HOW CAN COMPETENCE AROUND U VALUE CALCULATIONS AND INSULATION SPECIFICATION BE IMPROVED?
The matter of misleading U-value calculations – produced unintentionally or otherwise – is a question of the ethics and values that the construction industry wants to reflect, and of the quality of end product it wants to deliver to building users.
The above statement is highly relevant to the discussions and initiatives around competence that have been at the forefront of the construction industry in recent years. And we wrote that statement nearly seven years ago, in early 2018. At the time, we were highlighting an issue with insulation specification that had troubled us for some time, even then.
Accurate insulation specification reflects on everyone
Despite the best efforts of ourselves and others to promote greater awareness, and despite an updated BRE Report BR 443:2019 Conventions for U-value Calculations, inaccurate U-value calculations remain a stubborn issue as we approach 2025.
Getting U-value calculations right and having accurate material data in them is fundamental to good construction practice. It shows that all parties involved – from specifier, to Building Control Body, to contractor – understand the build-ups being specified and installed, and the importance of the outcomes being aimed for.
None of this is to say that all U-value calculations are poorly carried out. Most are accurate and fit for purpose. When we do see inaccuracies, however, they tend to be glaring and concerning.
If a U-value calculation on paper isn’t aligned with what is built on site, or what the client wants to achieve, then it compromises building performance. And beyond compliance with energy efficiency requirements, it risks compromising the comfort and health of building occupants. In the most extreme cases, it could even compromise safety.
Competence in floor insulation specification
Lately we have repeatedly seen an issue in ground floor specification: insufficient depth in the floor build-up to accommodate enough insulation to achieve the required ground floor U-value. Our technical services department has been contacted about multiple projects where the thickness of extruded polystyrene (XPS) needed is around double the depth of the insulation zone allowed for.
“This is not a problem unique to XPS,” explained Rob Firman, Technical and Specification Manager at Polyfoam XPS. “The qualities that make XPS such a good all-round insulation solution mean it is not the outright best-performing in terms of thermal efficiency. However, in the majority of cases we’re talking about, no commonly-available insulation material is capable of performing well enough.
“Put simply, people want insulation to achieve the impossible.”
Hoping to achieve the desired U-value with very limited space risks the specification of an insulant on thermal efficiency alone – without assessing whether its other qualities are suitable to meet the performance requirements for the intended application.
We have received technical enquiries from people who had PIR insulation specified, only for them to realise they need the additional loadbearing capability offered by XPS. Because of the slight difference in thermal performance, the consequence is they don’t have a deep enough insulation zone.
The project then faces the possibility of a compromise: meet the required U-value but risk having insufficient loadbearing capability; or have the right compressive strength but risk non-compliance in terms of energy efficiency.
When good advice is seen as an inconvenience
Alongside ground floors, XPS insulation is also commonly used in inverted roof constructions. This application provides another example of where competence and U-value calculations intersect. Just recently, we received a calculation where a declared lambda value was used for the insulation layer, rather than the material’s design lambda.
(Read more about declared and design lambda values for inverted roof insulation.)
“A calculation with this kind of ‘glaring’ error is still likely to be accepted by the Building Control Body,” said Gary Ferguson, Business Development Manager North and Scotland. “Unfortunately, the people checking them don’t have the time or resource to fully understand every nuance and query them.
“If we raise the issue, we’re more likely to be seen as the ‘difficult’ party and we have to go the extra mile to explain why the original calculation is wrong. That approach is backwards, and our industry needs to do better at pursuing accuracy and questioning if something is too good to be true.”
Demonstrating competence in U-value calculations is difficult. Schemes to assess the competency of people carrying out calculations have met with limited success, so there is unlikely to be a mark or ‘badge’ to help reassure those requesting or checking a calculation.
Conclusion: engage with manufacturers as early as possible
When we see inaccuracies in a U-value calculation and flag them up, it’s often at a very late stage. And because specifications have been set, and floors possibly even dug out already, our advice means an inconvenient change to the project.
Readers of this blog post might be specifying floor insulation for a small house extension or roof insulation for an industrial shed. The examples quoted above all come from such a diverse range of projects. Whatever the building size, the principle is the same: speak to us early in the project timeline if you can.
Preparing information about U-value targets, performance requirements and outline specifications helps to reduce the likelihood of shocks or inconvenience later. Often, the only ‘badge’ on a U-value calculation is the logo of a product manufacturer. Reputable manufacturers provide honest advice about your project – and nobody knows an insulation product better than the manufacturer themselves.
What to do next
Read our application pages for ground floors and roofs to find the right product from our range to suit your needs. You can also check our range of technical support information.
To discuss a specific project, to request a U-value calculation, or to get advice about a calculation you may have already had produced by others, contact us. If we think XPS is the right product for your requirements, we’ll explain why. If we think it would be an over specification, we’ll let you know why and leave the decision to you.