Housing Retrofit: Whole house ventilation

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)

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.

Pro In its simplest form PSV requires no energy use
Con Uncontrolled background ventilation can give rise to over-ventilation
Con Wind-induced air leakage will occur around inlets
Con Heat is lost through vented warm air
Con Requires space for ducting


Mechanical extract ventilation (MEV)

Mechanical extract ventilation (MEV)

Mechanical extract ventilation (MEV) is a system that continuously extracts air from ‘wet’ rooms. The moist air is drawn up ducts through to an outlet at or above the roof ridge. The 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. Air is drawn by a centrally located (eg in a cupboard or loft), extract system though other systems exist using multiple fans.

• NB: To achieve the EST ‘Best practice’ standard, ‘the whole system must have a specific fan power of 0.6W/l/s or less when running at each of its settings’.

Pro Provides controlled and controllable ventilation
Con Heat is lost through vented warm air
Con Requires space for ducting
Con Fan noise can be an issue if extract unit is not located sensitively
Con Wind-induced air leakage will occur around inlet
Con Requires electrical energy to operate


Mechanical ventilation with heat recovery (MVHR)

Mechanical ventilation with heat recovery (MVHR)

Appropriate for use where air leakage rate has achieved < 3 m3/m2.h @ 50 Pa or lower, mechanical ventilation with heat recovery (MVHR) systems provide the optimum form of ventilation control. Tried and tested, particularly in Germany where insulation and airtightness standards have been traditionally been higher, MVHR systems are becoming much more common in the UK market as the implications presented by the Code for Sustainable Homes become apparent for highly energy efficient buildings.

MVHR differs from mechanical extract ventilation and passive stack ventilation in that the fresh air supply is provided by the one mechanical source. As in the other systems, warm moist air is extracted from the ‘wet’ rooms via ducting, but before passing to the outside, the air passes through a heat exchanger where the heat is passed to the incoming fresh air that is ducted through to the ‘dry rooms’.

Though MVHR heat exchangers are very efficient – manufacturers are quoting 90-95% efficiency – there is, of course, a small heat lost incurred.

• Note: that when using the fan power assumed by SAP 2005 of 2W/l/s, calculations can result in figures that look only marginally more efficient than natural ventilation – and then only at the highest standards of airtightness. The ‘Best practice’ fan power for MVHR as defined by the EST is for less than 1W/l/s. Using the latter figure in the SAP calculations dramatically improves the efficiency of the MVHR system, making it efficient to an air tightness of down to q50 10.

Pro Provides controlled and controllable ventilation
Pro Recovers heat from extracted air, so reducing heating demand
Pro Can provide air filtration to incoming air
Con High capital cost
Con Requires a high level of air-tightness
Con Complex to install and commission
Con Requires space both for extraction and delivery
Con Filters need to be replaced at regular intervals
Con Occupants might find this type of ventilation control over-restrictive - there can be a steep learning curve for residents new to the MVHR concept
Con Requires electrical energy to operate




Building Regulations

Approved Document F: Ventilation 2006 edition

British Standards Institute (BSI)

• BS 5250: 2002 Control of condensation in buildings
• BSEN 13141-7: 2004, Ventilation for buildings - Performance testing of components/products for residential ventilation - Part 7: Performance testing of a mechanical supply and exhaust ventilation units (including heat recovery) for mechanical ventilation systems intended for single family dwellings.

Chartered Institute of Building Services Engineers (CIBSE)

Handbook of Domestic Ventilation, Roger Edwards, 2005
Mixed mode ventilation, 2000
• Improved Life Cycle Performance of Mechanical Ventilation Systems, CIBSE / DTI 2002

Building Research Establishment (BRE)

Passive stack ventilation systems: design and installation, Stephen et al, 1994
Digest 297,Surface condensation and mould growth in traditionally-built dwellings. BRE, 1985
Positive Input Ventilation in Dwellings, Stephen, 2000

Further information

The Chartered Institution of Building Services Engineers



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