External Wall Insulation System Designers meet with the BBA to discuss Dry Fix Systems

Mitch Gee – INCA Chairman

The BBA called certificate holding system designers to a meeting on the 7 August to discuss dry fix systems. The majority of UK external wall system designers were represented at the meeting.  Paul Valentine, the BBA’s Director of Technical Excellence presented to the group.

Paul explained that for the BBA to become aligned with European Technical Approvals there was a requirement for system designers to provide evidence that dry fix systems did not suffer from settlement. Such settlement is potentially a result of the flexing of fixings caused by the bending moment as a result of the weight of the render system. The majority of the audience believed this was not necessary, as dry fix EWI systems had been used installed in the UK for over 30 years without any evidence of failure caused by this phenomenon.

European Systems

Dry fix systems are not approved in continental Europe. This is because there is no market for dry fix systems in continental Europe. In Germany, the use of heavy dash systems is very rare and the market is predominantly thin coat render systems.

It is critical with thin coat render systems that the insulation is applied perfectly flat. To achieve this a bedding mortar is required.  In the early days of the UK market when EWI systems were predominantly thicker dash systems, there wasn’t the same need to ensure the insulation was fitted perfectly flat. Therefore in Germany there has always been need for a bedding mortar and no demand to develop dry fix systems.  Some may argue that with a heavy dash system the risk of settlement of would be greater due to the heavier render finish, although there is little evidence to support this.

I believe that the consensus of opinion that on high rise buildings, the additional resistance to wind load that an adhesive mortar might provide and its use in thin  coat render systems it may be considered as best practice, but this is certainly not the case for low rise buildings with thicker render systems.

Calculating the performance of a system

Notwithstanding this, Paul demonstrated three theoretical methods by which the performance of the system could be demonstrated, each one making an allowance for the insulation to assist in limiting the bending moment of the fixing. He described the three method as:

  1. Strut and Tie
  2. Cantilever
  3. Finite modelling.

Paul recommended the use of the cantilever model as it was the easiest to calculate. The strut and tie is more complicated, but would provide a more accurate result that is more favourable to the system designer. The most accurate method would be the finite modelling but its complexity would probably make it impractical for this purpose.

It will be possible for the BBA to arrange testing but this practical method of assessment would be costly and expensive and therefore unlikely to be adopted by any of the system designers.

Paul was open to other methods of calculation, and offered the option of the BBA undertaking the calculation or to receive the calculations submitted by system holders. Either way he did not expect the cost to be greater than a few hundred pounds.

View from INCA

I believe the industry would prefer not to have to undertake this exercise at all as they believe that there is no evidence that there is a problem. If it has to be done in the interests of European harmonisation then we expect the BBA to provide us with the most efficient and cost effective solution and provide the service for a reasonable cost.

The industry is pleased that the BBA is operating a more open policy of engagement and consultation before imposing their will. INCA understands that the BBA has a difficult job, consulting with the industry as a whole but having to retain client confidentiality, ensure all certificates are treated equally and to ensure continual updating. This is a challenging task, and as an industry we need to assist them in this endeavour.