Curtain walling

The preceding pages have identified the main mechanisms of water ingress and the main systems of cladding. Knowledge of these principles can be put to use in diagnosing leakage problems in cladding systems that may be unfamiliar.

Analysis of leakage patterns is seldom straightforward, as water can travel some distance within framing members before it becomes manifest internally. A methodical approach is needed, together with the courage to open up.

Stage 1 - initial reconnaissance

According to Field Marshal Montgomery, 'time spent in reconnaissance is seldom wasted'. To identify the cause of cladding defects an initial inspection is essential. But the first inspection should not be a simple viewing of the problem, it should be tackled with some measure of science and methodology. For example, it would be little use inspecting a building only to agree with the occupant that the cladding is leaking - what is needed is an analysis of the locations and an understanding of the weather conditions that cause leaks, their duration and magnitude and whether there are any common patterns.

This may seem obvious, but it is surprising how many surveyors fail to establish patterns and locations of leaks - these can often help to pinpoint potential problems. For example, the initial review may reveal that the leaks all occur at a certain level or always in line with a particular feature, a change in level or the interface between 2 different forms of cladding. Does orientation or level have a bearing? Are there any obvious defects? A quick check of the inner gaskets can reveal whether or not there is insufficient pressure - they should resist finger pressure, if they can be pushed inwards slightly, the external pressure plates may not be fixed to the correct torque.

Another common failing is to think in 2 dimensions rather than 3 - easier said than done, particularly as most graphical representations are drawn in 2 dimensions - usually because it is easier to do it that way.

Stage 2 - investigate the history and background information

Having gathered the initial information, attention needs to focus on identifying the type and design of the system. Here an examination of the operating and maintenance manuals should help. Unfortunately, in older buildings a lack of as-built information is common, but if it is possible to identify the manufacturer, it might be possible to obtain construction details, perhaps not of the actual building but at least of the system.

Many manufacturers have ceased trading, so if all else fails, archival research may yield further information. Journals such as the Architects Journal and Building often have building case files from back issues. These can provide some information as to the systems used, although this may be limited in terms of its reliability.

The purpose of this stage is to try and understand as much about the assembly process and design features as possible:

• How were joints to be formed?
• Was it mullion or transom drained?
• Are there rubber infiltration blocks?

and so on.

Stage 3 - site investigation

A visual inspection of the cladding installation will reveal useful information. However, for many systems it will be difficult to tell much without removal of at least the cover caps and probably some of the pressure plates too.

The object of this inspection is to check to see if the assembly of the frame, the fitting of setting blocks and the use of sealants is in accordance with the design and manufacturer's instructions. Check that pressure plates are installed the correct way up, i.e. that the slots for ventilation are not confused with the drainage slots. Check that the drainage paths are clear and not silted up and make sure that ad hoc amendments have not been made to drainage paths, such as the well intentioned but misinformed application of mastic sealant.

Water leakage can travel some distance such that the point of entry is not necessarily close to the point of egress. There are several methods of field testing that are generally recognised:

• Pressure spray in accordance with AAMA standard 501-94, Methods of test for exterior walls, American Architectural Manufacturers Association, USA.
• Test methods for curtain walling, CWCT, 1996.
• Curtain walling. Watertightness site test, BS EN 13051:2001.
• Methods of testing windows. Watertightness test under static pressure, BS 5368-2:1980.
• ASTM E 1105-96, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Curtain Walls and Doors by Uniform and Cyclic Static Air Pressure Difference.

The CWCT and AAMA test regime is essentially similar.

Field tests, if executed methodically, can be a useful means of identifying leakage paths, but require care and patience. They also need an observer on the inside, preferably equipped with a 2-way radio for communication. If the performance of the cladding installation is in dispute, it is best to stay within the parameters set in the standards; if the matter is non-contentious then it is possible to relax some of the requirements as long as the basic methodology is followed.

However, the 2-pressure spray methods, AAMA 501-94 and CWCT are rigorous in their application; they are designed for testing site installations during construction and unless used with care and judgment will create leaks in joints and gaskets that would perform adequately in normal service conditions. Similarly, if leaks are produced during the test, you need to define whether or not the degree of leakage is acceptable (the result of extreme conditions) or whether remedial work is necessary.

Applying a pressure test to a feature that is not designed to deal with it (such as a rainscreen cladding) will inundate the system and drainage paths leading to a false result. It is usually possible to make something leak, but whether or not this produces the right answer depends on a careful and methodical approach. It is easy, for example, to form a pre-conceived view as to why a failure has occurred and then inadvertently make the evidence fit the theory.

The water spray test is not suitable for all types of cladding and for rainscreens or unsealed patent glazing systems. A spray bar test would be better as this reduces the pressure of water hitting the surface and produces a curtain of droplets flowing down the wall in a manner that replicates actual conditions. BS EN 13051 sets out the procedure to be followed.

Checklist of critical points in rainscreen cladding

• Can water be discharged from the outer cladding face? Are there drips under external projections to throw water clear of the face?
• Is there a cavity behind the rainscreen of sufficient width to prevent capillary movements?
• Is there a continuous and effective air barrier? Are joints in the air barrier sealed and is it properly compressed?
• Can water drain away from the system? Is there a proper water path and is it clear from obstruction (moss, silt, etc.)?
• Is there adequate venting of the cavity? Does the area of venting take into account potential air leakages through the air barrier or changes of volume of the cavity (through flexible inner linings)?
• Is the facade compartmentalised to prevent lateral movement of air behind the rainscreen?
• Is the air barrier rigid or will it flex (and so vary the volume of the airspace)?
• Is the cladding sufficiently rigid to resist wind loads without excessive deflection?

The above checklist is based on The rain screen wall system, Ontario Association of Architect.