Curtain walling

From a design point of view, the metal framing members of the curtain wall (or windows) should be maintained at a temperature slightly higher than the interior surface of the insulating glass unit (IGU), so that condensation will form on the glass first rather than its frame. Although temperatures differ according to the construction of the IGU, for a 6-12-6 unit the temperature of the frame will need to be greater than 10-12°C. (Condensation on glazing frames, Technical Update 3, CWCT)

The trend toward highly insulating units, using Low-e coatings, warm-edge technology and gas filled voids, tends to raise the inner surface temperature of the glass. According to CWCT, including a Low-e coating can raise the centre pane temperature to 15.2°C meaning that condensation could occur on the frame first unless a highly insulated frame design is chosen. Good practice dictates the use of better insulated framing for high efficiency insulating glass units.

Slight condensation in the form of misting towards the lower parts of the glass or window may be acceptable if it only occurs on a few occasions each year. However, if the condensation is severe enough to occur as running water, this is unsatisfactory and remedial measures should be contemplated. Such conditions may be the product of user abuse, (for example, removal of or failure to maintain ventilation) and/or high internal humidity due to processes or equipment failure - something that is difficult to control at the design stage.

Case study: Scottish office building

The building comprised a multi-tenanted office of circa 10,000m2 arranged on a ground and 12 upper floors. The building's footprint reduced over 3 floors with a sloping glazed roof. The walls and roof were clad with a conventional drained and ventilated stick system which was designed to be mullion drained. Special shoes were built into the sloping roof mullions (rafters) to allow drainage into an aluminium gutter positioned at the transition of the sloping and vertical sections of cladding.

Water penetration was encountered at all floor levels to varying degrees, with most locations being confined to the areas below the sloping roof. Over the years, various unsuccessful investigations and trial repairs had led to accumulations of 'wet' mastic seals over pressure plates and gaskets. The window cleaning cradles had not been maintained and could not be used for inspection purposes; the only access being via a specially designed scaffold extending vertically, raking across the roof and then vertically upwards to the full height of the building.

Initially, selected cover strips and pressure plates were removed. Removal of the pressure plates released reservoirs of water that had been held in place by accumulations of silt and moss as well as the misguided applications of mastic, which in some cases had sealed drainage paths.

The aluminium gutter was tested and found to be watertight. However, the top lip of the gutter was formed of aluminium sheet held in place by the top pressure plate and gasket system to the vertical wall. When the pressure plate was properly compressed against the top glazing gasket, a good seal was formed between the glass and the gasket, but not between the gasket and the gutter. This led to water ingress into the system.

Tests using the AAMA procedure commenced at the bottom of the building, working upwards. Variable and random leakage patterns ensued suggesting that the basic concept of the system was sound, but that errors in construction might be responsible for localised faults.

To investigate fully, various IGUs were then removed and the glazing rebate inspected. According to the manufacturer's assembly details, a bead of sealant should have been applied to the mullion transom connections but this was missing in many areas. Also, the inner seals were assembled using pre-made corner pieces and straight gasket lengths. The contact faces between the corner pieces and the straight gaskets should have been sealed. They were not sealed, allowing water to be drawn into the building as a result of pressure differentials.

Attention was then turned to the drainage shoes at the base of the rafters. These shoes were designed to sit behind the face of the gutter and to drain into it via small pieces of hose cut in the side of the gutter. If the shoes leaked, water could only travel inside the building. This was solved by amending the gutter design and extending the rafters into the gutter itself to provide a fail-safe design.

Given the age of the glazing, and the need to remove all panels to achieve a satisfactory seal to the back gaskets and frame connections, all IGUs were replaced during the remedial works contract. The extent of water ingress was such that edge seals had been in contact with moisture for some period of time and so could have been compromised. All back gaskets were then sealed and fillet seals inserted at the transom and mullion connections. Following reassembly and reglazing, the entire areas were again field tested.

In summary, the water leakage was due to a combination of:

• poor initial assembly: a failure to seal the mullion and transom joints and a failure to make the back seals airtight;
• poor design in the detailing of the top gutter and the creation of a bespoke drainage shoe arrangement;
• poor maintenance: lack of the cradle meant that no cleaning had taken place, and the drainage paths had become compromised; and
• well-intentioned but inadequate repairs: failing to understand the mechanisms of water ingress and widespread use of sealant.

Case study: office building in coastal location

The building comprised a 1970s office block of rectangular plan constructed above an enclosed shopping centre near the coast. The cladding was steel framed, but single glazed. Coloured spandrel panels and fixed vision glass was held in place by external gaskets and pressure plates.

The cladding system leaked heavily. It relied upon a face seal with no provision for accidental leakage and no drainage paths. Over the years the system had fallen into disrepair and the frame had begun to corrode. The client needed to rectify the problem, but low rental income and poor cost recovery prospects had forced a series of compromises in repair strategies, usually involving copious amounts of sealant. Numerous pressure plates appeared to be loose and some cover caps were missing.

Water testing was unnecessary in this instance. Being single glazed, the provision of drainage channels around the glass was unnecessary for the purposes of durability, although if drainage could have been achieved it would have been beneficial.

In view of budgetary constraints, replacement of the cladding was impracticable and so a 2-stage external seal was provided. First, the old pressure plates were removed and discarded and the glass cleaned to remove all traces of the old mastic. Glass to metal joints were cleaned and sealed and a layer of butyl tape applied. New eagle gaskets and pressure plates could then be refitted, taking care to ensure that new fixing holes were drilled so that sufficient torque could be applied. The plates were then covered with drained cover caps. Internally, glass to metal joints around the inner seals were 'top capped' with silicone to form an internal air seal.