Curtain Walling in the UK: A Complete Guide for 2026

Curtain walling is the structural-glazing envelope that hangs off the slab edge of almost every modern UK commercial, residential and mixed-use building. It carries no vertical load from the structure above — it only carries its own self-weight plus the wind, thermal and impact loads acting on the facade. Done well it delivers daylight, acoustic separation, weathertightness and thermal performance for the next 40+ years. Done badly it leaks, fails CWCT testing, or holds up the entire programme.

This guide covers what curtain walling actually is, the three system types you'll see specified in the UK, the standards and regulations that apply post-Building Safety Act, what realistic U-values look like under Part L 2025, and how to procure it without burning programme.

What Is Curtain Walling?

A curtain wall is a non-loadbearing facade system fixed back to the primary structure — typically at every floor slab — using engineered brackets. The grid is formed by aluminium mullions (vertical) and transoms (horizontal), with infill panels of insulated glass units (IGUs), insulated spandrel panels, or opaque metal cassettes. Everything in front of the slab is the curtain wall package: glazing, gaskets, pressure plates, capping, brackets, fire stops and cavity barriers.

Because the system is non-loadbearing, the structure can be lighter and floor plates can be pushed closer to the perimeter — one of the reasons curtain walling has dominated commercial construction since the 1960s.

The Three System Types

Stick Curtain Walling

Stick systems are built up on site: mullions are fixed first, transoms span between them, then IGUs and spandrels are loaded into the grid and pressure-plated. Stick is flexible — site dimensions can be taken late, profiles can be cut to suit awkward geometry, and the per-m² material content is the leanest of the three. The trade-off is labour time on site, weather exposure during install, and a longer critical path because nothing can be installed until the structure at that level is ready.

Stick is the default choice on low-to-mid-rise schemes, on projects with bespoke geometry, and on refurbishments where existing tolerances need to be absorbed in real time.

Unitised Curtain Walling

Unitised systems arrive on site as factory-assembled, pre-glazed storey-height panels (typically 1.5–4m wide × one floor tall). Each unit is craned into position and locked into the unit below via a male/female stack joint. Because all the glazing, gasket compression and weathertightness work is done in factory conditions, quality control is significantly higher and on-site installation is roughly 3–5× faster per m² than stick.

Unitised is the default on high-rise, on schemes with tight cycle times, and anywhere on-site labour and weather risk dominate programme. The material premium over stick is usually recovered on programme and prelims.

Structural Silicone Glazing (SSG)

SSG removes the external pressure plate and cap: the IGU is bonded to the carrier frame with a structural silicone joint, giving a flush, frameless external appearance. SSG can be built in stick or unitised format. It's specified where architects want the cleanest possible glass-to-glass reading, and it's almost always factory-bonded under controlled humidity to satisfy ETAG 002 / EAD 090010-00-0404.

Stick vs Unitised vs SSG at a Glance

U-Values and Part L 2025

Part L 2025 (England) tightened notional U-values for non-domestic curtain walling to 1.6 W/m²K (whole-element, including frame). For dwellings the target is typically 1.4 W/m²K. To hit this with a standard double-glazed unit you need warm-edge spacers, argon fill, soft-coat low-E glass and a thermally-broken mullion with at least 24mm of polyamide isolator. Triple glazing is often specified on residential to give designers margin against the SAP calculation.

A realistic centre-pane U-value for a high-performance double-glazed IGU is around 1.0–1.1 W/m²K. Once you account for the spacer (Psi-value) and the frame, the whole-element U-value lands between 1.3 and 1.7 depending on grid density. Architects who push for slimmer sightlines pay for it in U-value — more frame per m² means worse thermal performance.

CWCT Testing — What's Actually Required

The Centre for Window and Cladding Technology (CWCT) publishes the Standard for Systemised Building Envelopes, which is the de facto UK test regime for curtain walling. There are two layers: system test data (provided by the system house — Schüco, Reynaers, Kawneer, Sapa) and project-specific testing on a Performance Mock-Up (PMU) for larger schemes.

A typical PMU sequence includes air permeability (EN 12153), watertightness static (EN 12155), wind resistance (EN 12179), watertightness dynamic (CWCT TN75 / AAMA 501), and impact (CWCT TN76). Most main contractors require a PMU for schemes above 5,000 m² of facade or anywhere the system is being modified from its tested standard.

Fire Safety: Building Safety Act and AD-B

Since the Building Safety Act 2022 and the post-Grenfell revisions to Approved Document B, curtain walling on relevant buildings (residential above 11m, certain other residential and institutional uses above 18m) must use materials achieving European Class A2-s1,d0 or better. That affects spandrel panel construction, insulation in the back-pan, cavity barriers around openings, and horizontal/vertical fire stops at every compartment line.

For relevant buildings the responsibility under the Building Safety Act sits with the Principal Designer and Principal Contractor through the three gateways. Curtain wall packages must be documented in the golden thread — including all material certificates, fire test evidence (BS 8414 system tests where applicable), and installation records.

Realistic Programme

On a unitised package, expect 16–22 weeks from contract for design, sign-off, sample approval and first deliveries, then on-site install at 80–150 m² per crane day per team. Stick installs around 25–45 m² per team per day depending on access. The critical path almost always runs through PMU sign-off and the first delivery of factory-assembled units — protecting both is where main contractors win or lose programme.

How to Procure It Well

• —Engage the specialist curtain wall contractor at RIBA Stage 3 (developed design), not Stage 4 — late engagement burns weeks.
• —Use a design-assist contract for the typical bay before tendering the package — the value of getting the brackets, fire stops and U-values right at concept stage is enormous.
• —Specify the system house openly (Schüco / Reynaers / Kawneer) rather than a closed proprietary system — keeps the tender competitive.
• —Insist on a sealed sample / visual mock-up signed off by the architect before factory production begins.
• —Build CWCT PMU into the programme as a critical-path milestone with a named test house — don't leave it floating.

Frequently Asked Questions

Get Curtain Walling Right on Your Next Project

Delta Facades is a specialist curtain wall installer working across London and the UK on commercial, residential, education and mixed-use schemes. We install stick, unitised and SSG systems from Schüco, Reynaers, Kawneer and Sapa, with full design-assist support from RIBA Stage 3 onwards.