Under pressure

Approved document Part L2 does not refer directly to curtain walling, but refers to the Centre for Window and Cladding Technology publications Guide to good practice for glazing frame U-values and Guide to good practice for assessing heat transfer and condensation risk for a curtain wall. These are two of four documents containing advice on the thermal performance of curtain walling: Standard for specifying and assessing for heat transfer and Standard for specifying and assessing for condensation risk will be available later this summer.

The following points on Part L are taken from the CWCT's Technical Update 5:

Using the elemental method to comply with Part L2, the building envelope has to provide certain minimum levels of insulation. For glazing in metal frames, the maximum allowable U-value is 2.2 W/m2K.

The elemental method in Part L allows a trade-off between different building components but limits the U-value of the opaque parts of a wall to a maximum of 0.37 W/m2K. However, this does not apply to curtain walling, as the glass is an integral part of the frame. For curtain walling, the opaque parts of the glazed facade can be greater than 0.37 W/m2K, as long as the whole facade complies with Part L's requirements.

To reduce the risk of condensation, Part L states that no part of the wall shall have a U-value greater than 0.7 W/m2K, but mullions and transoms in curtain walls have U-values in excess of this figure. The regulations allow the use of curtain-wall components if specifiers carry out a condensation risk assessment in accordance with the CWCT good practice guide.

Part L requires that air leakage measured through the building envelope should not exceed 10 m3/hr/m2 at 50 Pa. For buildings with a floor area greater than 1000 m2, Part L suggests air leakage should be demonstrated by tests. For buildings with a smaller floor area, the suggested approach is to show that air leakage has been limited by good design and workmanship.

Walls that meet the requirements of CWCT's Standard and guide to good practice for curtain walling will have air leakage rates of less than 0.45 m3/hr/m2 at 50 Pa. However, interfaces with other forms of construction, such as roofs and adjacent walls, require particular attention to detail in order to achieve compliance.

Other regulations

Changes to Part A of the Building Regulations could force specifiers to change the design of overhead glass panels. The aim would be to eliminate the risk of glass falling on people in the event of extreme weather. Possible solutions would be to specify highly laminated glass or incorporate full arrest systems into designs.

The acoustic Building Regulations Part E could dramatically alter the specification of curtain walling if it asks for the use of thicker glass to protect against external noises. Frames would have to be redesigned to take the weight of the heavier glass, and this would affect window as well as curtain walling manufacturers.

An announcement by the government on Part E is imminent, and the new regulation should come into effect from the autumn.