What thickness of insulation do I need for my inverted roof U-value?
Thermal insulation is just one component of a flat roof build-up, so the required insulation thickness depends on the other materials specified with it. For example, if a roof build-up features a structural deck of metal rather than concrete, even if everything else is identical there can be a notable difference in the thickness of the insulation layer needed to meet the target U-value.
Most inverted flat roofs follow a typical build-up: a concrete deck, waterproofing (which also serves as an AVCL), extruded polystyrene (XPS) insulation, water flow reducing layer (WFRL) and ballast layer. As such, it is possible to offer a good idea of what thickness of insulation will help to meet the proposed U-value. Such a figure should only ever be taken as a guide, however, and there is no substitute for project-specific U-value calculations.
Furthermore, the development of lower lambda XPS products is increasingly making it possible for target U-values to be met with a single layer of insulation. This blog post discusses typical inverted roof U-value targets, and the new products available to achieve them.
What building regulation targets are given for inverted flat roof U-values?
There is no single defined U-value target that all new-build flat roofs need to meet. The compliance calculations set out in national building regulations factor in a host of variables, including building fabric energy efficiency. This is true for both for domestic and non-domestic buildings.
Not only that, but the approach taken by different parts of the same regulations can make it confusing to understand what sort of U-value might be needed. Take Volumes 1 and 2 of Approved Document L in England as an example.
Volume 1 (dwellings) describes the notional specification for a new dwelling which, if followed precisely, would ensure compliance. The notional roof U-value given is 0.11 W/m2K.
Volume 2 (buildings other than dwellings) takes a different approach because of the possible variation in building types. It gives a maximum (i.e. worst possible) U-value for new or replacement flat roofs of 0.18 W/m2K.
How do these differing approaches compare to what is seen in real-world projects?
What inverted roof U-values do projects aim to achieve in practice?
Looking at the project enquiries that come through the Polyfoam XPS technical helpdesk, inverted roof U-values are typically in the range of 0.12 W/m2K to 0.15 W/m2K, in practice.
This does not necessarily mean a project you are working on definitely needs to achieve a target within that range. It is, however, a reasonable benchmark.
It confirms a trend that flat roof U-values are moving away from the once commonplace 0.18 W/m2K target (and highlights why Approved Document L Volume 2 uses that value as a worst case now). It also shows how dwellings don’t necessarily need to achieve U-values as low as 0.11 W/m2K if other areas of the specification allow.
What thickness of XPS insulation is required to achieve a 0.12 W/m2K U-value?
For an inverted roof construction that meets the typical specification described above, 250mm of extruded polystyrene (declared thermal conductivity: 0.031 W/mK; design thermal conductivity: 0.032 W/mK) is sufficient to achieve a 0.12 W/m2K U-value.
Thanks to new product developments and production capability, Polyfoam XPS now offers a 0.031 W/mK lambda across our entire range of XPS insulation. For inverted roof applications, where moisture correction has to be taken into account, this translates to a design lambda of 0.032 W/mK.
We are also pleased to be able to offer up to 250mm of insulation in a single thickness. This means there is no need to install double or triple layers of insulation on an inverted roof to reach the desired thickness, saving on installation time.
To discuss your project requirements and to obtain project-specific U-value calculations, contact us.
Could inverted roofs require lower U-values in future?
Meeting regulatory requirements today is not necessarily a guarantee of buildings remaining useable in the future, especially as we don’t know the extent to which climate change will affect how we live and work.
There may come a point where an existing roof would benefit from achieving a lower U-value in order for the built asset to remain viable. In such cases, we believe inverted roofs could have a vital role to play.
An inverted roof build-up is loose laid. Because the insulation is neither mechanically fixed nor adhered, it could help designers and specifiers to allow for future flexibility. It’s not unreasonable to think that the roof finish and ballast layers could be lifted temporarily and an extra insulation layer added, prior to reinstating the layers above.
Issues like thresholds and their heights would need to be considered during the initial design. That’s why you will see us talking more about circular economy and end-of-life scenarios as part of considering the use of XPS through all stages of construction projects. For example, there are sections dedicated to this subject in The Polyfoam Orange Book.
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