Siniat Weather Defence^TM does act as a breather membrane when installed and sealed in line with the installation procedure. The board and sealing have been tested to show negligible air leakage (0.002 m3/m2/hr). The prevention of water penetration test demonstrated performance at the highest level - Class W1 to EN13859-2 matching that of standalone breather membranes.  The product also has vapour resistance of 0.49MN.s/g  which sits within the defined level of performance range of 0.25 - 0.6MN.s/g found in BS 5250:2011 for breather membranes. The site design team must consider these performance levels and determine if that is suitable for the project.

Siniat consider breather membranes unnecessary in normal UK exposure conditions when combined with façade elements, e.g. a typical façade systems and insulation, that deflect >95% of rain from reaching the board but if increased performance is required over the stated values above then a separate membrane must be considered.

The waterproofing at edges of runs of Weather Defence^TM, or at other junctions, e.g windows, may need to be provided by others and these suppliers would advise on weather-tightness of these areas. Complex details and junctions may require the use of a breather membrane for effective weather-tightness where multiple materials meet and/or the geometry is complex, but this would be at the decision of the project designers. Note; some external insulation manufacturers do recommend breather membranes to their products.

Ultimately, the use of a breather membrane will need to be determined by the client, principal contractor and/or designer in consideration of all the complexity of the building and any additional assurances offered by the waterproofing suppliers. We’re not stopping designers from using breather membranes with our product, we’re simply advising it may not be necessary in most/some cases.