|Manual operated smoke puffer||Electronically operated smoke puffer|
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 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).
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.
|1. Gaps at ceiling-to-wall joint at the eaves||10. Gaps around skirting board & floor|
|2. Gaps around windows||11. Gaps around internal partition/ ceiling junction|
|3. Leaky windows||12. Gaps in & around electrical fittings|
|4. Leaky doors||13. Gaps around loft hatch|
|5. Leaks at threshold||14. Gaps around soil stack|
|6. Open chimneys||15. Gaps around ceiling light fittings|
|7. Leaks around flue penetration of ceiling||16. Vents penetration roof/ceiling|
|8. Gaps in & around suspended timber floors||17-21. Gaps around waste pipe & flue penetrations|
|9. Open fire/stove||22. Gaps around wall-to-floor joint|
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:
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:
|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 . 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 , 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 , 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.