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What is Airtightness?

What is airtightness?

Airtightness is a measure of uncontrolled air-loss through areas like:

  • the building fabric (ie: gaps in plaster, membranes, tapes or boards)

  • around the edges of windows

  • the eaves or roof

Why is airtightness important?

Traditional buildings commonly have areas where air can pass through the building fabric. For example, chimneys, around windows and through porous materials (such as stone or brick). This can result in the following issues:

  • Warm air escapes the building in winter. This means the building looses heat even when all of the windows are closed.

  • Hot air enters the building in summer. During heat waves this can contribute to overheating.

  • Traditional buildings are often over-ventilated. If too much air passes through the building, moisture is removed from the internal atmosphere and humidity drops to uncomfortable levels. This leads to conditions such as dry skin and can aggravate respiratory conditions like asthma. This is especially the case in winter as cold air holds less moisture than warm air.

One solution for this is to mechanically ventilate the building. Mechanical ventilation is becoming increasingly common in the UK because:

  • It ensures there is a constant supply of fresh air; avoiding a build up of carbon dioxide.

  • Humidity is controlled at comfortable levels (ie: 40-60%). It is not too dry or too moist.

  • Filters within the mechanical ventilation system remove pollen, exhaust fumes and other pollutants that have a long term impact on health.

  • In winter, up to 90% of the heat in the building can be recovered. This is known as MVHR: Mechanical Ventilation with Heat Recovery. This means that the building is getting a constant supply of warm, fresh air without drafts from opening windows. In the summer months, the heat recovery function can be turned off to save energy.

  • As the windows do not need to be opened, the internal environment is protected from traffic noise, sirens and other sounds of the city

  • A note on openable windows: all rooms can still have opening windows and occupants can use them if they wish. The important thing to bear in mind is that, should the occupant want to have a closed house and fresh air (for example, on a day with high pollen levels), this is possible with mechanical ventilation.

For mechanical ventilation to work, the building needs to have a good level of airtightness. This ensures that:

  • The supply and extract of air can be controlled with no uncontrolled gain/loss of air through the fabric

  • The humidity can be controlled with no uncontrolled gain/loss of moisture through the fabric

  • Pollutants are effectively filtered out

  • Heat is recovered during the winter; saving energy

  • Noise pollution does not penetrate through gaps in the fabric

Airtightness is measured in m³/hr@50pa. This translates to:

  • metres cubed of air (m³)

  • per hour (/hr)

  • at 50 pascals of pressure

An airtightness test would typically consist of the following steps:

  • The controlled ventilation systems in the building are sealed (ie: extract vents, mechanical ventilation duct routes, etc)

  • The building is pressurised to 50 pascals above atmospheric pressure

  • The rate of air-loss is measured in m³/hr to establish one reading

  • The building is de-pressurised to 50 pascals below atmospheric pressure

  • The rate of air-loss is measured in m³/hr to establish a second reading

  • The m³/hr rate is an average of the pressurised and de-pressurised readings

Typical airtightness requirements are:

  • Passivhaus: 0.6m³/hr

  • Building Regulations: 8m³/hr for a new dwelling (varies for different building types)

What if I don't want mechanical ventilation and want to simply open the windows (ie: naturally ventilate)?

There is lots of evidence supporting airtight buildings and the use of mechanical ventilation systems. This is especially the case in homes where a mechanical ventilation system can offer increased air quality, a quieter environment and improved health for occupants. Low energy standards like Passivhaus cannot be met without mechanical ventilation.

However, some clients may want to have a naturally ventilated building for a number of reasons:

  • Personal preference

  • Cost

  • Building typology. Buildings that are infrequently used such as outbuildings, warehouses or storage units may not require a constant supply of air from a mechanical ventilation unit

In these instances, it is important that the building is not too airtight as there needs to be a good supply of background ventilation. General advice is to aim for an airtightness of around 5m³/hr but this will vary on a case-by-case basis.


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