VENTILATION

All buildings require a constant source of fresh air to ensure the health of its occupants and the buildings health by reducing the risk of condensation, mould and rot. Natural Ventilation is the most common type found in UK houses but it has the disadvantage of often taking too much heat out of a building.

Home energy use is currently responsible for 27% of the UK's CO2 emissions. In a typical home 20 per cent of all heat loss is through poor ventilation and draughts. It is imperative that as many steps as possible are taken to ensure none of the heat in a building goes to waste.

Passive Stack Ventilation (PSV) systems use the heated air of a building to draw air through a building. As the hot air rises out of a building it pulls fresher air into it. This has the advantage of a completely silent ventilation system and also does not require any fans to operate, reducing the energy requirement of the building.

Wind based ventilation systems are more powerful the PSV systems. These are designed into a building so that the external air pressure on one side of the building is lower than the side facing the wind. Wind is then channelled through the building, replacing old air and drawing it out into the lower pressure of the outside air on the non-wind facing side.

Mechanical ventilation systems require the use of fans to move air around a structure. These can range from small extractor fans to large central air conditioning systems. These ventilation units often use a lot of power, so it is essential that they are working at optimum efficiency. However they are far easier to control than natural systems. Whole House Mechanical Ventilation Systems with heat recovery (WHMV) combined with an airtight house are often rated as the most effective forms of ventilation.

Fitting heat exchangers to a ventilation system will substantially increase their efficiency. These systems work by exchanging the heat of the air leaving a building into the air coming in, pre-warming it and reducing the gap between fresh air temperature and the internal air temperature. The environmental benefits of a heat exchanger will always be balanced by its electrical consumption.

fan air conditioning

A good control system will also cut back on the amount of energy a mechanical system uses. These can be set to watch for specific factors such as CO2 level, humidity and other pollutants. This allows the air ventilation to work at an appropriate speed, trickling air through normally or boosting air through when large amounts of air pollutants are found (i.e. cooking or smoking). One of the main factors that affect the efficiency of a building's ventilation systems is its air tightness. This is because air permeating through the structure can cause draughts and alter the channels of airflow around and inside a building.

Air leakage also affects the insulation of a building because if air can get in, heat can get out. When starting a new build it is essential to design a building with an air barrier to prevent as many leaks as possible. Quality control must be performed at every step of the way to make sure air leakage is reduced. This can be done by insuring that workmanship standards are kept as high as possible and that adequate materials are used.

Air leakage is quantified as 'air permeability' and is calculated by inducing a building to positive and negative pressure. By assuming a normal pressure of 50Pa the difference is the amount of air escaping/invading the property. This is measured in cubic meters or air per hour per meter squared of a building's area. Ideally a rating of 3 meters cubed should be allowed in or out through air leakage although up to 5 is acceptable.

Common areas that air leaks are found in are:

Underfloor ventilator grilles.
Gaps in and around suspended timber floors.
Leaky windows or doors.
Gaps around windows.
Gaps at the ceiling-to-wall joint at the eaves.
Open chimneys.
Gaps around loft hatches.
Vents penetrating the ceiling/roof
Bathroom wall vent or extract fan
Gaps around bathroom waste pipes
Kitchen wall vent or extractor fan
Gaps around kitchen waste pipes
Gaps around floor-to-wall joints (particularly with timber frame)
Gaps in and around electrical fittings in hollow walls

Many of these gaps can be filled using the multitude of sealants available from DIY stores. Mastic seals should be applied to all door and window frames and cracks in the floor/roof/framework should be repaired immediately. When a house has been made as airtight as possible it allows the ventilation to do its job to the required standard. This allows smaller and more efficient ventilation and heating systems to do the same job as a larger system in a leaky house.

As a more complete system than passive cooling and ventilation, mechanical ventilation with heat recovery (MVHR) can provide a good solution for zero or near zero emission buildings.

The combination of an airtight building and a good controllable ventilation system with heat recovery units in place will allow a building to drastically reduce the heat lost. It is one of the few times when a man-made system will be favourable to a natural system for green building due to the ability to be controlled, and only provide the level of ventilation required.

Examples of energy efficient homes are at the Innovation Park at BRE Watford.

See the Building Research Group (BRE) www.bre.co.uk