Airtightness – Part 1

Airtightness is an essential part of creating a healthy, comfortable, energy-efficient living environment. Air leakage is one of the most significant contributors to inefficiently heated buildings. Research confirms that air leakage can account for up to half of all heat losses in modern buildings and reduce insulation performance by as much as 480%. Considering Ireland is one of the dampest and windiest climates in Europe, it’s no surprise that airtightness is now seen as a fundamental element in the construction of low energy, healthy buildings.

Decreasing the air leakage rate in buildings is one of the most cost effective means of constructing low energy, sustainable buildings. It is essential of course that a very airtight building must have an effective ventilation system; be it by mechanical or natural means.

What is an airtight dwelling?

Air leakage is the uncontrolled flow of air through gaps and cracks in the fabric of a building (sometimes referred to as infiltration or draughts). This is not to be confused with ventilation, which is the controlled flow of air into and out of the building through a purpose built ventilation system that is required for the comfort and safety of the occupants.

The term airtightness refers to the minimisation of these gaps and cracks. Of course one can open and close windows or doors in an airtight house as they choose. The difference with an airtight house is that when the windows are closed, the curtains stop moving in front of it!

In the past the majority of heat loss from our buildings could be attributed to little or no insulation and a very low level of airtightness. As insulation levels have increased over the decades, the proportion of heat loss in buildings due to air leakage has increased dramatically. Whereas ventilation is intended, air leakage or draughts are not. The key is to “Build Tight and Ventilate Right”.

One of the most cost effective means of reducing heating bills, increasing the energy efficiency in buildings and ensuring that the insulation can perform effectively is to use quality insulation while making sure the building envelope is as airtight as possible. In this way addressing air leakage can be pinpointed as one of the most effective and cheapest means of increasing the energy performance of our buildings.

An insulation layer is only as effective as it is sealed. If the insulation is continuously exposed to air movement, this will lead to a significant reduction in performance. For example, if one were to wear a woolly jumper on a cold windy day it will not insulate effectively, whereas if one were to wear a light windshield over the jumper, then it actually insulates effectively. Insulation in our homes is very similar. Figure 1 highlights common areas in buildings where air leakage occurs (red arrows), and where intended openings may be placed (green arrows).

Figure 1: An leaky ineffcient building and an airtight building (Red arrows indicate air leakage and green arrows intended openings)


Air leakage can occur due to a combination of poor building design, poor workmanship and inappropriate materials been used on site. It can account for up to 50% of all the heat losses through the external envelope of a building. An airtight dwelling does not mean it is hermetically sealed, rather that it has reduced air leakage to an absolute minimum and of course, has an effective means of ventilation. Ireland is not a country exposed to extremes in low temperatures; however we are exposed to extremes in wind pressure.

Airtightness is a precursor to maximising thermal performance both in winter and summer, optimising acoustic performance, improving comfort levels and reducing the risks associated with condensation within building elements, particularly in timber based constructions.

How do I measure airtightness?

The only way to measure the level of airtightness a building has achieved is by conducting a blower door test. A blower door test involves installing a large fan within a temporary door frame in an external door opening and inducing a pressure differential. The building may be depressurised or pressurised.

There are various units used to measure airtightness. The Irish building regulations dictate that buildings should be tested to EN Standard 13829:2001, to measure the leakage rate of the building envelope.

An airtightness measurement usually involves a combination of depressurising and/or pressuring a building to a pressure difference of 50 Pascal’s (50Pa). A pressure difference of 50 Pa can be compared with the equivalent pressure induced by a wind speed of approximately 22mph on a building.

Once the pressure differential reaches 50 Pa, leaks can be readily identified in the external envelope and an accurate measurement of the air leakage rate or permeability of the external envelope of the building can be calculated. The air permeability of a building at a pressure differential of 50Pa is referred to as the Q50 of a building and is measured in m3/hr (of airflow) per m2 (of total external envelope area).

The standard upper limit level of airtightness in buildings in Ireland and the UK at the moment, is to achieve a Q 50 of less than 10m3/hr/m2 for all new buildings. This literally means that when we exclude ventilation and design openings, and exert a pressure difference of 50Pa on the external envelope of the building, 10m3 of air per hour passes through every square meter of the external envelope…by no means energy efficient and certainly not airtight!

According to the Airtightness Testing and Measurement Association (ATTMA), for airtightness best practice, buildings which are mechanically or naturally ventilated should achieve an air permeability of less than 3m3/hr/m2 should be achieved. This is quite a high level of airtightness. However the most stringent requirement for airtightness can be found in Passiv Haus which requires a Q50 of less than 0.6m3/hr/m2. This indicates how leaky a building which records an air permeability of 10m3/hr/m2 is!

The cost for airtightness testing can vary depending on a number of factors such as:

  • the size of the building,
  • its location relative to the tester,
  • and the number of buildings to be tested. If there are a number of buildings on one site the tester may provide a discounted price.

The costs for testing residential buildings can vary from about €500 to €1000. The cost for commercial buildings can be significantly higher.

Figure 2: A pro clima Wincon airtightness quality control unit and a blower door fan

As the blower door test is carried out when the building is complete, it is generally very difficult and much more costly to seal up leaks at this stage. Ideally, an intermediate airtightness quality assessment test should be conducted prior to the internal lining been applied. This can be conducted by using a blower door or pro clima Wincon airtightness quality control unit (see figure 2). At this stage areas where air leakage is occurring can be easily identified and sealed cost effectively.

How do I locate airleakage?

Air leakage points can be highlighted by using a fog emitting device, thermal imaging or, when depressurising the building, by simply using you hand in which case you may feel the air leak. When the climatic conditions are suitable (when the temperature difference between inside and outside is sufficient), thermal imaging may be used as means of highlighting air leakage.

The illustrations below demonstrate some thermal images which highlight air leakage paths. Blue coolers in the images highlight the cooling in the areas of airleakage and warmer areas area coloured yellow/red. A scale at the bottom left of each image provides an indication of the temperature at a specific location. The blue areas indicate where the majority of heat loss and potentially condensation may occur.

Figure 3: Air leakage at gable wall in dormer roof

Figure 4: A dormer roof before blower door test. Notice that the insulation layer highlighted in orange/yellow

Figure 5: A dormer roof while a blower door test is running. Notice the effect of air leakage by the increase in blue areas of cooling. The insulation layers’ efficiency has been dramatically reduced.

What means of ventilation are there available?

An essential component in an airtight building is to ensure an effective controlled ventilation system is employed. There are various ventilation systems ranging from natural, to mechanical, a mixture of mechanical and natural or even mechanical ventilation with heat recovery. Whether buildings are naturally or mechanically ventilated a high level of airtightness ensures these systems perform to their highest efficiency.

While a natural ventilation system ensures sufficient fresh air is supplied to the living space, as the stale heated indoor air is replaced by fresh possibly cooler external air, the heat from the outgoing indoor air is lost. This is referred to as a “ventilation loss”. A mechanical ventilation system with heat recovery not only exchanges the stale indoor air with fresh outdoor air, but it also recovers the heat from the outgoing stale air and exchanges this into the cool fresh incoming air. Hence the “ventilation losses” can be dramatically reduced. If a building is very “leaky”, then cool external air will leak into the building, which will then dramatically reduce the efficiency of the heat exchange unit. In this way ventilation and airtightness should be considered collectively.

How do I achieve high levels of airtightness?

There are three key steps to maximising the airtightness of a building:

1. Design for Airtightness

The Architect designs the building bearing in mind key airtightness details.

2. Building for Airtightness

Both the installer and all personnel on site who interact with the airtightness layer must install the airtightness materials correctly. Coordination of work on site and communication between professions is essential. Examples of the airtightness layer may be a vapour check/barrier on the warm side of the insulation or the plaster onto the inside of a block work and all connections to structural components and service penetrations.

3. Test for Airtightness

The only way to confirm we have achieved the level of airtightness specified by the architect is to carry out a blower door test to measure airtightness.

Achieving high levels of airtightness is very much down to good building practice, and using the proper materials to achieve a durable airtight seal. Some of these details are highlighted in the selection of images below.

Figure 6: A pro clima service grummet sealing a pipe penetration of the airtightness layer

Figure 7: A “service batten” on the inside of the airtightness layer which ensures penetrations of the membrane are not damaged improves airtightness durability

Figure 8: Airtightness sealing of collar ties in roof with pro clima TESCON PROFIL corner jointing sealing tape

Figure 9: Overlaps of the Airtightness membrane (INTELLO PLUS), sealed with a suitable overlapping sealing tape (TESCON NO 1)

The next part of this feature will centre on detailing for airtightness and construction durability, and considering building health. With airtightness the devil is without doubt in the detail!

Niall Crosson
Technical Engineer BTech, MEng Sc, MIEI

Pictures and graphics provided courtesy of Ecological Building Systems