IDENTIFYING & QUANTIFYING AIR LEAKAGE   

LEAKAGE IDENTIFICATION

The principal techniques available to identify the main areas of air leakage within a building are:

• Smoke detection
• Thermal imaging

Both methods have distinct advantages and disadvantages, and neither technique alone will provide a comprehensive list of all air leakage under all circumstances. Additional techniques may be used to augment these procedures, but the most powerful tool in determining points of leakage and leakage paths is a combination of techniques, coupled with the experience and vigilance of the tester.

Smoke detection
For domestic applications this technique most commonly involves the use of a manual or electronically operated hand-held smoke puffer, although much larger smoke generators can also be used. The building is either pressurised or depressurised, and the smoke puffer is used inside the building to locate the main areas of air leakage. In most instances, detection is undertaken from inside the dwelling under pressurisation, as it is much easier to identify where the smoke leaks out of the habitable space, rather than into the habitable space.

The following video clip illustrates a number of common air leakage paths that are found within current new build UK housing. All of the leakage paths have been identified during pressuisation.

Leakage identification video

An important point to note about smoke detection techniques is, that in most cases, it is only possible to identify the point where the smoke leaks out of the habitable space, and not the path that the smoke takes from the inside to the outside of the building.

In addition to leakage identification, smoke detection techniques can also give an indication of the relative severity of the air leakage, by observing the velocity of the smoke as it exits through the identified areas. If the leakage detection is performed at similar pressure differentials, for instance 60 to 75 Pa above external pressure, then an indication of the relevant importance of each point can be obtained.

A 360° virtual tour of a dwelling where the main air leakage points have been identified under pressuisation using hand-held smoke puffers can be viewed by clicking on the image below:

Thermal imaging
Infrared thermal imaging using an infrared camera can also be used to identify the main areas of air leakage within the building fabric. However, this technique is considerably more complex and problematic than smoke detection. In order for meaningful results to be obtained using this technique, the building has to be in a steady thermal state and a sufficient temperature differential must exist between inside and outside of the building. In addition, care must be taken when interpreting the thermal images obtained from the camera, as a number of other factors, such as direct sunlight and uninsulated primary pipework, can affect the images achieved.

Thermal imaging can also provide additional information which is not always possible to recognize purely by smoke detection. For instance in the example below thermal imaging has been used in conjunction with leakage detection to identify air leakage at the floor / wall junction of the bay window. However, limitations as to when and where it can be used as a detection technique often prohibit its use. Also, without appropriate skills and knowledge of the construction, it is possible to misinterpret the images obtained.

Thermal imaging used in conjunction with leakage detection

Knowledge of leakage paths, rather than points of air leakage alone, becomes increasingly important if reparatory work is required to reduce the air permeability of a dwelling. Secondary sealing (such as using a mastic to seal the floor/skirting junction in the above example) may eliminate air leakage at that point, but will not prevent air leakage from around the window sill into the void behind the dry lining.In favourable conditions, it is possible to use thermal imaging not only to detect air leakage, but also to provide an indication of the paths by which air moves throughout the whole dwelling.

Thermal imaging used to illustrate air leakage paths

It is important to realise that there are a number of limitations to thermographic leakage detection. These are listed in detail in BSI (1999) and BSRIA (2002).

COMBINING TECHNIQUES

Whilst smoke detection under pressurisation will indicate points of air leakage from the habitable space (exfiltration), infrared thermal imaging used during depressurisation will display uncontrolled cooler air entering the dwelling (infiltration). By combining both techniques it is possible to determine whether air leakage is direct to outside or via a more complex indirect path (see photograph and thermal image below).

Smoke detection under dwelling pressurisation can easily indicate points of, and relative intensities of, air leakage at the floor / wall junctions.
Thermal imaging under dwelling depressurisation can distinguish between the direct and indirect leakage paths where conditions allow.

Additional information can be obtained by augmenting the smoke detection and thermal imaging with supplementary techniques. For instance, a handheld hot-wire or hot-bulb anemometer can also be used to measure air leakage flow rates and the temperature of air entering a building at that point under dwelling depressurisation. However, such techniques should only be used for comparative measurements with a single test, as the high number of variables usually renders methods like this as purely qualitative rather than quantitative.

Handheld anemometers can be used to compare relative significance
of leakage paths by measuring air velocity and temperature

MAIN AIR LEAKAGE PATHS

Air will leak through porous building materials and unintentional cracks, gaps and openings in the building envelope. The main air leakage paths in UK dwellings have been well documented (see Stephen, 1998 & 2000 and EST, 2005) and are illustrated below (in no particular order of importance). As the illustration below shows, air leakage can occur either directly or indirectly.

Most common air leakage paths

DIRECT AIR LEAKAGE POINTS

These are points in the building envelope where air leakage occurs directly through the primary air barrier from inside the insulated envelope to outside or vice versa. Common direct air leakage points include:

• Around trickle ventilators and through poorly closing trickle ventilators
  
• Around and through loft hatch
  
• Through gaps at bay windows
• Around poorly fitting windows and doors
  
• Around sliding mechanism of patio doors
  
• At thresholds
  
• Around services at the point where they penetrate through the primary air barrier

Trickle ventilators	Loft hatch	Bay windows
Patio doors	Thresholds	Services
	Click to view enlarged images

INDIRECT AIR LEAKAGE POINTS

These are points in the building envelope where air leakage occurs indirectly through the primary air barrier via a series of interconnected voids from inside the insulated envelope to outside or vice versa. Experience indicates that the majority of air leakage within UK dwellings occurs indirectly rather than directly. Common indirect air leakage points include:

• At ground floor/external wall junction
  
• Under kitchen & utility room units
  
• Around staircases
  
• Into intermediate floor voids
• Into service voids, e.g. behind bath panels
  
• At intermediate floor perimeters
  
• At service penetrations where they penetrate the drylining and / or internal finish

Ground floor / external wall junction	Kitchen / utility room units	Staircases
Intermediate floor voids	Intermediate floor perimeters	Service penetrations
	Click to view enlarged images

It should be noted that the above photographs illustrate the point of air leakage and not the complex indirect or “hidden” air leakage path. It is not uncommon for the point where the air leaks through the primary air barrier to be some distance away from the observed entry or exit point inside the dwelling.

For example, the photographs below show a common fault, where an oversized core drill has been used to bore a hole for a soil pipe. In this example a Ø150mm hole has been made for a Ø110mm soil pipe, leaving a gap around the pipe of over 8000mm² [1]. The soil pipe has been sealed around at the external brickwork, for weatherproofing, but no attempt has been made to plug the gaps at either the blockwork or dry lining.

The build sequence of a soil pipe penetration through a dry lined cavity masonry wall to an external soil stack

When the tiles are fitted there has still been no attempt to seal these gaps [2], allowing air to move freely between the cavity and the void behind the plasterboard. As the cavity is effectively ventilated, this means that there is a gap of over 80cm² around each soil pipe allowing air infiltration. As the void behind the dry lining is linked to intermediate floor voids, and hence all other voids throughout the dwelling, air from anywhere within the property could be moving through these holes.

Once the tiling has been finished and the soil pipe sealed around [3], it is often extremely difficult to tell that these “hidden paths” exist; possibly only by thermography, and even then relying on favourable environmental conditions.

If this dwelling was to fail to meet the required air permeability target, and reparatory work required, how would gaps like this get sealed? Simply sealing around both the soil pipes in this dwelling, where they pass through the primary air barrier, could make the difference between a pass and a fail in an air tightness test; saving time, effort and money.

QUANTIFYING AIR LEAKAGE

Component air leakage in UK dwellings. After Stephen (2000)

 

In UK dwellings, experience indicates that the majority of air leakage within both new and existing dwellings tends to occur indirectly rather than through easily identifiable direct gaps and cracks in the building envelope. This is reinforced by work undertaken in the late 1990’s by the BRE who attempted to quantify the average component air leakage attributable to the main air leakage paths in UK dwellings using reductive sealing techniques (see Stephen, 1998 & 2000). The results of this work are illustrated in the figure below.

Although this work is based upon a very small sample of dwellings (35) from the BRE’s database of air leakage, the results suggest that the vast majority of component air leakage could not be attributed to a single component. Instead, it could be attributed to the numerous “hidden paths”, through cracks and gaps that exist throughout the building.

These “hidden” air leakage paths are often complicated, making it very difficult, if not impossible, to trace and seal them effectively. Therefore, it is much more effective to design and construct airtight dwellings in the first instance, rather than try to carry out post construction tightening (most commonly taking the form of secondary sealing) once the dwelling is constructed.