AIRTIGHTNESS & VENTILATION   

All buildings require ventilation. This is necessary for:

• Human respiration
• The health and comfort of the occupants
• The control of condensation and humidity
• Fuel burning appliances
• The dilution and disposal of pollutants

Ventilation is the process of changing air in an enclosed space. In order to maintain optimum air quality in buildings, a proportion of the air contained within any enclosed space should be continuously removed and replaced with fresh outside air.

If the incoming ‘fresh’ air is contaminated by pollutants, then measures have to be taken to remove them. Buildings are ventilated by a combination of infiltration and purpose-provided ventilation.

• Infiltration – The uncontrollable air exchange between the inside and outside of a building through a wide range of air leakage paths in the building structure
• Purpose-provided ventilation – The controllable air exchange between the inside and outside of a building by means of a range of natural and/or mechanical devices

Ventilation requirements for UK dwellings are typically:

• Recommended - between 0.5 and 1.0 air changes per hour (ach)
• Minimum - between 0.3 to 0.5 ach

Methods of achieving sufficient purpose-provided ventilation are contained in Approved Document Part F of the Building Regulations (ODPM, 2006).

It is important to realise that the level of airtightness achieved within a building will have an important influence on the overall ventilation rates that will be achieved (higher levels of airtightness, lower ventilation rates) and the type of ventilation strategy that should be adopted. Typical levels of airtightness suggested for various different ventilation strategies are shown below.

Ventilation strategy as a function of air leakage

Therefore, the aim of good ventilation design should always be to minimise the uncontrolled infiltration by making the building as airtight as possible, and then supply sufficient purpose-provided ventilation. In other words:

‘build tight, ventilate right’

It should be remembered that a dwelling cannot be too airtight, but it can be under ventilated.

Under average weather conditions, the air infiltration attributable to air leakage can be approximated using a simple ’rule of thumb’, which involves dividing the air leakage rate in ach @ 50Pa by 20. For instance, if the air leakage of a dwelling was 10 ach @ 50Pa, then the air infiltration due to air leakage would equal 10/20 = 0.5 ach. However, it has to be remembered that this is only a simple ‘rule of thumb’ and will not always be applicable to all types of buildings all of the time.