'Natural environmental ventilation' is defined as environmental ventilation driven by the natural forces of wind and temperature1. It is intentional and, ideally, controlled. It should not be confused with infiltration, which is the unintentional and uncontrolled entry of outdoor air through cracks and gaps in the external fabric of the building.
Natural environmental ventilation is not only about openable windows. Rather, it is a holistic design concept. The design focuses on using passive means for ventilation based on a combination of wind pressure differentials and stack effects (buoyancy forces induced by the temperature difference between inside and outside). Fresh air is supplied to building interiors even when the windows are closed. Designs often incorporate atria or internal stairwells to take advantage of these effects.
Winter environmental ventilation
The key issue for winter environmental ventilation is the control of indoor air quality. Experience shows that windows are usually kept shut during winter in naturally ventilated buildings. The requirements for winter indoor air quality control can usually be met by installing controllable trickle ventilators2.
Summer environmental ventilation
Summer environmental ventilation usually focuses on minimising overheating. Because of this, airflow rates typically exceed those required to satisfy fresh-air needs. Design still has to ensure good air distribution to enhance comfort conditions and freshness.
Opening sizes are very different to those needed in winter. Spaces can be ventilated either by single-sided or cross-flow ventilation.
Single-sided ventilation - large, natural ventilation openings such as windows and doors are situated on only one external wall and the exchange of air takes place by the turbulence of the wind and local stack effects
Cross-flow ventilation - this occurs when the inflow and outflow openings in external walls have a clear internal path between them.
Stack ventilation, on which the principle of atrium environmental ventilation is based, is used to describe those strategies where natural, temperature- induced driving forces (sometimes assisted by using low-energy fans) promote an outflow from the building, thereby drawing in fresh cool air via ventilation openings at low levels. (Purpose-designed ventilation openings can allow windows facing busy roads to be kept closed to minimise possible vehicular noise and air pollution3.) Atrium ventilation is essentially cross-ventilation as far as occupied spaces are concerned. Air enters these spaces from the building perimeter side and leaves from the atrium side.
If the airflow is temperature-driven, it is said that this method of ductless ventilation works best for offices that are open-plan and at least three floors high. Buoyancy is improved as the stack height increases. This approach sets minimum design dimensions for total stack height and for the height from the top-floor ceiling to the top of the stack. Air inlets may need to be larger at higher levels to compensate for the reduced driving forces.
Solar chimneys, which have glazed elements in the chimney structure, can enhance stack pressure. The positioning and height of the chimney have to minimise any outflow reversals.
Wind pressures can also be used for ventilation by placing the outlets in a negative wind pressure zone relative to the inlets. Design has to determine the different sizes of ventilation openings in each floor of the building to give the ventilation rates required for that storey. A relatively simple design procedure to do this is now available4.
Fire safety in atria
The fundamental objective is for the occupants to escape in safety. Fire design should also allow the fire services to gain safe access to the fire, extinguish it, and therefore minimise property damage and consequential loss.
All parts of the building separated from the atrium by fire-resisting and smoke-resisting construction can be assumed to be catered for by the usual requirements for non-atrium buildings (eg bs 5588: Part 115 in the case of office buildings). We are only concerned here with those spaces in the building which form part of the same fire compartment as the atrium void, and which by definition form a multi-storey space lacking internal compartmentation.
We need to design into the atrium and its surrounding spaces sufficient provisions to control the creation, movement and venting of smoky fire gases. Any stair in the atrium void cannot be regarded as a safely protected escape route unless special measures have been taken. Furthermore, escape routes within an adjacent space should allow people to move away from the atrium void towards a protected stairwell.
Much of the seeming complexity of smoke-ventilation designs in atrium buildings comes about because of the existence of different smoke-control strategies and because the buildings themselves differ in both construction and use. It is helpful to categorise these differences.
The sterile tube - the atrium facade is both fire-resisting and smoke- resisting. In this case the atrium void is a single-storey room with a high ceiling, requiring no special measures
The closed atrium - the atrium facade is not fire-resisting and may not be smoke-resisting, ie the atrium facade may be 'leaky'. This will usually be the case where the natural environmental ventilation depends on air movement from adjacent occupied spaces into the atrium through an otherwise closed facade
The open atrium -where there are large openings between the adjacent spaces and the atrium void
Mixtures of the above.
Evacuation strategy also has a major influence on construction. Evacuation is either simultaneous from all storeys, or phased, where the building is evacuated in a controlled manner, usually two storeys at a time. The protected stairwells needed for phased evacuation can be narrower, but must be usable for a longer time, than for simultaneous evacuation.
A second way of categorising a building is by its use. The recently published bs 5588 Part 76 recognises the following:
Category A - occupants are awake and predominantly familiar with the building. This category will include almost all office buildings
Category B - occupants are awake but unfamiliar with the building. This will include shopping malls and most public-assembly buildings
Category C - occupants who are likely to be asleep. There are three subdivisions. Flats, halls of residence and hotels are included
Category D - occupants requiring medical or nursing care. This includes hospitals.
In general safety requirements become more onerous from A to D, largely because evacuation of occupants is likely to take progressively longer times.
Alternative smoke-control strategies
Do nothing - where the atrium will take a much longer time to fill with smoke to a dangerous condition than the occupants need for escape, doing nothing becomes a viable strategy. However, good practice requires that the smoke-control designer should be able to demonstrate this by calculation.
Note that Category A buildings such as offices, with simultaneous evacuation and less than 30m tall, are allowed to adopt the 'do nothing' strategy provided that:
- the atrium has a smoke clearance system (see below)
- the building uses a simultaneous evacuation strategy
- the atrium has a large enough 'dead volume' at its highest levels to extend the 'time to danger' as it fills with smoke.
Smoke clearance - this entails having enough smoke ventilation to remove smoke from the atrium after the fire has been brought under control. This is useful for bringing the building more quickly back into everyday use after the fire has been extinguished.
Smoke exhaust ventilation from the atrium - this uses the buoyancy of the smoky fire gases to form a buoyant layer safely above people's heads, or at some other specified height. The designer must calculate the exhaust capacity (if fans are used) or ventilator areas and the inlet area for replacement air, as well as other parameters needed to keep the smoke layer above the design height.
Temperature control - this is a variation on smoke exhaust ventilation (above) where, instead of specifying the vertical position of the smoke layer's base, the designer specifies the maximum temperature of the smoke- layer gases. This allows the use of materials in the atrium facade which cannot survive high gas temperatures and often allows a trade-off between different fabric and fire-design costs.
Smoke ventilation from each storey separately - smoke ventilation from the atrium can prove impractical (or impossible) where the clear height below the smoke layer's base is too large. In practice this limit is reached when this height is more than 8-18m (depending on many factors). In such cases, especially for an open atrium, it can be better to prevent smoke from entering the atrium altogether - for example, by using automatic- drop smoke curtains along the edge of the atrium void at each storey and designing a smoke-ventilation system for each storey. Note that there is no intrinsic limit to the height of the atrium with this strategy.
Atrium depressurisation - this uses essentially the same principles as natural environmental ventilation to keep the leakage paths across the atrium facade under 'suction', thus preventing smoke leaking from the atrium into the adjacent spaces. The strategy can be combined either with smoke exhaust ventilation from the atrium to maintain a clear layer low down in the atrium, or with temperature control to protect the materials in the atrium facade. The natural limits to this strategy are reached when the predicted forces required to open an escape door from the atrium against the suction forces become too large for people to manage. In practice this is not a serious problem and the strategy can be used for atria many storeys in height.
Note that many of these methods use active devices, usually triggered by a signal from a smoke-detection system. Detailed design guidance can be found elsewhere7. Minimum smoke-control requirements for different categories of construction and of use are given in bs 5588: Part 76.
Reconciling fire and smoke design
The essential similarity between natural environmental ventilation and atrium depressurisation for smoke control makes it easy, in principle, to mix the two approaches.
Environmental ventilation in summer - ventilation air paths can be allowed for in designing the atrium depressurisation strategy for smoke. This is because the roof ventilation area (or fan capacity, if used) and the low-level inlet provisions for the smoke control are likely to be much greater than the air-path areas needed for environmental ventilation.
Another consequence of these larger air-movement quantities for smoke is that there is likely to be a need for automatic devices to implement smoke control. That may include automatic door openers to provide all or part of the inlet air requirement. This will be particularly true if a smoke exhaust ventilation or temperature-control system is combined with the atrium depressurisation strategy. In general however, there is not likely to be any need for special devices or dampers to automatically close off the environmental air leakage paths on any storey. It is good practice for the smoke-control system designer to demonstrate by calculation that this is the case.
Environmental ventilation in winter - this is easier to combine with an atrium depressurisation strategy as there will be fewer and smaller leakage paths than in the summer.
There is no serious practical limit to the height of the atrium where an atrium depressurisation strategy has been used. But there is a practical limit to the height of the clear-air layer that can be created beneath the smoke layer's base. This means that the only practicable smoke ventilation strategy for an open atrium more than 8-18m tall is to use smoke ventilation from each separate storey.
The intended fire-safety strategy needs to be clarified at the earliest stage of the design. If left too late, then more difficult and probably more expensive, less aesthetic and less functional compromises are likely. There is almost always a technical solution to reconciling ventilation and fire design in atria.
Corinne Williams and Howard Morgan are in the Centre for Fire Protection Systems at the bre. Earle Perera is director of the bre's Building Performance Assessment Centre.
This article is part of a detr pit project on 'Overcoming conflicts between environmental natural ventilation and fire safety in atrium office-type buildings'. The bre's project partners are Ove Arup and Partners and Colt International.