Corrected thermal calculations for inverted roofs

Regular readers of this column (yes, we do assume there are some!) may remember our April 2020 QI@QI titled ‘Crucial considerations in calculating U-values in Inverted roofs’. In case you don’t remember this first pandemic/lockdown/furlough period article (which we were not fined for) it covered the use of a WFRL, the difference between Declared and Design Lambda values and ETAG 031. It also referred to the BRE and BBA recommendations. So why are we raising the topic again this month?

Well, as the demand for compliance demonstration increases and the new Part L 2021 guidelines have been released, we feel it is important for our readers and customers to understand some ‘nuances’ in compliance demonstration and U-value calculations that could raise questions with competitive tendering.

3rd Party Certification

The British Board of Agrément (BBA) was founded in 1966 as a construction industry approvals body and has been producing BBA Certificates for roofing and insulation products ever since. In today’s world, BBA Certification has never been more demanded by specifiers and building control authorities.

But the BBA is not the only 3rd party approvals body working in the UK. KIWA is a Dutch company with offices worldwide, including the UK. As approvals bodies you would think the outputs of their certification are the same, but you would be wrong as the two organisations take different approaches to ETAG 031 Lambda values.

KIWA makes no reference to a Design Lambda value to be used in the U-value calculation, simply stating a Declared Lambda value. The BBA on the other hand apply a 2.5% correction to the Declared value to provide a Design value.

The answer is simple you say; “I’ll use the KIWA value and use thinner insulation”. If you did you would be ignoring the guidance note in BS 6229:2018 that recommends increasing the design thickness of the insulation by not less than 10%, which is not clear in the KIWA BDA Agrément as it only cross refers to the requirement to design in accordance with BS 6229:2018.

Confused? Well to add to the confusion the BBA do not agree that an arbitrary 10% increase in thickness is correct as it does not take into account rainfall factors across the UK, and your insulation could be too thick in some locations and not thick enough in others, like Scotland and Northern England.

What’s the solution? Well, the notes in BS 6229:2018 reference ‘until further evidence and test data is made available’ because both 10% (BS 6229:2018) and 2.5% (BBA) are arbitrary and not based on scientific data. QI is supporting the LRWA in establishing this test data, and in the meantime, we recommend following BBA guidance as it is the most common inverted insulation certification and has been used for many years without issue.

Next month’s QI by QI – What is happening with market pricing?