The much-delayed Future Homes Standard is finally here, with an updated and renamed Approved Document L: Energy and Greenhouse Gas Emissions reflecting a clear shift in focus. From March 2027, new homes will need low-carbon heating, renewable energy sources and a highly efficient building fabric. New reporting requirements will also mean that homes must perform as designed.
Even with the introduction of solar PV and heat pumps, it is important not to lose sight of the role of the building fabric. It remains critical to delivering high-performing homes, and established best practice for specification still applies.
THREE TARGETS, BUT FABRIC COMES FIRST
New homes must meet three performance targets. The Target Emission Rate (TER) covers operational CO2 emissions. The Target Primary Energy Rate (TPER) measures total primary energy use. The Target Fabric Energy Efficiency Rate (TFEE) assesses the thermal performance of the building fabric.
At first glance, it may seem that fabric only affects the TFEE, while the TER and TPER are addressed through low-carbon technologies. In reality, the fabric plays a much wider role. Insulation does not just control internal temperatures. It reduces the overall energy demand of the building and supports the efficient operation of low-carbon heating systems.
The maximum U-values in the Future Homes Standard and the updated Approved Document L remain the same as those introduced in the 2021 Part L update. However, the new guidance makes it clear that these values are unlikely to be sufficient in practice. Lower U-values will often be needed in key elements of the fabric to meet the TFEE, which can only be met through insulation measures.
AS-BUILT PERFORMANCE
The shift towards proving performance in the finished building, rather than only at design stage, was first made in the 2021 update to Part L with the introduction of the BREL report. This report is required at the design stage and again once the building is finished, placing far greater emphasis on delivering the design intent on site.
The Home Energy Model (HEM) will eventually replace SAP 10.3 as the primary calculation methodology. This will enable a more detailed and dynamic assessment of building performance, further aligning predicted and actual outcomes.
NOTIONAL ISN’T OPTIMAL
Although the new Approved Document L outlines the importance of the notional dwelling, following this is only one path to compliance. In practice, the notional dwelling can be restrictive, and specifiers should look for the optimal approach, considering the whole fabric and other performance factors. Specifiers must hit the TFEE, but how the building achieves that is flexible and should be treated on a case-by-case basis.
DEEPER LOFTS, WIDER WALLS
For many house types, one of the simplest ways to achieve compliance is by specifying deeper loft insulation and wider wall cavities. Loft insulation has long been recognised as both an essential part of the building fabric and a cost-effective way to improve overall performance. It is now common to see loft insulation specified at depths of up to 500mm, and this should be considered for future home-compliant designs.
Wider wall cavities are also becoming standard practice. A survey conducted in late 2025 of Professional Builder readers found that 66% of respondents who build with masonry cavity walls plan to use 150mm cavities when the Future Homes Standard comes into effect, with a further 23% considering it.
One advantage of 150mm cavities is that they allow the use of more forgiving insulation materials while still meeting thermal requirements. In the same survey, 54% of respondents identified mineral wool, either rock or glass, as their preferred cavity wall insulation, compared with 38% who favoured rigid boards such as PIR or EPS.
BEYOND LAMBDA
There are additional considerations for fabric specification beyond the lambda value of the insulation. Airtightness and the control of thermal bridging are essential to meeting the targets set out in Approved Document L. Achieving this requires careful design and detailing, particularly at junctions and around penetrations.
Material choice can also support better outcomes. Flexible or loose-fill insulants, such as glass mineral wool, can make it easier to maintain thermal continuity and achieve a consistent fit around complex details.
Fire safety is another factor that should not be overlooked. Fires associated with solar panels are a recognised risk, and this should be considered when designing the building fabric. Specifying non-combustible insulation can help limit the spread of fire if one occurs. Glass mineral wool is typically classified as A1 or A2-s1,d0 under the Euroclass reaction to fire system, although ratings can vary between products.
SPECIFYING FABRIC FOR FUTURE HOMES
In practice, for many house types, one of the simplest ways to achieve compliance is by specifying deeper loft insulation and wider wall cavities. Thoughtful specification, buildability and attention to detail will determine whether targets are achieved in reality or only on paper.
As performance assessment becomes more rigorous and increasingly aligned with as-built outcomes, the margin for error is reduced. Prioritising robust, practical fabric solutions from the outset will be key to closing the performance gap and ensuring that homes perform as intended. www.knaufinsulation.com