Cross ventilation
Ventilation is necessary in buildings to remove ‘stale’ air and replace it with ‘fresh’ air:
- Helping to moderate internal temperatures.
- Reducing the accumulation of moisture, odours and other gases that can build up during occupied periods.
- Creating air movement which improves the comfort of occupants.
Very broadly, ventilation in buildings can be classified as ‘natural’ or ‘mechanical’.
- Mechanical (or ‘forced’) ventilation tends to be driven by fans.
- Natural ventilation is driven by ‘natural’ pressure differences from one part of the building to another.
Natural ventilation can be wind-driven (or wind-induced), or it can be buoyancy-driven ‘stack’ ventilation. For more information about buoyancy-driven ‘stack’ ventilation, see Stack effect.
Approved Document O: Overheating, published by HM Government in 2021, defines cross-ventilation as: ‘The ability to ventilate using openings on opposite façades of a dwelling. Having openings on façades that are not opposite is not allowing cross-ventilation, e.g. in a corner flat.’
Cross ventilation occurs where there are pressure differences between one side of a building and the other. Typically, this is a wind-driven effect in which air is drawn into the building on the high pressure windward side and is drawn out of the building on the low pressure leeward side. Wind can also drive single-sided ventilation and vertical ventilation.
Whereas cross ventilation is generally more straight-forward to provide than stack ventilation, it has the disadvantage that it tends to be least effective on hot, still days, when it is needed most. In addition, it is generally only suitable for narrow buildings.
If there are windows on both sides, then cross ventilation might be suitable for buildings where the width is up to five times the floor-to-ceiling height. Where there are only openings on one side, wind-driven ventilation might be suitable for buildings where the width is up to 2.5 times the floor to ceiling height.
Beyond this, providing sufficient fresh air creates draughts close to openings, and additional design elements such as internal courtyards are necessary, or the inclusion of elements such as atrium that combine cross ventilation and stack effects, or mechanically assisted ventilation.
Cross ventilation is most suited for buildings that are:
- Narrow.
- On exposed sites.
- Perpendicular to the prevailing wind.
- Free from internal barriers to air flow.
- Provided with a regular distribution of openings.
It is less suitable where:
- The building is too deep to ventilate from the perimeter.
- Local air quality is poor; for example, if a building is next to a busy road.
- Local noise levels mean that windows cannot be opened.
- The local urban structure is very dense and shelters the building from the wind.
- Privacy or security requirements prevent windows being opened.
- Internal partitions block air paths.
Some of these issues can be avoided or mitigated by careful siting and design of buildings. For example, louvres can be used where ventilation is required, but a window is not, and ducts or openings can be provided in internal partitions, although these will only be effective if there is sufficient open area, and there may be problems with acoustic separation.
Cross ventilation can be problematic during the winter when windows may be closed, particularly in modern buildings which tend to be highly sealed. Trickle ventilation, or crack settings on windows can be provided to ensure there is adequate background ventilation. Trickle ventilators can be self-balancing, with the size of the open area depending on the air pressure difference across it.
In straight-forward buildings, cross ventilation can often be designed by following rules of thumb for the openable area required for a given floor area, depending on the nature of the space and occupancy. The situation becomes more complicated when cross ventilation is combined with the stack effect or mechanical systems, and thermal mass and solar gain are taken into consideration. Modelling this behaviour can become extremely complicated, sometimes requiring the use of local weather data, software such as computational fluid dynamics (CFD) programs and even wind tunnel testing.
Ventilation in buildings is regulated by Part F of the building regulations and soon to be Part O.
[edit] Related articles on Designing Buildings
- Air infiltration testing.
- Approved Document F.
- Approved Document O.
- BREEAM Potential for natural ventilation.
- Building services.
- Computational fluid dynamics.
- Condensation.
- Humidity.
- Interstitial condensation.
- Mechanical ventilation.
- Natural ventilation.
- Passive building design.
- Dew point.
- Single-sided ventilation.
- Solar chimney.
- Stack effect.
- Thermal comfort.
- Types of ventilation.
- UV disinfection of building air to remove harmful bacteria and viruses.
- Ventilation.
Featured articles and news
Key takeaways from the BSRIA Briefing 2024
Not just waiting for Net Zero, but driving it.
The ISO answer to what is a digital twin
Talking about digital twins in a more consistent manner.
Top tips and risks to look out for.
New Code of Practice for fire and escape door hardware
Published by GAI and DHF.
Retrofit of Buildings, a CIOB Technical Publication
Pertinent technical issues, retrofit measures and the roles involved.
New alliance will tackle skills shortage in greater Manchester
The pioneering Electrotechnical Training and Careers Alliance.
Drone data at the edge: three steps to better AI insights
Offering greater accuracy and quicker access to insights.
From fit-out to higher-risk buildings.
Heritage conservation in Calgary
The triple bottom line.
College of West Anglia apprentice wins SkillELECTRIC gold.
Scottish government launch delivery plan
To strengthen planning and tackle the housing emergency.
How people react in ways which tend to restore their comfort.
Comfort is a crucial missing piece of the puzzle.
ECA launches Recharging Electrical Skills Charter in Wales
Best solutions for the industry and electrical skills in Wales.
New homebuilding skills hub launch and industry response
Working with CITB and NHBC to launch fast track training.