Improving airtightness in a house extension and dwellings
While the main focus of this guide is the achievement of an airtight dwelling, ventilation
must not be forgotten. It is necessary for a comfortable and healthy environment as it removes or dilutes
pollutants that accumulate in the building.
There are many different types and sources of pollution within the home, for example:
• Volatile organic compounds (VOCs) such as formaldehyde, which can give off gas from carpets and furniture.
• House dust mites.
• Oxides of nitrogen.
• Carbon monoxide (CO).
• Carbon dioxide (CO2).
• Tobacco smoke.
• Food smells.
• Body odours.
It is generally acknowledged that moisture is the most significant source of pollution within a dwelling, with
high quantities generated by common activities such as cooking, showering, bathing and laundry. Moisture from these
activities can result in condensation and, in extreme cases, mould growth.
To prevent moisture spreading around the home, some form of local extraction will be required (fans in the
bathroom and kitchen), with the overall objective being to keep relative humidity levels below 70 per cent for most
of the time.
The ventilation rates required to provide effective moisture control are higher than those needed simply to
provide oxygen for occupants to breathe. A typical ventilation strategy, therefore, involves the rapid removal of
moisture at its point of production and the provision of secure, controllable background ventilation, often by
trickle ventilators. Openable windows allow occupants to air a room rapidly and so remove stale air.
As a first step to providing effective, controllable and energy efficient ventilation, air leakage must be
Further guidance on effective ventilation can be found in GPG 268 (see back page for more information).
Rooms with open-flued combustion appliances, such as gas fires, must have a direct fresh air supply for safe
operation. Separate provision for such a supply should be made and a combustion product spillage test undertaken
whenever airtightness work is carried out.
Remember: build tight – ventilate right!