STA Advice Note 18 – what do specifiers need to know?

John Mellor of Protect Membranes explains the key subject of thermal performance for breather membranes installed to structural timber walls, as detailed in the Structural Timber Association’s STA Advice Note 18.

To assist specifiers involved in the growing structural timber construction market, the Structural Timber Association (STA) after consultation with the NHBC, has released STA Advice Note 18 to provide guidance on the selection of external breather membranes to cavity facing structural timber wall panels.

One of the key focus areas of STA Advice Note 18 covers thermal performance, which plays an important and increasingly significant role in the energy efficiency of a building. Where reflective, insulating membranes are used within a still airspace to improve the wall’s thermal performance and to generate low U-values as calculated in the Standard Assessment Procedure (SAP), these should have been tested to BS EN 15976 to demonstrate emissivity values in both unaged and aged conditions. STA Advice Note 18 recommends that aged thermal resistance values are used as a basis for specification of external breather membranes and should be installed in accordance with the good practice of fixing at the correct stud centres.

The aged ‘R’ value result (m²K/W) and surface emissivity (Ɛ) should also be used when assessing a reflective breather membrane’s total thermal resistance to BS EN ISO 8990 and BS EN 15976. Any printed logos on the reflective membrane surface should also be factored into the final thermal performance claims in line with BE EN 16012 and the result should incorporate printed logos. As stated in STA Advice Note 18, overall surface emissivity is not affected if the print area utilised on the membrane is below two per cent ink coverage.

STA Advice Note 18 also states that the fixing of breather membranes to a timber frame wall stud is also important in terms of ensuring the required thermal performance. Membranes fitted to timber frame wall panels should be installed with a maximum of 600mm horizontal fixing centres and up to a maximum of 500mm vertical stud centres. Designers are advised to avoid the specification of membranes with wider fixing centres as these cannot be maintained on-site and excessively loose material could bridge the cavity. Therefore thermal resistance values would then need to be adjusted.

It is important that the chosen reflective breather membrane manufacturer can demonstrate that thermal resistance results are based on aged values, incorporate printed logos and refer to the correct fixing centres. Protect is one such supplier with reflective membranes independently certified by BM TRADA with aged thermal resistance and surface emissivity results, using 600mm horizontal and 500mm vertical stud centres.

Visit the company’s LinkedIn page at to watch a video blog that summarises these key specification tips and recommendations detailed in STA Advice Note 18, regarding the thermal performance of external reflective breather membranes.

As the construction industry gears up to getting fully back to work as lockdown restrictions start to ease, Glidevale Protect is open for business, with our employees working from home to support our customers. We can help with U-value calculations, design guidance, building regulations advice, site take-offs, product specification details and clauses as well as structured quotes. If you require technical support on existing projects or wish to discuss new specifications, we can be contacted on info(Replace this parenthesis with the @ sign) or call us 0161 905 5700. Please leave a message and we will aim to return your call within the hour (during working hours).

John Mellor is Product Manager at Protect Membranes, sister company to Glidevale. A copy of the STA Advice Note is available from him at johnm(Replace this parenthesis with the @ sign)