Failure to account for unprotected insulation degradation could undermine new Home Energy Model
By David Lennan, Chairman, National Warm Homes Council
The government’s consultation on its new online calculation method to accurately assess the energy use and energy efficiency of homes in the UK, replacing the Standard Assessment Procedure (SAP), closed on 27 March and will input into changes in Buildings Regulations planned for 2025. The draft consultation plans are a positive step in the right direction. But does the new Home Energy Model (HEM), which will replace SAP, go far enough in ensuring assessments are robust and fit for the future?
Changing how we understand and manage home energy efficiency and consumption is on the critical path to net zero and increased levels of home decarbonisation. People want to live in modern homes genuinely future-ready that minimise environmental and social impacts.
Reform of SAP has been long overdue, with the industry seeking changes for the last decade, but the regulatory environment has barely moved. That is why the consultation has been welcomed.
However, it appears that the proposed HEM misses key reforms needed to ensure assessments are indeed robust and accurate. One missing element from the core calculation is the degradation of unprotected insulation, which signficantly affects the accuracy of the HEM.
The problem appears to be that the HEM treats insulation as a constant value, based upon the theoretical thermal resistance of insulation. In reality, insulation significantly degrades over time, with a corresponding thermal impact that the HEM simply does not account for. This is a particular issue for loft insulation, which is not usually protected when installed, and has a very high rate of degradation as a result. Thus, without counter measures in place, the HEM will simply fail to consistently provide an accurate, real-world assessment of energy performance.
Ofgem has previously recognised this fact within the methodology for the ECO scheme, whereby the lifetime effectiveness of loft insulation was derated by 35% to reflect actual performance, plus an additional 15% for comfort-taking
Data has shown that the real-world performance of insulation will be negatively impacted by human behaviour. The vast majority of homeowners use their lofts for storage, with surveys from 2012 and 2023 showing that around 80% of people use their lofts for storage.
Survey data also shows that of those householders who use their lofts for storage, 78% said their loft was half full or more. This means that most loft insulation will undergo significant compression. Research from the National Physical Laboratory (NPL) has further shown that the standard 270mm height of loft insulation compressed to joist height results in at least a 50% drop in thermal resistance. This means the U-value performance of insulation that has been compressed is significantly negatively impacted, and that this happens in most UK lofts.
Conversely, insulation that is properly protected with raised loft boarding, for example, will be undamaged and will retain near-constant performance, indefinitely, as the material is inert. Insulation protection products are made from specialist materials (recycled plastic components) with thermal qualities, which means they do not conduct heat ensuring that not only is the insulation protected but also that thermal bridges are not created.
In the absence of insulation protection, the HEM could account for insulation degradation in the following ways:
- For new builds, or the retrofitting of insulation, the inclusion of a standard degradation factor that applies to insulation over time.
- For existing dwellings, the amount of degradation should be calculated from the time that the insulation was installed.
To aid the assessment of insulation, a ‘checklist item’ could be added to the HEM, which would remove the degradation factor in new builds when insulation protection is fitted. For existing dwellings, a roof U-value calculator would be necessary whereby an assessor would input the height of the uncompressed insulation – the proportion that is compressed – and the average height of the compressed proportion, and the calculator would output a revised U-value.
This would enable the compression of insulation to be accounted for in the overall energy efficiency assessment for the property.
Protection of insulation safeguards its performance over its lifetime and thereby ensures the long-term energy efficiency of homes. Otherwise, its real-world, in-use performance will be significantly diminished, particularly by insulation compression. This is what is absent from the HEM core calculation and critical for the government to urgently address.
Given the HEM will form the basis of many long-term government policies on insulation schemes, net zero and broader sustainability goals, getting it right now is of utmost importance. Failure to account for the degradation of unprotected insulation risks undermining the HEM, consumers paying increased costs for their energy bills and a missed chance for the UK to achieve net zero ambitions earlier.
