Airtightness and ventilation: why a sealed building still needs air

What airtightness means

Airtightness is how little air leaks uncontrollably through the building envelope, through gaps around windows, doors, services, junctions, and through materials. Uncontrolled leakage wastes heat, causes drafts and can carry moist air into the structure.

Why it matters

• Energy. Less leakage means less heat is lost.
• Comfort. Fewer drafts and more even temperatures.
• Moisture. Warm, moist air leaking into cold parts of a wall or roof can condense and cause damp or rot. See Damp, condensation and mould.
• Sound. An airtight building often keeps out more noise.

Uncontrolled leakage wastes heat, causes drafts and can carry moist air into the structure.

How it is achieved

• A continuous air barrier. A layer, such as a membrane, board or plaster, that runs unbroken around the heated space.
• Careful junctions. Sealed joints, tapes and gaskets where walls meet floors, roofs, windows and doors.
• Sealed penetrations. Pipes, ducts and cables passing through the barrier are sealed.
• Planning before building. The air barrier is drawn on the drawings, and the builders know where it is.
• Testing. A fan test (often called a blower-door test) pressurises or depressurises the building to measure leakage. Many countries require a test for new homes, and it is best done early enough to find and fix leaks.

Why ventilation is still needed

People, cooking, washing and drying create moisture and pollutants. Fresh air is needed for health and comfort. In a leaky building, air gets in by accident. In a tight one, it must be provided on purpose.

Ways to ventilate

• Natural ventilation. Opening windows and trickle vents. Simple, but depends on the weather and on people's habits.
• Extract ventilation. Fans in kitchens and bathrooms remove moist air, and fresh air enters through vents.
• Mechanical ventilation with heat recovery (MVHR). Fans supply fresh air and extract stale air, and a heat exchanger passes heat from the outgoing air to the incoming air. It is often used in very airtight homes. It must be designed, installed and maintained properly, with filters changed.
• Passive stack and hybrid systems. Use stack effect and wind, sometimes with fans.

The choice depends on the climate, the building, the occupants and the regulations.

Things to watch

• Noise and maintenance of mechanical systems.
• Filters that need regular changing.
• Summer overheating. A tight building can heat up, so shading and purge ventilation matter. See Designing for a hotter future.
• Air quality outside, near roads or industry.
• Occupants' understanding. People need to know how their building is meant to work.

In short

Make the building tight, then ventilate it on purpose. Plan both together from the start, test the result, and keep systems maintained.

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