SFS Wall Build-Up: UK Specification Reference (External & Internal Walls)

Why a wall build-up reference matters

When a structural engineer sizes an SFS frame they need to know the build-up. When a thermal designer calculates a U-value they need to know the build-up. When the fire engineer signs off the Gateway 2 evidence pack they need to know the build-up. When the cost consultant prices the package they need to know the build-up. And when the SFS subcontractor turns up to site to install the wall they need to know the build-up — to the millimetre.

This document is the standard reference we use at BAS Frames for the common SFS wall build-ups we install across UK Build-to-Rent, student accommodation, hotel and commercial projects. Use it as a starting point for a specification, a sanity check against an existing specification, or a copy-paste source for repetitive specification work. Every build-up below is documented to a level a structural engineer, fire engineer and thermal designer can review and challenge — that’s the standard a properly-engineered SFS specification needs to meet in 2026.

Where this document gives a range, the actual figure depends on project-specific factors: wind load, height above ground level, fire engineering strategy, acoustic performance target, and the exact products selected within each material class. Use the ranges as guidance; sign off the specifics with your design team and the relevant manufacturers.

Anatomy of an SFS external wall

Every SFS external wall is built up in layers, each playing a specific role in the wall’s overall performance. Read from outside (the visible cladding finish) to inside (the painted plasterboard the building occupants see), the standard layer list is:

1. Outer leaf — visible cladding finish (brick, render, rainscreen, terracotta or metal)
2. Cavity — minimum 50mm drained and ventilated, with vertical cavity battens
3. Breather membrane — vapour-permeable, water-resistant, lapped and taped
4. Sheathing board — typically cement-bonded, 12-15mm
5. SFS frame — cold-formed steel studs and tracks, typically 100mm or 150mm deep at 600mm centres
6. Insulation — mineral wool, between studs, typically 100-150mm
7. Vapour control layer (VCL) — heavy-duty polythene or specialist VCL membrane, lapped and taped
8. Internal lining — fire-rated plasterboard, single or double layer

Different cladding finishes change the outer two layers. Different fire ratings change the plasterboard. Different acoustic specs change the stud arrangement. Different thermal targets change the insulation depth. The middle of the build-up — frame, sheathing, breather, VCL — stays broadly the same across all standard SFS external walls.

Anatomy of an SFS internal partition wall

Internal partition walls use the same SFS frame technology but with simpler weatherproofing requirements (none) and a focus on fire and acoustic performance:

9. Internal lining (Side A) — fire-rated plasterboard, single or double layer
10. SFS frame — typically 70mm, 92mm or 146mm studs at 600mm centres
11. Acoustic infill — mineral wool, typically 50-75mm (sometimes full-depth)
12. Internal lining (Side B) — fire-rated plasterboard, single or double layer

Service zones — for electrical, mechanical and plumbing — are accommodated within the frame depth or via dedicated services voids on one or both sides of the partition. Specification gets more complex when service zones intersect fire-rated walls, because every penetration needs intumescent or fire-rated sealing to maintain the wall’s fire rating.

Standard build-up 1: 100mm SFS infill + rainscreen cladding

The most common UK external wall build-up for Build-to-Rent and student accommodation in 2026. Light, fast to install, A2-fire-compliant, hits Part L 2025.

• Outer leaf: A2-rated rainscreen panel (fibre cement, terracotta, A2-rated aluminium composite, or natural stone) on aluminium carrier rails
• Cavity: 50mm minimum, drained and ventilated, with aluminium cavity battens at panel-appropriate centres
• Breather membrane: A2-rated vapour-permeable membrane (e.g. mineral-based or fibreglass-reinforced), lapped 150mm and taped
• Sheathing: 12mm cement-bonded particleboard or mineral-fibre sheathing board, fully fixed to studs
• SFS frame: 100mm × 1.5mm or 2.0mm C-studs at 600mm centres, U-tracks at floor and ceiling with deflection head detail
• Insulation: 100mm stone wool slab (A1-rated, density 60kg/m³ or higher for fire performance)
• Vapour control layer: 500-gauge polythene with all laps taped
• Internal lining: 1 × 15mm Type F (fire-rated) plasterboard, screw-fixed at 300mm centres

Performance: U-value approximately 0.22 W/m²K (needs increasing for Part L 2025 — see build-up 2). Fire rating 60 minutes from inside. Acoustic Rw approximately 45 dB. Total wall thickness approximately 250mm.

Standard build-up 2: 150mm SFS infill + brick outer leaf (Part L 2025-compliant)

The premium UK external wall build-up for mid-rise residential where brick aesthetics are required and Part L 2025 compliance is mandatory.

• Outer leaf: 102.5mm facing brickwork, retained on stainless steel windposts and brick ties (every 4th course vertically, 600mm horizontally)
• Cavity: 100mm clear, drained and ventilated, with weep vents at all DPC levels
• Breather membrane: A2-rated breather membrane on cement-bonded sheathing
• Sheathing: 12mm cement-bonded particleboard, fully fixed to studs
• SFS frame: 150mm × 2.0mm C-studs at 600mm centres, with deflection head detail at all slab interfaces
• Insulation: 150mm stone wool slab (A1-rated, 60kg/m³ minimum density)
• Vapour control layer: 500-gauge polythene with all laps and penetrations taped
• Internal lining: 2 × 12.5mm Type F plasterboard, screw-fixed at 300mm centres (achieves 60 minutes fire from inside)

Performance: U-value approximately 0.18 W/m²K (meets Part L 2025 for dwellings). Fire rating 60 minutes from inside. Acoustic Rw approximately 50 dB. Total wall thickness approximately 380mm (excluding brick which adds 102.5mm + 100mm cavity).

Standard build-up 3: Internal partition wall — 60-minute fire rated

Standard 60-minute fire-rated SFS partition for apartment intra-tenancy walls and commercial partitions. Single-stud, mineral-wool-filled, double-board on each face.

• Face A: 2 × 12.5mm Type F (fire-rated) plasterboard, screw-fixed at 300mm centres, joints staggered between layers
• SFS frame: 70mm × 0.55mm or 0.7mm C-studs at 600mm centres, in 70mm U-tracks at floor and ceiling
• Acoustic infill: 50mm mineral wool slab (lightweight grade)
• Face B: 2 × 12.5mm Type F (fire-rated) plasterboard, screw-fixed at 300mm centres, joints staggered between layers

Performance: Fire rating 60 minutes (REI 60 to BS EN 13501-2). Acoustic Rw approximately 48 dB. Total wall thickness approximately 120mm.

Standard build-up 4: Internal acoustic separating wall — apartment to apartment

Required between apartments under Part E (Resistance to the Passage of Sound). Achieves Rw 55 dB minimum (DnT,w +Ctr ≥ 45 dB on completion testing). Uses a twin-stud arrangement to prevent flanking sound transmission.

• Face A: 2 × 15mm Type F or specialist acoustic plasterboard, screw-fixed at 300mm centres
• SFS frame A: 70mm × 0.7mm C-studs at 600mm centres, in 70mm U-tracks
• Air gap: 25mm
• SFS frame B: 70mm × 0.7mm C-studs at 600mm centres (offset from frame A), in 70mm U-tracks
• Acoustic infill: 100mm mineral wool slab between both frames (50mm in each)
• Face B: 2 × 15mm Type F or specialist acoustic plasterboard, screw-fixed at 300mm centres

Performance: Acoustic Rw approximately 58 dB (DnT,w +Ctr 50 dB+ on completion testing — comfortably exceeds Part E). Fire rating 90 minutes. Total wall thickness approximately 250mm.

Acoustic build-ups: meeting Part E

Part E of the Building Regulations sets minimum acoustic standards for residential separating walls. The key targets for new build:

• Separating walls between dwellings: DnT,w + Ctr ≥ 45 dB (airborne sound, weighted with low-frequency correction)
• Separating floors between dwellings: DnT,w + Ctr ≥ 45 dB, plus L’nT,w ≤ 62 dB (impact sound)
• Internal walls between rooms in the same dwelling: Rw ≥ 40 dB

Hitting these in SFS requires the twin-stud separating wall shown in build-up 4 above for the apartment-to-apartment case, and the single-stud build-up 3 (with appropriate plasterboard upgrade) for internal partitions. Acoustic performance is verified on completion via Pre-Completion Testing (PCT) — a sample of party walls and floors is tested by an acoustician, and non-compliance triggers remedial work or design review.

Common acoustic failure modes in SFS construction:

• Rigid connections between the two stud frames in twin-stud separating walls — even a single screw inadvertently fixed across both frames creates a sound bridge that wipes out the airborne performance
• Service penetrations not properly acoustic-sealed — every cable, pipe and socket box needs intumescent acoustic mastic at the penetration
• Continuous header tracks at the top of the wall transferring sound through the soffit — break the track at the wall line and seal the gap with acoustic mineral wool

Thermal build-ups: meeting Part L 2025 and the Future Homes Standard

Part L 2025, in force since June 2025, sets external wall U-value targets of 0.18 W/m²K for new dwellings. The Future Homes Standard (transitioning 2026-2028) brings this down to 0.15 W/m²K. Both standards require detailed thermal bridging (psi-value) calculations alongside the nominal U-value calculation.

Achieving Part L 2025 in SFS is straightforward with 150mm of stone wool insulation between 150mm studs (see build-up 2 above). The challenge is the thermal bridging through the steel studs themselves — steel is a much better thermal conductor than the surrounding insulation, and the studs create repeated thermal bridges through the wall.

Mitigations:

• Thermal break tape between the SFS frame and the sheathing board — typically a 3mm closed-cell foam tape that interrupts the thermal bridge
• Continuous external insulation — a layer of insulation outside the sheathing, with the cavity battens carrying through it. Adds 30-50mm to the wall thickness but significantly reduces thermal bridging
• Perforated stud profiles — Metsec and several other suppliers offer perforated C-studs (‘thermal studs’) where the web of the stud is punched with slots to lengthen the thermal path through the steel

For Future Homes Standard targets (U=0.15), expect to use 200mm insulation depth in 200mm studs, perforated stud profiles, thermal break tape, and continuous external insulation. The wall gets thicker (typically 320mm in the frame itself, plus outer leaf and cladding cavity). Floor area takes a hit — typically 1.5-2.5% of GIA depending on the building footprint.

Fire build-ups: meeting 60, 90 and 120-minute ratings

SFS wall systems achieve fire resistance ratings of 60, 90 or 120 minutes depending on the plasterboard build-up. Performance is to BS EN 13501-2 (REI = load-bearing capacity / integrity / insulation). The standard build-ups:

• REI 60: 1 × 15mm Type F plasterboard each face (basic spec) or 2 × 12.5mm Type F each face (more robust to construction tolerance issues)
• REI 90: 2 × 15mm Type F plasterboard each face, joints staggered between layers
• REI 120: 2 × 19mm Type F plasterboard each face, or specialist fire boards (e.g. calcium silicate-based) as per the manufacturer’s test certificate

These specifications assume single-stud partitions with mineral wool infill. Twin-stud separating walls (as in acoustic build-up 4) achieve higher fire ratings for the same plasterboard specification because of the additional insulation depth and the air gap interrupting heat transfer.

Critical: a tested fire-rated wall system is tested as a system — substituting one component (a different plasterboard, a different sheathing, a different fire-stop product) can invalidate the test certificate. For Higher-Risk Buildings under the Building Safety Act, document the exact products specified and supplied at every step. See our SFS fire resistance ratings guide for full detail.

Service zones and penetrations

Services — electrical cables, plumbing, ductwork, sprinklers — run within or alongside SFS walls. Three approaches:

• Within the frame depth: Cables and small-bore pipes pass through the perforations in the stud webs. Suitable for light electrical and data only; not for plumbing or HVAC.
• Service voids: A dedicated 25-40mm void on one or both sides of the SFS frame, between the studs and the plasterboard, carrying first-fix electrical and plumbing. The plasterboard is fixed via metal carrier sections or independent stud framing.
• External services: Larger plumbing or HVAC runs accommodated in dedicated service risers or external bulkheads, not within the wall depth.

Every service penetration through a fire-rated wall needs intumescent fire-stop sealing — and every penetration through an acoustic separating wall needs acoustic sealing. These are common failure points. Coordinate the M&E and fit-out trades carefully with the SFS install programme, and inspect every penetration before the second-side plasterboard goes up. Once the wall is closed up, fixing a missed penetration is significantly more expensive.

Downloadable specification pack

If you specify SFS regularly and want these build-ups as ready-to-paste specification text (with the technical citations and product references already populated), we maintain an SFS Wall Build-Up Spec Pack — a 32-page PDF covering all standard build-ups plus the regulatory citations, fire and acoustic test certificate references, and standard detail drawings. Request the spec pack and we’ll send it over by return.

For project-specific specification advice — fire engineering, acoustic strategy, thermal bridging calculation, BIM coordination — our engineering team is available for pre-construction consultation at no cost on tendered projects.

Frequently asked questions

The most common questions from architects, specifiers and engineers about SFS wall build-ups and their performance.

What is a standard SFS external wall build-up?

A standard SFS external wall build-up is: 100mm or 150mm SFS infill frame, cement-bonded sheathing board, breather membrane, 50mm cavity with vertical battens, then the outer leaf (brick, render, rainscreen, terracotta or metal). Internal face receives mineral wool insulation, vapour control layer and plasterboard.

What U-value can SFS walls achieve?

SFS external walls typically achieve U-values of 0.18 to 0.13 W/m²K with standard insulation thicknesses. Hitting Part L 2025 targets (U=0.18) is straightforward; hitting Future Homes Standard targets (U=0.15 or lower) requires deeper insulation or higher-performance products. Detailed thermal bridging calculation is mandatory at psi-value level.

What fire rating do SFS walls offer?

SFS wall systems can achieve 60, 90 or 120 minutes of fire resistance depending on the build-up. A typical 60-min spec uses 15mm fire-rated plasterboard each face; 90-min uses 2 x 15mm; 120-min uses 2 x 19mm or specialist boards. The frame itself is non-combustible; performance comes from the boarding.

What’s the acoustic performance of SFS partition walls?

Standard SFS partition walls achieve Rw 45-53 dB depending on construction. Single-stud 70mm partition with 15mm plasterboard each face achieves ~45 dB. Double-stud or staggered-stud systems with insulation between studs reach 53-58 dB, meeting Part E for apartment separating walls.

Do SFS walls need vapour control layers?

Yes. SFS external walls require a vapour control layer (VCL) on the warm side of the insulation to prevent interstitial condensation. The breather membrane on the cold side allows moisture to escape. Skipping the VCL is a common installation failure that leads to condensation, mould and steel corrosion within 3-5 years.