10.1 General
The occupation of buildings, with associated activities and processes, produces moisture. This needs to be removed to outside air by ventilation, to avoid high relative humidities that can result in problems of condensation and mould growth.
For the purposes of this standard, the function of ventilation is to control the humidity of internal air to between 40 % r.h. and 70 % r.h. However, short periods when the relative humidity is outside this range need not necessarily lead to problems of discomfort or mould growth.
Some building uses, such as swimming pools and laundries can have normal relative humidities in excess of 70 %. These types of buildings require specialist knowledge in design and use, but the design principles of Clause 10 nevertheless apply.
Building occupants are sensitive to discomfort from draughts and therefore have a tendency to reduce ventilation during cold weather. The effect of modern building, coupled with the provisions in the Building Regulations is to reduce ventilation rates due to air leakage.
From the point of view of controlling condensation, the ideal ventilation system will provide either: a) finely controllable background ventilation; and mechanical extraction of water vapour from moisture producing areas such as kitchens and bathrooms; or
b) continuous ventilation either by use of passive stack ventilation or a mechanical ventilation system. Further information on ventilation provision is given in CIBSE Guide B, Ventilation and air
conditioning [10]. The meteorological data needed for ventilation design is available in CIBSE Guide J, Weather, solar and illuminance data [11].
10.2 Natural ventilation
In many existing buildings, the only control the occupier has of ventilation is the opening of windows. In winter, this method is unlikely to be used unless the window incorporates provisions for controllable background ventilation. Where such trickle ventilation is not provided, controllable slot ventilators should be installed in the windows or in the walls. The ideal place to install these ventilators is in the top section of an opening light in the window.
These background ventilation openings should be installed in all occupied rooms. Every room should have background ventilation openings of at least 4 000 mm2. Habitable rooms in dwellings should preferably have background ventilation openings of at least 8 000 mm2. In occupiable rooms in buildings other than dwellings, the background ventilation provision of 4 000 mm2 should be increased by 400 mm2 per square metre of floor area where the area of the room exceeds 10 m2.
While such small ventilation openings will normally provide adequate background ventilation, they are unlikely to be able to cope with high amounts of water vapour production. Natural ventilation should therefore be supplemented by mechanical extract ventilation or PSV in moisture producing areas to remove water vapour to the outside of the building.
10.3 Passive stack ventilators
Passive stack ventilation (PSV) is a ventilation system using ducts from the ceiling or walls of rooms to terminals on the roof which operate by a combination of the natural stack effect, i.e. the movement of air due to the difference in temperature between inside and outside and the effect of wind passing over the roof of the dwelling. In order to maximize the stack effect, ducts should be as near vertical as possible, and should never be at an angle of more than 45° to the vertical. Ducts should be insulated, to prevent condensation, where they pass through unheated spaces such as lofts. Humidity sensitive grilles are available that provide increased flows when water vapour is being generated.
Trickle ventilators should be provided in the rooms where moisture is not produced so that air is drawn through these dry rooms to exit via the moisture producing areas through the stack ventilator. Ideally, when using natural ventilation, outside air should be drawn in through dry rooms located on the windward side of the building to exit via the moisture producing areas, which should be sited on the leeward side of the building. Further information on the performance of PSV is available in BRE IP 13/94 [12].
10.4 Forced ventilation
Forced ventilation is more reliable than natural ventilation because air change rates and directions can be better controlled. Where mechanical ventilation is provided in kitchens and bathrooms, it may be used for short periods to assist natural ventilation.
With warm air systems the incorporation of a percentage of outdoor make-up air can induce satisfactory ventilation throughout the building (see 10.2).
With heating systems that do not provide air movement, mechanical extract ventilation is recommended in all moisture producing areas such as kitchens or bathrooms. However, extractor fans should not be installed in the same room as an open flued heating appliance burning solid fuel or oil as they can draw flue gases into the room.
The design of the forced ventilation system should be such that the action of fans will reduce moisture at source, reduce the spread of moisture to the rest of the building, and cause outdoor air ventilation of other rooms.
Fans should be of sufficient power to ensure that they can extract, if necessary, against the prevailing wind. The sizing of fans depends upon the rate and amount of moisture generated in the room. It should be borne in mind that when fans are not operating, the air movement patterns induced by the wind will be
re-established.
When air movement into moisture producing areas is from other parts of the building, care should be taken to ensure that these rooms are not significantly cooler than the rest of the building as this could exacerbate the risk of condensation in these areas. It should also be noted that air extracted will be replaced by incoming air which in turn should be heated.
There is evidence to show that extractor fans are frequently not used and consideration should be given to the use of humidistat controls. However, it should be borne in mind that certain types of humidistat controls require periodic maintenance.
Speed controls on fans are an advantage so that extraction can be reduced to a trickle when large amounts of moisture are not being generated. By operating fans in this way, the fans can assist in providing background ventilation to the whole building.
A variety of mechanical ventilation systems are available (see BRE Digest 398 [13]), the simplest being central mechanical extract ventilation which removes moisture laden air from several rooms and using ductwork, discharges to the outside of the building from a single remote fan. Mechanical ventilation systems with heat recovery provide outdoor air to the whole building and extract ventilation from all moisture producing areas such as kitchens and bathrooms.
Incorporating a heat exchanger means that heat contained within the exhaust airstream can be partially recovered and used to warm the incoming outdoor air. Individual heat recovery room ventilators are also available which provide outside air and extract from individual rooms in buildings.
10.5 Mechanical supply ventilation systems
Supply ventilation systems are fans that are often mounted in the loft, which supply air continuously to the centre of a house, usually through the ceiling of the landing. Besides providing a continuous supply of outside air, these systems benefit from some degree of solar gain on the roof, supplying air that is warmer than outside. Care should be taken to ensure that the fans are as quiet as possible and are not mounted in a way to cause resonance. The inlet air supply vent should be arranged so that the effect of cold draughts is minimized. Trickle ventilators should be provided in all rooms to allow for air to disperse to outside. This system is unsuitable for unventilated lofts.
10.6 Mechanical ventilation with heat recovery
Whole house mechanical ventilation with heat recovery (MVHR) is a ventilation system that combines supply and extract ventilation in one system. Whole house units comprising of two fans and a heat exchanger are often mounted in the loft (wall mounted units are also available), the supply fan continuously provides outdoor air to all habitable rooms using a system of ductwork. The extract fan removes warm moist air from kitchens and bathrooms via a system of ductwork and passes it across the heat exchanger before it is discharged to outside. The heat recovered preheats the incoming outdoor air. All ducts in the loft should be insulated to minimize condensation, and care should be taken to ensure that the fans are as quiet as possible and are not mounted in a way that could cause resonance.
Individual room heat recovery ventilators are mounted in external walls and each ventilator comprises of two fans and a heat exchanger. Warm air is extracted from the room and passes across the heat exchanger before being discharged to outside. The heat recovered preheats incoming outdoor air to the room. These units normally have two speed settings, low speed to provide continuous background ventilation and high speed boost.
10.7 Heated drying cupboards
Where a heated drying cupboard is provided it should be ventilated to disperse moisture directly to the outside air. Alternatively, it could be located in e.g. a bathroom, vented to that room and a humidistat controlled extractor fan fitted in the room.
10.8 Dehumidifiers
Electric dehumidifiers that work on a closed refrigeration cycle both dry and heat the air. As some latent heat is released when the water condenses, the heat output is 10 % to 30 % greater than electricity consumed (typically 200 W to 300 W). An essential feature of all dehumidifiers is that the amount of water they extract is dependent on the temperature and vapour pressure of the air. They are more effective in warmer dwellings where condensation problems are caused by high vapour pressures, than in more typical condensation prone houses where problems are caused by low temperature. They tend to be fairly obtrusive and too noisy to run in bedrooms overnight and are not acceptable to all householders. However, their portability means that they can be used on a trial basis and moved elsewhere if need be. Some
dehumidification systems can be mounted in the loft or a cupboard, but that makes the installation more expensive.