Thermally efficient building envelope using pre-finished steel cladding
Pre-finished steel cladding systems can be used to form the basis of a highly thermally efficient building envelope, with low fabric U-values and excellent levels of air-tightness.
Work carried out by Corus and Oxford Brookes University (OBU) has demonstrated that air leakage rates through correctly installed pre-finished steel cladding side and end lap joints account for less than 2% of the allowable air infiltration rates.
U-value requirements of Part L 2006 and the 2010 revisions
Modern pre-finished steel cladding systems can be specified to easily meet the U-value requirements of Part L 2006 and the 2010 revisions due for implementation in October.
This places the emphasis on the performance of building details and junctions and how they have been designed and installed.
How do I ensure air-tightness of the building envelope is maintained at the cladding interfaces and junctions?
There are 2 main types of interface to consider.
1) Pre-finished steel to pre-finished steel
2) Pre-finished steel to an alternative material such as brickwork or concrete.
Pre-finished steel to pre-finished steel cladding joints have been assessed as part of a comprehensive test programme with OBU.
The key issue is to firstly identify the liner sheet of the cladding system as the air-tightness layer. If the correct compressible foam filler blocks and sealant are used and the detail has been correctly installed, test data indicates that the leakage rates from these joints will not be a significant factor in the overall building performance.
Pre-finished steel to alternative materials need to be treated on a case-by-case basis. The key points are to firstly ensure that the different cladding materials have been installed to the specified tolerances, to ensure good alignment at the interface and minimise excessive gaps. The interface detail which is used should have been designed for the application, using the appropriate sealant and filler blocks.
How do I ensure that the building details have been correctly designed and installed to minimise heat losses?
Pre-finished steel has a very high thermal conductivity, details must be carefully designed to ensure that they do not penetrate through the insulation cavity, creating a thermal bridge through the cladding system. Poorly designed details will lead to excessive heat losses and may result in condensation forming within the detail. This could lead to corrosion and premature failure of the cladding.
Corus work with a number of supply chain partners, who provide building specific details and calculations of the associated thermal heat loss and condensation factor. Details have been designed by the cladding system manufacturer to ensure that they can be practically installed on site and then assessed using a thermal modelling package.
Use of a reputable cladding installation contractor with good site practices and inspection procedures will ensure that the cladding and details are installed as designed.