For architects and sustainability consultants designing external walls in 2026, thermal performance is no longer a tick-box exercise. Part L 2025 has tightened U-value targets, the Future Homes Standard is in sight for 2026/2027, and the Building Regulations are converging on net-zero-ready building fabric.
For SFS through-wall systems, the question is no longer “can we meet the U-value?” — modern SFS facades can readily achieve U-values of 0.18 W/m²K and lower. The harder questions are: how do we achieve them efficiently, what does thermal bridging do to the design U-value, and how do we specify build-ups that pass not just the calculation but the in-service test?
This guide covers the achievable U-values for SFS through-wall systems, the thermal bridging factors specifiers commonly miss, and how to specify SFS facades that genuinely deliver on Part L 2025.
For the foundational context, our pillar guide on SFS construction and how it integrates thermal, acoustic and fire performance explains why through-wall design is a holistic exercise, not a layered checklist.
What Part L 2025 requires for new build external walls
Under Part L 2025 (the 2025 amendments to the Building Regulations Approved Document L), notional building U-value targets have tightened significantly:
- New dwellings: Notional external wall U-value of 18 W/m²K (down from 0.26 W/m²K under previous regulations)
- New non-domestic buildings: Notional external wall U-value of 18 W/m²K
- Building target (using SAP/SBEM compliance): must be at or below the notional building’s overall energy use
The notional U-value is not a strict limit — buildings can comply via the overall energy budget — but in practice, designers target 0.18 W/m²K or better at element level to achieve compliant overall performance without compromise elsewhere.
The Future Homes Standard, scheduled for full implementation in 2026/2027, tightens this further, with a strong push toward U-values around 0.13–0.15 W/m²K for external walls in new dwellings.
Achievable U-values for SFS through-wall systems
SFS through-wall systems can readily achieve the U-values required by Part L 2025 and beyond. Typical build-ups and their calculated U-values:
Standard SFS through-wall (target: U = 0.22–0.28 W/m²K)
- 100mm SFS framework with 100mm mineral wool quilt insulation in cavity
- 50mm rigid PIR insulation continuous outboard of sheathing
- Sheathing board, breather membrane, ventilated cavity, cladding
- Internal plasterboard lining
- Calculated U-value: ~0.22 W/m²K
Enhanced SFS through-wall (target: U = 0.18 W/m²K)
- 150mm SFS framework with 150mm mineral wool insulation in cavity
- 80mm rigid PIR continuous outboard
- Sheathing, membrane, ventilated cavity, cladding
- Internal plasterboard lining
- Calculated U-value: ~0.18 W/m²K
Future Homes Standard SFS through-wall (target: U = 0.13–0.15 W/m²K)
- 200mm SFS framework with 200mm mineral wool in cavity
- 100mm rigid PIR continuous outboard
- Sheathing, membrane, ventilated cavity, cladding
- Internal plasterboard lining (insulated PIR-backed if needed)
- Calculated U-value: ~0.13–0.15 W/m²K
These figures are indicative — actual U-values depend on the specific products specified, the proportion of framing-to-insulation in the wall, and the design detail of openings and corners. The SFS infill walls technical guide covers the design parameters in more depth.
Why thermal bridging matters more than the headline U-value
Here is the critical insight that distinguishes SFS thermal design from masonry: steel is a far better thermal conductor than concrete or brick. A steel stud passing through your insulation layer is a thermal bridge — a high-conductivity pathway that loses heat regardless of how well-insulated the cavity is.
In a typical SFS wall, the steel framing might cover 8–12% of the wall area. Without continuous outboard insulation, the effective in-service U-value can be 30–50% worse than the simple calculation suggests.
The solution is continuous insulation outboard of the framing — a layer of rigid insulation (typically PIR or mineral wool) on the cold side of the steel, breaking the thermal bridge. This is why the build-ups above all include outboard insulation: without it, the U-value calculation becomes meaningless in practice.
Modern SFS through-wall design therefore uses three insulation layers conceptually:
- Cavity insulation between the studs — fills the framework depth
- Outboard continuous insulation — breaks the thermal bridge at the studs
- Optional insulated internal lining — for ultra-low U-value targets
Skipping the outboard layer is the most common — and most expensive — thermal mistake in SFS specification.
Other thermal bridges to watch
Beyond the steel-stud thermal bridge, other linear thermal bridges in SFS design include:
- Window and door reveals — the cold steel jamb stiffener creates a thermal bridge unless the reveal is insulated
- Floor-to-floor junctions — the slab edge can be a major bridge, especially where SFS is flush-fixed rather than oversail
- Roof-to-wall junctions — the parapet detail commonly leaks heat
- Balcony connections — thermal break connectors are now standard for compliant detail
Linear thermal bridge calculations (Psi values) feed into the SAP/SBEM compliance calculation. A facade with an excellent flat U-value but poor junction detailing can fail SAP. The fix is design-stage coordination between architect, structural engineer and SFS subcontractor — exactly the work covered by our SFS design service.
Air-tightness — the silent partner of thermal performance
A wall with a perfect U-value performs poorly if it leaks air. Part L 2025 requires significantly improved air permeability — 5 m³/(h·m²) at 50 Pa for new dwellings, with stretch targets at 3 m³/(h·m²) and below for Future Homes Standard compliance.
SFS through-wall systems can achieve excellent air-tightness when designed with:
- Continuous breather membrane with all joints taped (Illbruck, Siga, or equivalent)
- Air-tightness tape at all junctions — slab edges, openings, services
- Sealed deflection head detailing — see our SFS deflection heads guide
- Wet-applied air barrier paint at high-risk junctions
- Whole-building air pressure test at handover, not as an afterthought
In our experience, the air-tightness specification is the most commonly underspecified element of an SFS package. Get it right at design stage and the cost is modest. Discover the leakage at the post-construction air test and the remediation cost is substantial.
Specifying U-values that actually deliver
When writing the SFS thermal specification, include:
- Target U-value for the through-wall element
- Approved insulation products and thicknesses (specify by product, not just by thermal conductivity)
- Continuous outboard insulation requirement — non-negotiable for SFS thermal performance
- Linear thermal bridge requirements at junctions (target Psi values for window reveals, slab edges, parapets)
- Air permeability target at element and whole-building level
- Product certification — third-party Agrément certificates, BBA approval, or equivalent
Our SFS supply service sources thermal materials with full BBA / third-party certification and traceable performance data. For complex thermal coordination, our team works alongside the project’s M&E and sustainability consultants from RIBA Stage 2.
The Future Homes Standard — what’s coming
The Future Homes Standard (FHS), scheduled for 2026/2027, will require approximately 75–80% reduction in regulated CO₂ emissions from new homes compared with 2013 baseline. For external walls, this means:
- U-values around 0.13–0.15 W/m²K for the dominant building element
- Heat pump-ready building fabric — the wall must be airtight and well-insulated enough for low-temperature heating systems to perform
- No fossil fuel heating in new homes — which puts even more pressure on fabric performance
- As-designed and as-built performance gap closure — standard performance testing post-construction
For SFS specifiers, this means:
- Designing now to FHS targets even on Part L 2025 projects, to avoid stranded assets
- Continuous outboard insulation as default, not as an upgrade
- Air-tightness specification as a primary, not secondary, design parameter
- As-built compliance documentation — increasingly enforced
The SFS systems being installed today on Part L 2025 projects will be expected to perform throughout the FHS era. Specifying for 0.18 today often makes more sense as specifying for 0.15 — the cost premium is small at design stage and significant at retrofit.
Practical takeaways
- U-value targets are achievable with SFS — the system supports any thermal performance target up to and including Future Homes Standard.
- Continuous outboard insulation is non-negotiable. Skip it and the calculation becomes fiction.
- Coordinate junction details at design stage. Linear thermal bridges destroy the headline U-value if not detailed.
- Specify air-tightness as a primary requirement. It is the silent partner of every U-value.
- Document as-built performance. What you specified is not necessarily what you got — testing closes the loop.
For project-specific advice on achieving Part L 2025 or Future Homes Standard U-values with SFS, get in touch with your structural drawings and target thermal performance, and our team will propose a tested through-wall specification with full compliance documentation.
Related reading
- SFS infill walls — technical design guide — the foundation for thermal-related design decisions.
- SFS fire resistance ratings explained — fire and thermal specification must be coordinated, not stacked.
- SFS cost per m² UK 2026 — how thermal specification choices affect package cost.
