Airtightness of a house

airtightness of a two-storey extension

Airtightness of a house is crucial for optimal energy performance. Your aim should be to minimise how much air can flow through gaps in the structural fabric ie walls, floors, roofs, doors, windows etc. With improved airtightness, you will need less energy to heat and cool your home.

Building Regulations
Current building regs require that all new dwellings achieve an air leakage of 5m3/hm2 or less. For new builds, the regulations set an air leakage limit of 10m3/h/m2 at 50 pascals. In plain speaking, no more than 10m3 of air can flow through one square metre of the structural envelope’s surface area in one hour.

This regulation refers to uncontrolled air leakage through the building fabric rather than leakage through controllable ventilation sources such as trickle vents, extract fans or ventilation systems. Testing of the leakage rate is achieved by sealing all controllable ventilation sources and using a large fan to suck air out of the building.

The rate of 5m3/hm2 is considered a healthy rate for a naturally ventilated house, one with trickle vents, extractor fans etc. . That is, ventilated only with extract fans, trickle vents and windows. Anything less will probably require a mechanical ventilation system.

The Passivhaus standard requires all new builds to reach a maximum of just 0.6 air changes per hour. In this instance, a mechanical ventilation system is always specified.

Ensuring Airtightness
With SIP and closed panel build systems, it’s not too difficult to achieve a result of around 3-5m3/h/m2 as long as the joints between panels are properly taped up. A wet plaster finish can be a useful option.

Airtightness should be designed in, especially detailing service penetrations, loft hatches, external doors, pet access. To ensure airtightness, inform contractors what is required to achieve airtightness and then inspect regularly throughout the build. If you insist that poor workmanship will need to be rectified at their own cost, the message will be taken on board.

Points of weakness
The most common leakage points are:
• breaching of cavity wall by floor joists
• plasterboard installed with dot and dab – use parge coating or perimeter sealing
• penetration of walls and floors by pipework – use gunned sealants or appropriate flexible foam
• windows and door frames – use seals and strips
• eaves cupboards and loft access
• light fittings in theceiling
• Water, drainage, gas pipes, boiler flues and electricity cables