Curtain walling sits at the heart of most commercial facade packages. Getting the specification right at RIBA Stage 2 and 3 saves significant programme time and cost further down the line. This guide walks through the key decisions — system type, thermal performance, fire rating, and how to structure your NBS specification — drawing on our experience delivering curtain wall packages across London and the UK.
Types of Commercial Curtain Walling
- Stick system — aluminium mullions and transoms assembled on site; suitable for low to mid-rise; lower unit cost but higher site labour
- Unitised system — factory-assembled panel units craned in floor by floor; faster programme, better quality control; preferred for high-rise and time-critical projects
- Structurally glazed (SG) — silicone-bonded glass gives a flush, frameless external appearance; requires BS 6262 compliant structural silicone and ETAG 002 assessment
- Pressure-equalised / drained — active drainage channels manage water ingress at joints; essential for exposed elevations and high wind-driven rain zones
Thermal Performance Requirements
UK Building Regulations Part L (2021) set a maximum area-weighted U-value of 1.6 W/m²K for curtain walling on new commercial buildings. In practice, most well-specified systems achieve 1.0–1.4 W/m²K using thermally broken aluminium profiles and argon-filled double glazing. BREEAM credits and net-zero targets are increasingly driving specs below 1.0 W/m²K, where triple glazing and warm-edge spacer bars become relevant.
Key performance metrics to specify: overall U-value (Ucw), centre-of-glass g-value (solar heat gain coefficient), visible light transmission (VLT), and condensation resistance (CRF). Request test evidence to EN 13830 and EN 12152/12153 for air and water permeability.
Fire Compartmentation Requirements
Post-Grenfell, fire performance of external wall systems is heavily scrutinised. For buildings over 11m, Regulation 7 and the Building Safety Act 2022 require third-party certified products. Curtain walling fire ratings are classified as EI (integrity and insulation) or EW (integrity only). Typical requirements are EI30 between floors for residential above commercial and EI60 for fire-fighting shafts and stairwells.
Specify fire-rated curtain walling systems with test evidence to BS EN 13830 and BS EN 1364-3/4. Ensure junction details between the curtain wall and floor slabs are designed with appropriate cavity barriers and intumescent seals — these are frequently the weakest link and must be coordinated with the structural engineer.
Writing the NBS Specification
- Use NBS section H11 (Curtain Walling) as your base; cross-reference H21 (Aluminium Curtain Walling) for system-specific clauses
- Define performance by output specification (U-value, air, water, wind load) not proprietary product — allows competitive tender
- Require submission of test evidence, system approval documentation, and fabricator details at tender stage
- Include a mock-up clause — a physical sample panel reduces RFI volume and programme risk on larger packages
- Specify warranty periods: system manufacturer minimum 10 years, installer minimum 2 years workmanship
BDF Architectural provides early-stage technical support including U-value calculations, system selection and NBS clause assistance. Contact our commercial team to discuss your project.
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