The rules

Unlike mainland Europe, the UK already has a curtain-walling standard that has been in use since 1993. Before that year, no country had a national standard for curtain walling, and most walls were specified using window standards.

All specifiers have little to fear from the new CEN standard, which will be largely based on current UK practice. The CEN standard will set out the methods of testing and the requirements for a simple curtain wall, and is only really appropriate for simple stick curtain walls with glass and opaque panels into a glazing rebate.

The current CWCT standard goes beyond this and makes specifice reference to the Building Regulations.

It is also the only standard in Europe that covers ventilated rainscreen curtain walls. Next year's updated CWCT standard will also have a single standard covering all curtain walling and rainscreen walls.

The CEN standard will comprise a product standard and supporting standards, whereas the CWCT standard is a single document. The latter comprises full requirements of the wall, methods of testing to verify performance to the specified level and a guide to good practice, covering details that might affect durability. This includes quality of materials and workmanship standards, which the European standard does not have.

There will be some minor differences in the standards that are worth bearing in mind. These include deflection limits and the volume of water sprayed on to test specimens (see table below). More key differences arise when the dynamic watertightness test is considered.

CWCT has always required a dynamic watertightness test for walls where the design wind load is greater than 600 pa. This is based on North American practice and uses an aeroplane engine to drive air and water on to a test specimen. It requires a jet of air with a diameter of at least half the height of the test specimen.

The CEN dynamic water test, on the other hand, uses a small fan that is moved over the face of the wall. This is a sufficiently onerous test on a curtain walling joint, provided the joints on both sides of a framing member are tested individually. However, the CEN test may be too onerous on the normal framed wall. This means that walls with ventilated cavities may fail the European dynamic test with the small fan but pass the CWCT test using the aeroplane engine. The reason for this is the small fan will force air into one drainage opening and out at another causing unrealistic air and water movement. With a larger aeroplane engine all drainage openings will be simultaneously pressurised. The aeroplane engine also, incidentally, provides a good shake-down test for any lightweight components.

The specifier will be free to use either standard. CWCT currently advises that walls are specified and tested in accordance with the CWCT standard. If a contractor proposes to use a system that has been tested in accordance with the new CEN standard, the specifier should ensure that the lower water spray rate gives a sufficiently robust test and that an appropriate dynamic watertightness test has been carried out.

CWCT
www.cwct.org.uk