OBSERVATIONS OF CONSTRUCTION PRACTICE   

By observing and recording buildings at key stages during their construction, areas can be identified that may contribute to or lead to direct and indirect air leakage once the dwellings is complete. These areas include:

PLASTERBOARD DRY LINING

Experience suggests that it is extremely difficult, if not impossible, to achieve a completely airtight seal around the edges of plasterboard dry lining on external and separating walls and all openings when using adhesive dabs. For instance, it is not uncommon to find that the dabs that are used to seal the perimeter of the dry lining are not continuous (as illustrated in the two animations below). If this is the case, free movement of air can occur into the space between the plasterboard and the inner leaf of blockwork. This creates a network of penetrating voids behind the plasterboard dry lining that can link together various other leakage paths within the dwelling.

Discontinuous ribbons of plasterboard adhesive on an external wall

Even when considerable efforts are made to apply a continuous strip of adhesive to the perimeter of the plasterboard dry lining (as illustrated in the animation below), the resulting ribbons are very seldom completely solid. In addition, a channel for air movement is left around the perimeter of the wall between the ribbons of adhesive and the perimeter junctions with adjacent walls, the ceiling and the floor.

Continuous ribbons of plasterboard adhesive on an external wall

The air movement that can potentially occur behind unsealed plasterboard dry lining can be illustrated by thermal images taken of the ceiling/wall junction during depressurisation. The images below show cold air from the loft (blue in colour) being pulled down through the gaps in the plasterboard ribbon at the ceiling/wall junction [1] and the air being pulled down into the framed partition wall [2].

Thermal images of ceiling/wall junctions during depressurisation test

BUILT-IN JOISTS

When joists that form intermediate floors are built-in, rather than supported on joist hangers, achieving an airtight seal is difficult. In part, this is due to joint movement and shrinkage but is also the result of the construction processes adopted. Problems occur because of the difficulty of achieving access to the junction between the blockwork and the built-in joists and access where joists run parallel to the external wall. Restricted access makes it difficult to mortar the joints in the external face of the blockwork and mortar and seal around the built-in joists.

For example, where a substantial height of brickwork in the outer leaf is constructed before the inner blockwork above the position of the intermediate floor, difficulties arise in accessing the outer face of blockwork because the construction of the blockwork has to be carried out from the intermediate floor, which is used as a working platform. This makes it difficult for bricklayers to achieve full perpends and bedding layers and strike off excess mortar, as shown below. The excess mortar [1 & 2] has to be chipped away leaving uneven surfaces that are difficult to seal with mastic around the joists. The gaps at perpends are also difficult to fill because of their proximity to the floor decking [3].

Excess mortar around built in joists and gaps at perpends

Problems also occur where the joists run parallel with the walls (see photograph below). The joists are offset from the inner leaf of the external or party wall to allow electrical cables to be run from one floor to the next. The offset is so small that it is not possible to apply mortar and mastic to the area between the joist and the wall to seal this junction.

Offset joist running parallel with the wall

AROUND WINDOW SILLS

Experience suggests that it is also extremely difficult to prevent air leakage around window sills.

Leakage paths around sills consist mainly of air moving between the living space and the cavity around the edges of the sill boards.

The photographs below show a window in position and sealed at the junction between frame and jamb, but not sealed at the sill/jamb junction [1]. After the sill board is fitted the gap remains [2]. Invariably the resultant gap is not fully sealed by the continuous dabs of adhesive behind the reveal board leaving an air leakage path at the base of the jamb [3].

Window sill at various stages of construction

There is also potential for air movement underneath the sills. The animation below shows a gap between the blockwork and the dry-fitted sillboard. Gaps still remain under the sill board after patching and after dry lining.

Daylight visible through gaps beneath a windowsill

INTERNAL PARTITION / CEILING JUNCTION

Common construction practice is to erect the room partitions on the top floor before installing the plasterboard ceiling. This practice creates numerous potential air leakage paths into the loft space at the junctions between the partition walls and top floor ceiling, particularly at junctions between partition walls. An example of the problems that this can create and that remain even after the installation of the plasterboard ceiling is illustrated below [1]. An effective way of avoiding this problem is to erect the ceiling before the partition [2].

Alternative sequencing of construction of partitions and ceiling

BEHIND BATH PANEL & SHOWER TRAY

Another problem is that service penetrations that are subsequently hidden behind boxing or panels (for example the bath panel, shower tray, shower pod, in an under sink unit, in airing cupboard or in an under stairs cupboard) are often left unsealed, whilst visible penetrations in the same dwelling have been sealed. This suggests a lack of understanding of the importance of these areas, with the selection criteria being used when deciding to seal or not being one of cosmetic appearance rather than airtightness.

An example is shown in the animation below. The positioning of the shower tray creates a potential air leakage path from beneath the shower tray, through the metal stud partitioning, directly into the ventilated loft-space. Potential air leakage points behind a bath panel, through the floor and at the floor/wall junction.

Potential air leakage path from beneath shower tray & bath panel

SOIL STACKS

Observations suggest that one of the biggest contributors to overall air leakage is air movement via the soil stacks. The importance of this leakage path derives from both the number of times it occurs and also from the apparent speed and volume of airflow relative to other leakage paths.Leakage due to soil stacks can occur at the ground floor termination of the stack either in the kitchen or in a ground floor bathroom. The other main route into the soil stack is in upper floor bath or shower rooms, where the soil pipes penetrate the boxing in around the stack. The actual path into the soil stack is generally hidden behind kitchen cabinets, bath, and bath panel or shower tray. Where the soil stacks are internal and terminate in the loft space or roof, the stacks provide a direct infiltration route from the inside of a dwelling to the attic.

Unsealed soil stack penetrating through intermediate floor

SECONDARY SEALING

This is where visible gaps in surface finishes are sealed in an attempt to limit air movement within construction voids, such as behind plasterboard dry lining, with the intention of reducing overall air leakage. In most cases, the sealing provides a secondary defence against air leakage and does not involve sealing at the primary air barrier.

Anecdotal evidence from a very small sample of reasonably airtight dwellings at Stamford Brook (see Miles-Shenton, Wingfield & Bell, 2007) found that the impact of secondary sealing on airtightness can result in a reasonably significant reduction in air leakage. However, more importantly, the work also found that secondary sealing is prone to degradation over a relatively short time period. In a number of the dwellings, drying, shrinkage and settlement at the intermediate floor perimeter was observed, resulting in a gap forming between the intermediate floor and the skirting board.

Gaps between the intermediate floor and the skirting board

Large gaps were also observed around other elements, such as loft hatches, stairs and window sills, where a less flexible sealant (decorators’ caulk) had been used. In addition, adhesive failure of the sealant was also observed. Further inspection of the failed sealant revealed that was most likely attributable to poor surface preparation, caused by applying the sealant to dusty surfaces or over debris.

Gaps at wall string on stairs and below window sill

The important point to note about secondary sealing is that although it may have some benefit in the short-term in reducing air leakage, it is not a long-term robust solution.