BS 6229:2025 update: what is the 2.5% insulation increase for inverted roofs?
The updated code of practice for flat roofing, BS 6229:2025, recommends an insulation thickness increase where the design lambda of inverted roof insulation is the same as the declared lambda.
The new recommendation for a minimum 2.5% insulation increase replaces a controversial note in BS 6229:2018 that suggested insulation thicknesses may be increased by 10% regardless of circumstance.
Like its predecessor, BS 6229:2025 Flat roofs with continuously supported flexible waterproof coverings. Code of practice describes best practice for the design, construction and maintenance of flat roofs. The changes made to the standard reflect an effort to improve the in-service performance of flat roofs, making it more likely that design intent is reflected in a constructed roof.
BS 6229 is not an adopted European or international standard. But as a code of practice developed by the UK flat roofing industry, its guidance and recommendations carry weight.
Why has inverted roof insulation been debated in BS 6229?
As implied by their name, inverted roofs feature the thermal insulation layer above the waterproofing layer. This exposes the insulation to moisture, affecting the insulation’s thermal conductivity and the overall thermal performance of the roof.
The inclusion of a water flow reducing layer (WFRL) membrane over the insulation can significantly reduce the volume of rainfall penetrating the insulation layer and reaching the waterproofed deck. Even with clear industry guidance, test methods and calculation corrections that account for rainwater penetration, there is longstanding debate over whether theory reflects what happens in practice.
The 2018 version of BS 6229 only complicated this debate. It introduced a note (i.e. not a formal recommendation) saying that insulation thicknesses could be increased by 10% to allow for water-induced reduction in thermal performance.
“Some in the flat roofing sector interpreted the note as a recommendation that had to be adopted, leading to significant increases in inverted roof insulation thickness,” said Rob Firman, Technical and Specification Manager at Polyfoam XPS. “This approach ignored the ways in which inverted roof U-value calculations accounted for the effect of water, including the use of design thermal conductivities rather than declared thermal conductivities.”
To make matters worse, certification bodies offering alternative third-party approvals to BBA certificates took an entirely different approach. They issued inverted roof system certificates that claimed no corrections needed to be applied to insulation, on the basis that the informal 10% insulation increase would be adopted – something that was far from guaranteed.
BS 6229:2025 takes new approach to inverted roofs
In 2018, we produced a detailed white paper looking at the testing, assessment and certification of inverted roof systems.
Our conclusion showed that the 10% increase suggested by BS 6229 at the time made an appropriate adjustment for roof systems using the declared lambda of the insulation. Doing so produced a similar outcome to an inverted roof calculation featuring all of the typical corrections, including design lambda.
We are therefore pleased to see the approach adopted by BS 6229:2025. The note suggesting a 10% insulation increase has been removed.
Recognising the impact of correction factors and design lambdas, the standard now has no additional recommendations for inverted roof U-value calculations that adopt a typical approach. In the majority of cases, therefore, there should be no need to adopt an additional or alternative percentage increase for the insulation layer.
That is the key message to take away. What BS 6229:2025 does recommend is that, where declared and design lambdas are the same for a roof’s insulation layer, a minimum 2.5% insulation thickness increase should be adopted.
This recommendation sits alongside other new recommendations for the installation of WFRLs, and separate recommendations for inverted blue roofs (which we will cover in another blog post).
What else has been updated in BS 6229:2025?
One of the primary goals when revising BS 6229:2025 was to bridge the gap between how flat roofs are designed and how they perform across their whole life.
“Completed flat roofs have often been let down by poorly constructed structural decks with incorrect falls,” said Gary Ferguson, Business Development Manager (North and Scotland). “When it is unclear with whom the responsibility lies to deliver a correctly finished deck with the right falls, roof systems (such as inverted roofs) are installed without the fundamental issues being addressed first.”
BS 6229:2025 has therefore been updated to better align it with other industry standards, and improve consistency and clarity in its recommendations. Alongside the thermal performance of inverted roofs discussed above, other significant changes include the following.
- Updated guidance on drainage falls, including the introduction of ‘absolute zero falls’ for blue roofs.
- Clear emphasis on deck quality and definition of responsibility for avoiding back falls.
- Alignment of definitions with other standards to reduce ambiguity around terminology. BS 6229:2025 now uses terms such as balcony and terrace in a manner consistent with wider industry guidance.
What to do next
When we first wrote about the 2018 update to BS 6229, we advised that “design professionals should be familiar with BS 6229 and apply its principles from the outset” of projects. That advice remains valid for the 2025 edition, and arguably carries more weight with the greater expectations on designers and specifiers to deliver roofs that perform in service.
For support in applying the recommendations of BS 6229:2025 to your projects, contact us. You can also download our inverted roof BBA certificate, and request a copy of The Polyfoam Orange Book of XPS Insulation – our comprehensive specification guide to specifying extruded polystyrene for inverted roofs and beyond.Book Guide to XPS Insulation, visit Polyfoam XPS on stand H57 at Futurebuild, May 12th to 14th 2026. Find out more at https://www.futurebuild.co.uk/