet at or above the roof ridge. Warm air is replaced by fresh air that is drawn in through background ventilators located in ‘dry rooms’ (usually trickle ventilators in eg window frames) and through the building fabric in less airtight buildings.
PSV enhancement:
Humidity controlled outlets
Whereas over the period of a typical day a simple PSV system will provide adequate extraction suffice to control humidity (approximately the same as a mechanical fan), peaks of humidity are likely to occur. These peaks can be remedied by installing humidity-sensitive outlets that increase air-flow under humid conditions.
Mechanical enhancement (‘Assisted PSV’)
PSV systems are to a large extent dependent on external wind conditions to create suction using the ‘Venturi effect’. This dependence can often result in under-ventilation when wind speeds are low and when in summer where the temperature differential is low. This is particularly aggravated where high levels of airtightness have been achieved. For PSV to perform adequately (EST ‘Best practice’) in an efficient house it will be necessary to introduce in-duct either running continuously (trickle) or augment extraction only when required.
