Building Fabric Performance Under the FHBS
The FHBS sets demanding fabric standards that represent a significant improvement over Part L 2021. These U-values, combined with tighter airtightness targets, form the foundation of a zero-carbon ready building envelope.
External Walls — 0.18 W/m²K
FHS wall U-values of 0.18 W/m²K (down from 0.26 under Part L 2021) typically require enhanced insulation thickness in cavity wall or external wall insulation systems. This level of thermal performance is achievable with current construction methods but requires careful attention to insulation continuity and thermal bridging at junctions.
Roof — 0.13 W/m²K
Roof U-values of 0.13 W/m²K (down from 0.16 under Part L 2021) require increased insulation depth, typically 300mm+ of mineral wool between and over rafters, or equivalent rigid board. The improvement reduces heat loss through what is typically the largest exposed surface of a dwelling.
Floor — 0.13 W/m²K
Floor U-values of 0.13 W/m²K (down from 0.18 under Part L 2021) require careful ground floor construction, typically with rigid insulation boards below or within the slab. Edge insulation is important to prevent thermal bridging at the wall-floor junction.
Windows — 1.2 W/m²K (Triple Glazing)
Window U-values of 1.2 W/m²K effectively mandate triple glazing as standard for new FHS homes. The U-value calculation now uses actual installed window sizes rather than standard test sizes, improving accuracy and closing the gap between design and real-world performance. Some structural elements have been excluded from the calculation following consultation feedback.
Airtightness — 3 m³/h/m² at 50Pa
The airtightness target of 3 m³/h/m² at 50Pa is approximately three times tighter than the 2021 standard (8 m³/h/m²). At this level of airtightness, controlled mechanical ventilation (MVHR) is essential — natural ventilation through gaps and cracks is no longer sufficient to maintain indoor air quality. This dramatically reduces uncontrolled heat loss.
Thermal Bridging
The default y-value for assessing thermal bridges has been removed in the Home Energy Model, requiring junction-by-junction modelling or use of calculated psi-values. SAP 10.3 will continue to allow y-values during the dual running period. This change incentivises careful detailing at junctions and improves the accuracy of heat loss predictions.