Poor airtightness can be responsible for up to 40% of heat loss from buildings. This CPD, sponsored by Isover, discusses how “smart” membranes can help achieve airtightness and moisture management within buildings

Multi comfort project the king's school, worcester (2)

At the King’s School in Worcester, a “smart” Vario membrane from Isover was specified to create an airtight envelope for the new school hall

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Airtightness is the measure of the uncontrolled ventilation through gaps and cracks in the building envelope. Poor airtightness can be responsible for up to 40% of heat loss from buildings.

In the UK, airtightness values are expressed as air permeability in the unit m3/h.m2 at 50Pa (q50). This refers to the volume of air in m3 that can flow per hour through each m2 of the building envelope at a pressure differential of 50Pa.

The PassivHaus standard uses a slightly different measure: air changes per hour in the unit h-1, at 50Pa (known as n50). This refers to the number of times the volume of air in the building is changed each hour at a pressure of 50Pa.


There are a number of benefits to minimising uncontrolled ventilation through a building envelope. These include:

Energy efficiency: Reducing gaps in the building fabric lessens the need for internal heating in colder weather and helps the heating system to maintain a consistent temperature within the building. This in turn reduces carbon emissions and heating bills for occupants. However, it is important to note that, as airtightness increases, the need for controlled ventilation to ensure occupant comfort and safety also increases.

Energy efficiency targets for new dwellings are a key element of the Building Regulations, which give an airtightness value for a guideline “notional building” as well as an absolute limit. It is not necessarily acceptable to use the limiting value and achieve compliance unless the designer compensates in some way – for example, by specifying higher insulation values for the envelope. The Building Regulation requirements for airtightness in dwellings are:

Building RegulationTerminology usedLimiting value (m3/h.m2@ 50Pa)Notional dwelling value (m3/h.m2@ 50Pa)

Part L1A – England and Wales

Air permeability



Section 6 – Scotland

Infiltration rate

10 (recommended worst case)


Compliance is demonstrated through the testing of a sample number of properties on completion.

Blower door test (part l1 a)

Blower door testing is used to measure the airtightness of buildings, and can also be used to measure airflow between building zones, to test ductwork airtightness and to locate areas of air leakage in the building envelope

Protecting the building fabric: An airtight structure reduces the risk of condensation within the building fabric by limiting moisture transport into the structure via convection. This will improve the longevity of the building.

Improved indoor air quality: Indoor air quality has a major influence on the health, comfort and well-being of building occupants. Poor air quality has been linked to Sick Building Syndrome, reduced productivity in offices and impaired learning in schools. A building with high levels of uncontrolled ventilation also leaves occupants more exposed to outdoor contaminants such as volatile organic chemicals (VOCs), nitrogen dioxide, ozone and carbon monoxide.


Airtightness is achieved by designing and installing a continuous air barrier around the heated volume of the building. This air barrier will normally be on the warm side of the insulation. Junctions between different materials in the air barrier should be well sealed, and penetrations should be minimised. Where penetrations cannot be avoided, they must also be well sealed.

Poor airtightness can be diagnosed using tealights, candles or smoke pens to spot cracks and gaps in fabric, or by conducting a thermographic survey.

The ventilation strategy must be considered at the same time as airtightness levels. Too much air leakage leads to draughts, heat loss and wasted energy. However, too little air infiltration can result in inadequate ventilation and poor indoor air quality.

According to the Energy Saving Trust, the total ventilation rate for a home should be between 0.5 and 1.5 air changes per hour. This includes controlled and uncontrolled ventilation and is sufficient to:

  • Control condensation
  • Maintain adequate air quality
  • Ensure safe operation of gas appliances

An air permeability of 10m3/h.m2 at 50Pa, the maximum allowed by the Building Regulations in England and Wales, leads to approximately 0.5 air exchanges per hour in uncontrolled ventilation alone. For buildings with an air permeability of between 7 and 3m3/h.m2 at 50Pa, natural ventilation is likely to be sufficient to maintain adequate air quality. If the air permeability is below 3m3/h.m2 at 50Pa, mechanical ventilation will be needed.


The issue of moisture management is closely related to airtightness and ventilation. All air contains moisture and the amount of moisture in the air is measured as relative humidity. In a healthy internal climate, the relative humidity is typically between 40-60%, although bathroom and kitchens often exceed these levels. Other contributors towards the relative humidity include:

Building fabric: There is usually considerable construction moisture present following a building’s completion.

Occupants: Four people living together can generate up to 10 litres of water per day, adding significantly to the water vapour present in the atmosphere. Also, activities such as showering, bathing, laundry and cooking all increase internal humidity levels.

Leakage: Leakage from rainwater, groundwater and plumbing all contribute towards internal humidity levels.

Moisture accumulation within the building fabric can damage a building in a number of ways:

  • Timber damage and rot
  • Corrosion of steel and concrete reinforcement
  • Damp and mould growth
  • Staining of internal surfaces
  • Reduction in insulation performance


Moisture movement in buildings takes two main forms: convection and diffusion.

  • Convection is the rapid movement of moisture-laden air through gaps in the building structure. It decreases as airtightness increases.
  • Diffusion is the slow movement of moisture-laden air through building membranes (or other building materials). Even the smallest hole in the fabric can lead to large amounts of moisture entering the building through convection, thereby damaging the structure. For example, over a heating season in a cold climate, a small 25mm2 hole in a wall or roof – such as a penetration for a cable – allows 100 times more moisture to move into the building fabric via convection compared with the typical rate of diffusion through plasterboard.


The correct specification (and subsequent installation) of the airtightness and moisture management membrane is key to minimising the flow of moisture into the building fabric. If no vapour barrier is included water vapour can become concentrated in the building structure, and there is a higher risk of interstitial condensation (with increased risk of mould growth or structural damage) within the building fabric.

A traditional vapour control layer, such as a polythene sheet, can create an airtight finish and block any moisture transport by diffusion. However, in practice it is very difficult to create a completely airtight finish and this does not allow any moisture that is trapped within the structure to dry out. A typical polythene sheet allows the envelope to dry out by up to 3g/m2 of water per year via diffusion.

More recent developments in “smart” or “intelligent” membranes offer active moisture management as well as zero air penetration. These are referred to in this way because they react to their environment, providing a moisture barrier during periods of high internal humidity, but then allowing the building fabric to dry out in warmer seasons, when internal relative humidity is typically low. This results in a lower risk of interstitial condensation.

Smart membranes are able to vary their resistance to diffusion. In winter, the pores in the membrane close to increase the diffusion resistance. Moisture inside the building is prevented from moving into the structure. In summer, the pores in the membrane open so that moisture that has built up within the structure can escape, preventing damage to the building. Leading smart membranes allow the envelope to dry out up to 340g/m² of water per year via diffusion. 

Vario xtra safe smart membrane

This image shows the installation of a Vario XtraSafe smart membrane from Isover. Smart membranes react to environmental conditions to better manage moisture levels at different times of year


The King’s School in Worcester required a new multi-purpose hall. This building was to replace the existing school hall and would also be used for external lettings and events.

The Saint Gobain Multi Comfort criterion for airtightness in non-residential new-builds is 0.6V/h @ 50Pa (N50 standard). That is 0.6 air changes, by volume, per hour when the building is subjected to a pressure differential of 50 Pascal. This is roughly equivalent to 1m³/h.m² @ 50Pa for the UK Q50 standard.

Airtightness was a key requirement and an airtightness champion was appointed, so that each trade involved in the project knew of the impact of their activity on the following trades. Many potential airtightness pitfalls were designed out early in the project and a “smart” airtightness and moisture membrane was specified – Isover’s Vario membrane – to provide high levels of thermal efficiency with adaptive moisture management.

The final result of the fabric airtightness test for the King’s School, was just 0.48V/h @ 50Pa creating a high-performance structure and a comfortable living space.


Multi comfort project the king's school, worcester (1)

A “smart” Vario membrane from Isover was specified for the new school hall at the King’s School in Worcester

Multi comfort project the king's school, worcester (3)

The final result of the fabric airtightness test for the King’s School was just 0.48V/h @ 50Pa

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