Air

Like heat energy, air is constantly moving across the globe, driven by horizontal and vertical pressure differences. These two pressure differences are known as ‘wind-driven’ and ‘stack effect’ at the small scale of buildings.

‘Wind-driven’ pressure is where the wind blows in one direction across the building, creating positive pressure on one side and negative pressure (suction) on the other (leeward). This differential can be exploited naturally by opening windows on both sides of the building, allowing air to move through the house. This is called ‘cross-ventilation’ and is very effective at providing fresh air.

The ‘stack effect’ links air movement to temperature. Warmer air is less dense and tends to rise, and where this occurs, cooler air comes in below to fill the vacuum. Again, this phenomenon can be harnessed to form a very effective ventilation system. In warmer weather, windows (or roof lights) opened at a high level will allow warm air to rise up and away, drawing in cooler air from low-level windows (ideally to the north) and keeping the building and the occupants cool.

Ventilation

Ventilation is the term used to describe air movement, which is designed, intentional and controllable. Every building must have this to ensure an adequate supply of fresh air and a way to remove stale/moist air. The main mechanisms employed in older buildings were openable windows and chimneys/fires/stoves, which worked in concert with cross and stack ventilation, as noted above. Although chimneys are often blocked, these two mechanisms remain in many older buildings. In contrast, in most modern buildings, ventilation is also managed through mechanical extract fans with trickle vents in windows providing the replacement air. This is a regulatory requirement in all new buildings.

The term most commonly used within the UK industry is ‘air permeability’, and the air permeability of a building is measured in m3/hr/m2 at a standard pressure difference of 50 pascals. That is, the volume of air (m3) escaping per hour for each m2 of external surface area. In recognition of the problems it can cause, new buildings must be tested to ensure a suitable level of air permeability. The importance of airtightness is discussed in more detail in 'Airtightness', which also addresses some of the controversies surrounding the subject and looks at what can be done to reduce infiltration.

Whether natural or mechanical, the purpose of ventilation is fourfold:

to remove excess moisture
to remove unwanted pollutants from internal activities (including carbon dioxide)
to provide oxygen
in warm weather to cool occupants.

There is some debate about how much ventilation is required: it can be derived by considering how much (stale/moist) air needs to be removed from a space or by considering how much (fresh) needs to be provided – in other words, by considering the extraction or supply rate.

The problem, however, is that not all air movement in buildings is designed. Much of it is not designed, not anticipated and not particularly welcome! Infiltration is the term used to describe air getting in and out of the building which wasn't designed, not intended, or controllable. It is due to gaps and cracks in the building and around things like services penetrations, and so is affected by the quality of construction. It is driven by wind pressure and temperature differences, just the same as ventilation, but, unlike ventilation, it can't be controlled and leads to several problems, like heat loss. Infiltration is also known as air leakage or, more commonly, draughts, while the solutions are known as addressing airtightness, reducing air leakage/permeability, or simply as draughtproofing.