Natural ventilation: cross ventilation
Provided by Nick Baker: Research Associate, The Martin Centre, University of Cambridge
What, Why, When, How, Extras
Fig 1: The wind-induced pressure distribution is complex but generally positive on the windward side and negative on the roof and leeward side.
Fig 2: Wind generates complex pressure distributions on buildings, particularly in urban environments. This assists ventilation, provided that openings are well distributed and flow paths within the building are available.
Wind-induced ventilation uses pressures generated on the building by the wind, to drive air through openings in the building. It is most commonly realised as cross-ventilation, where air enters on one side of the building, and leaves on the opposite side, but can also drive single sided ventilation, and vertical ventilation flows.
When wind meets an obstruction such as a building, it is deflected and due to its momentum this creates positive and negative pressures over the surface of the building (fig 1). The pressure distribution map is complicated and non-uniform, even over an individual surface (fig 2), but is generally positive on the windward side, and negative over the roof and leeward side. Ventilation air will flow between any two points on the envelope at a different pressure provided there is an opening in the envelope at that point.
Wind-induced ventilation is one of the two mechanisms by which natural ventilation is driven, the other being the stack effect.
Natural ventilation as an alternative to mechanical ventilation has several benefits: low running cost, zero energy consumption, low maintenance and probably lower initial cost. It is also regarded as healthier, having less hygiene problems with ducts, and filters etc, and the “naturalness” in the way that it connects with outside, often in conjunction with windows, is seen as a psychological benefit.
Ventilation in buildings has three main purposes:
- 1. To maintain a minimum air quality (1 – 2 ac/h)
- 2. To remove heat (or other pollutant) (2 – 15 ac/h)
- 3. To provide perceptible air movement to enhance thermal comfort ( 0.5 – 2 m/s)
In winter, typically only (1) is required, though in highly insulated buildings (2) will also be required sometimes. (2) is the typical summer condition in the UK, and (3) would be required when the outside air temperature is already at the lower comfort limit. The air change rate (ac/h) is indicative only, and for (3), the required air movement will often lead to very high air change rates of 50 to 100 ac/h.
Wind-induced ventilation can provide types (1), (2) and (3).
Like stack ventilation, wind-induced ventilation takes place whether we want it or not, since no building envelope is completely airtight. Older buildings were very porous and reasonable air quality was maintained by uncontrolled leakage through the envelope – this is normally referred to as infiltration.
Historically, greater levels of pollution, such as the smoke from indoor fires, were dealt with initially by opening windows (the name being derived from wind-eyes), and later by stacks. Traditional buildings in warm climates, adopt more specialised elements to encourage high rates of ventilation (e.g. types (2) and (3), such as louvres, wind towers, and wind catchers).
In modern buildings the correct strategy is to build an airtight envelope and ventilate with intentional, controllable openings. These openings will typically be windows, but will also include various forms of controllable slots, grilles and louvres, where the ventilation function has been separated from the daylight and view function. Wind-driven ventilation can be adopted whenever a building is exposed to the prevailing wind, unless exceptional site conditions exist, such as noise and pollution, prohibiting the use of openings in the envelope adjacent to the occupants. In dense urban areas, close to the ground, and in heavily vegetated sites, the prevailing wind may be of such a low speed, as to only make a minimal contribution, and mechanical air supply may have to be considered.
- Wind speed and direction is very variable. Openings must be controllable to cover the wide range of required ventilation rates and the wide range of wind speeds.
- The more the opening area is distributed, the more likely it is that there will be a pressure difference between openings to drive the flow – i.e. many small openings are better than one large opening
- As with stack ventilation, the internal flow path inside the building must be considered.
- For cross-ventilation, bear in mind that the leeward space will have air that has picked up heat or pollution from the windward space. This may limit the depth of plan for cross-ventilation.
- If windows are used, consideration must be given to their controllability and ergonomic design, and the effect of air flows to the immediately adjacent occupants
Fig 4(a-c). (Click image to enlarge)
Fig 4(d). (Click image to enlarge)
- Step 1: Wind climate, assessing the potential of the site: In open sites, regional wind data can be consulted, and this will give a reasonable indication of the direction and frequency of useful wind. There are procedures for modifying the unobstructed windspeed (ref ..).
In urban environments it is more difficult since due to turbulence, the wind is variable in both speed and direction, in time and space. In these situations, the rule to provide as many controllable openings as possible, well distributed over the building envelope, should be adopted, rather than make assumptions about prevailing wind direction.
- Step 2: Spatial configuration: Four spatial types of wind-induced ventilation can be identified as illustrated in figs 4(a – d):
- 4a. Single-sided ventilation: This relies on two properties of the wind pressure distribution over a single room facade – firstly the spatial distribution of pressure (see fig 3). This will cause air to flow between two openings at different pressure, even if they are both at a positive pressure. Secondly, fluctuations in wind speed due to turbulence, will create a pumping action, where small inflows will be followed by outflows, via a single opening.
Fig 3. For a given total area, ventilation is improved when openings are well distributed horizontally and vertically. This is because the openings are more likely to be at different pressure. It also leads to better distribution within the room.
- 4b. Cross-ventilation single space: This is the classic case where a single banked room has openings on the windward and leeward side. With deep open plan layouts, the main limitation will be providing sufficient fresh air via the windward openings, to meet the demands of the whole floor (or rather its occupants), without causing disturbance to the occupants sitting close to the window. Related to this, is the fact that the air quality will diminish as it picks up pollutants (or heat) as it crosses the floor. For this reason, it is unlikely that cross-ventilation of this type is applicable to floor plans greater than 5 times the floor-ceiling height.
A variant on this is where the windward or leeward side of the room may be partitioned to form a corridor. Clearly there must be openings in this partition, equal to the window openings.
Fig 5. Ventilation flowpath.
4c. Cross-ventilation with double (or more) banked rooms: This can be achieved by openings in the corridor partition, but is generally unsatisfactory since the ventilation of the leeward room, relies on the occupant of the windward room, and, according to the use type of the building, may also have acoustic and hygiene problems.
The solution is to provide a bypass route. This pressurises the corridor with fresh air, and allows independent control to the occupant of the leeward room. The duct could be within a ceiling zone, or a useable annex to the circulation space, between two windward rooms.
Fig 6. Duct supply. (Click image to enlarge)
4d. Wind induced supply and/or extract by stack: This is regarded as an advanced technique and generally suits large deep-plan buildings (see case studies). It is almost always used in conjunction with thermally induced stack effect. Two subtypes can be identified. Firstly, centre in edge out, where the air intake is usually via an atrium and the outlet is by stacks or windows on the perimeter. Secondly, edge in centre out, is where the stack (or often an atrium) is used as extract, drawing air in from openings in the perimeter. In both cases, the stack effect, and the wind-induced (suction) pressure cooperates to extract air from the building.
These advanced systems require careful design using physical or computational simulation techniques. It is recommended that the specialist literature is consulted. (See case studies)
The following rule of thumb may be used to assess the potential for single-sided and cross ventilation. The depth of plan over which ventilation can be expected to work is specified in terms of the floor to ceiling height.
||Depth to floor/ceiling ht H. |
|single sided, single opening
||1.5 H |
|single sided, multiple opening
||2.5 H |
||5 H |
- Step 3: Sizing of openings: As with stack flow, the flow rate is dependent on the pressure difference (in this case generated by wind) and the flow resistance of the openings, which itself is dependent primarily on the opening area. However, there is little point making precise calculations, since the pressures over the building will vary over a large range, due to fluctuations in windspeed and direction. The simpler approach is to use a rule of thumb, which relates the total openable area to the area of floor over which ventilation is to be provided.
For the purpose of ventilation type (2), i.e. removal of heat gains, use the following guidelines. Note that the greater the gains, the more ventilation is needed, so there is always a benefit in reducing gains as much as possible – using low energy equipment and eliminating unwanted solar gains by shading.
||Total area opening % floor area |
|low (< 15 W/m2)
|med (15 – 30 W/m2)
|high (> 30W/m2)
This is the total opening area, assuming inlet and outlet are roughly equal.
- Step 4: Control: The openable area recommended above may seem large, but they are meant to be able to cope with times of low windspeed and high ambient air temperatures. As the windspeed drops, they will also operate under buoyancy driven flow (stack effect).
In summer, for heat removal (type 2) ventilation, the opening and closing of windows will be the conventional means of control. Whilst this is commonly done manually, by the occupants, and is thus an important adaptive opportunity, for larger buildings it can be automated it can be automated and under the control of the Building Energy Management system (BEMS). Ideally, where windows are accessible, manual override should be permitted, with the BEMS acting in a “caretaker” role to ensure that windows are closed when no longer required.
For winter (type 1) ventilation, i.e. minimal ventilation to maintain air quality in winter, much smaller openings are needed, and to make them more controllable, they are normally provided by separate slot ventilators or a “crack” setting on the window.
Slot ventilators are available which are self-balancing – that is as the wind pressure increases, they automatically close up to prevent over-ventilation. These are strongly recommended, since over-ventilation will increase heat loss and nullify the efforts to make the envelope airtight.
As with ventilation type (2), openings can be under intelligent control. Whereas in type (2) ventilation, the control parameter would be temperature, in the case of type (1) ventilation, the control parameter would be CO2 concentration. Many innovative buildings exist with controlled natural (and hybrid) ventilation systems of this type.
There is a close relation to stack effect (buoyancy driven) ventilation since many of the design requirements – flow paths, distribution of openings etc, are common to both. The overall natural ventilation design must be developed with both strategies simultaneously.
As with stack ventilation, the requirement for large openings may present problems with noise control. Also, the need to provide flow paths within the building may conflict with acoustic separation beteen internal spaces. However, the provision of by-pass ducts can help reduce this.
Closely related to problem of acoustic separation is fire compartmentation. This may be solvable with fire dampers and automatic fire doors.