Measures such as sealing gaps in floors and walls will make your home more air-tight and reduce wastage of heat and money. But with this comes the need to ensure that a supply of fresh air is maintained, to safeguard the comfort and safety of the occupants, and to prevent potential condensation and mould formation affecting the fabric of the building. The mantra often applied is 'Build tight, ventilate right'. Remember that 'air leakage', or draughts, represent uncontrolled air flow, whereas ventilation is controlled to match the needs of the building and its occupants - in the right place at the right time.
It seems obvious that fresh air is vital, but how much is needed and how often? Ventilation rate is the number of times per hour the entire volume of air in a room (or other space) is replaced by fresh air (air changes per hour, or ach). It can also be measured in litres per second. A ventilation rate of between 0.5 and 1.5 ach is generally recommended. This will:
- supply ample oxygen for any people in the room;
- control condensation by keeping relative humidity below 70%, which is vital to prevent growth of mould and proliferation of house-dust mites (both serious allergens for asthmatics);
- prevent a build-up of indoor pollutants, such as volatile organic compounds (VOCs), which are emitted by paints, varnishes, furnishings, and other items;
- disperse carbon monoxide and nitrogen dioxide generated by gas appliances and smoking, carbon dioxide from humans and other organisms, and (where applicable) radon gas;
- remove odours released by cooking, humans, pets, etc.
Note that this level of ventilation will not protect against the effects of tobacco smoke, for which there is no known safe level.
Ventilation systems: the options
A range of ventilation systems are available, varying widely in cost, complexity, and effectiveness. Also, some are applicable more as built-in features for new-build dwellings and may not be appropriate for existing buildings.
Passive stack ventilation (PSV)
This works due to the natural mechanism of the 'stack effect' caused by temperature differences between the inside and outside: cold air enters the lower part of the building and rises to exit at the top. Flow can reverse in summer when the interior is cooler than the outside. Vents in the ceilings of 'wet rooms' such as the kitchen and bathroom connect via ducts to outlets on the roof, and allow the warm moist air to escape. This is replaced by cooler dry air entering via trickle vents in 'dry' rooms, such as living rooms and bedrooms, and air leakage, which disperses to all parts of the building. Dispersal is assisted by gaps at the bottom of internal doors.
- Suitable for retro-fit during major refurbishment
- No running costs and no electrical connections needed
- Silent in operation
- Low maintenance - periodic cleaning of grilles and checks of ducting and roof outlets
- In some designs, heat recovered from the stale air is used to warm incoming fresh air.
- Requires careful design and installation to work effectively
- Performance dependent on weather conditions - but flow can be assisted by an extract fan
- No heat recovery in simpler systems - although performance enhanced by humidity-sensitive inlets
Single room or whole house mechanical extract ventilation (MEV)
Extractor fans draw moist air from wet rooms via ducts to the outside. Dry air enters via trickle vents or leakage. The extractor can be a single unit located in the loft or a cupboard, extracting air via ducts to the roof, and providing continuous low-level extraction, or a boost setting if required.
A more flexible option is to install an extractor in each wet room. The fans operate automatically and independently, e.g. when the light or shower is used, or are controlled by humidity sensors. Heat recovery ventilators incorporate a heat exchanger, and can provide continuous 'trickle' ventilation of wet rooms with heat recovery, plus a high-speed boost setting for rapid extraction.
- Can be adapted to most existing buildings
- Easy to install and operate
- Provide rapid extraction when required
- Can be noisy
- Uses some electricity
- Prone to tampering by occupants
- No heat recovered - unless heat recovery ventilator fitted.
Whole-house mechanical ventilation with heat recovery (MVHR)
A single unit is responsible for both the extraction and supply of air. A heat exchanger recovers heat from outgoing air and uses it to warm incoming air before pumping it to the rooms. Hence, besides ventilation, the system helps to heat the house, and can save energy overall. But achieving such efficiency requires an airtight building. Such a system can also filter incoming air, which may be helpful to people with respiratory conditions, especially in polluted areas. MVHR systems are expensive and require skilled design and installation.
- Systems represent 'best practice' for new-build homes
- Reduced energy demand for space heating
- Filtration of incoming air can be accomplished
- High degreee of control
- High cost
- Requires high level of air-tightness of building
- Requires expert design and installation
- Regular maintenance needed
Note: images courtesy of greenspec (www.greenspec.co.uk).