What is the difference between KIWA and BBA certification?

Compliance is big news in construction right now with clients, main contractors, insurers and building control bodies all looking for suppliers and contractors to demonstrate that their roofing systems comply with the legal requirements of the Building Regulations. Compliance can be demonstrated by manufacturing a product to a British Standard or through independent 3rd party assessment certification.

Currently there are two independent third party assessment organisations providing product certification in the United Kingdom: The British Board of Agrement, commonly referred to as the BBA, and KIWA. Only the BBA are UKAS accredited as an assessment organisation, but post Brexit a decision is still to be made by Government as to whether approvals bodies need to be UKAS accredited.

Both organisations provide certification for inverted roof insulation in accordance with HAG 031, a European Technical Approval Guideline, but the BBA and KIWA take different approaches to the correction factor that is applied to the Lamdba (thermal) value of the inverted roof insulation. The application of the correction factor to the thermal calculation is critical as it determines the thickness of insulation needed to achieve the required U-value (thermal performance) of the roof.

The BBA take an approach whereby they apply a moisture correction factor to the Declared thermal conductivity of the insulation board, publishing a Design thermal conductivity that is lower than the Declared value. The Design value is the one that should be used to calculate the U-value of the roof construction.

KIWA publish the same thermal conductivity figure for both the Declared and Design values, stating that the correction should be carried out in accordance with the guidance in BS EN 6229:2018, whereby ‘not less than 10%’ is added to the calculated insulation thickness.

The benefit of the BBA method is that the correction factor is applied during calculation, providing a clear thickness requirement for the individual roof. The risk with the approach of adding ‘not less than 10%’ is that this relies on the individual doing the calculation to add the additional thickness after the calculation is made, something that could be missed if the calculation is included in a tender package without the additional thickness included. The client could end up with less insulation than they need to achieve the required U-value, something that would affect the thermal performance of the building.

Next month: The financial impact of multi-layer inverted roof schemes.