CWCT Testing Explained: What Sequence B Actually Proves About Your Facade
If you're specifying a curtain wall, rainscreen or window wall on a UK commercial or residential building, the spec almost certainly says 'to CWCT Standard for Systemised Building Envelopes' and requires a sequence B test on a mock-up. This guide explains what CWCT is, what sequence B actually tests, and what happens when a test fails.
What CWCT Is
The Centre for Window and Cladding Technology (CWCT) is the UK industry body that publishes the technical standards most facade specifications reference. Its 'Standard for Systemised Building Envelopes' sets out performance requirements and test sequences for curtain walling, rainscreens and window walls.
The standard defines several test sequences (A, B, C etc.). Sequence B is the standard performance sequence used on most UK projects — it tests the facade at levels typical of UK weather and structural loading.
What Sequence B Actually Tests
Sequence B is run on a mock-up (typically 2 or 3 bays wide, one or two storeys tall) built in a test chamber that can pressurise, spray water, apply wind loads and swing impactors. The tests, in order:
| Test | What it proves |
|---|---|
| Air permeability | The facade doesn't leak air above the specified rate at design pressure — critical for Part L compliance. |
| Static water tightness | No water penetrates the inside face when spray is applied at steady design pressure. |
| Wind resistance (serviceability) | Deflection under design wind load is within limits — no permanent damage. |
| Dynamic water tightness | No water penetrates when spray is applied with a pulsing pressure that simulates gusting wind. |
| Wind resistance (safety) | No structural failure at 1.5× design wind load. |
| Impact resistance | Soft-body and hard-body impacts don't cause dangerous failure. |
Every test is recorded, and the mock-up is inspected between tests. A failure at any stage means the sequence is paused, the cause investigated, the detail changed, and the affected tests re-run.
Where the Mock-up Comes From
The mock-up is built by the specified fabricator using the same components, gaskets, sealants and fixings that will be used on site. It's built either at the test laboratory or built off-site and shipped in. Building a mock-up realistic enough to be representative — but constructible in a test chamber — is a discipline in its own right.
On projects where the design is bespoke (unusual bay proportions, non-standard corner conditions, integrated shading), a project-specific mock-up test is normal. On projects using a proven fabricator/system combination, a recent generic test result may be accepted in lieu — this needs sign-off from the specifier.
What a Fail Means for Programme
A failed dynamic water test that traces to a gasket detail can usually be rectified in the test chamber inside a day. A failed static wind test that traces to a bracket sizing issue means the fabricator goes back to shop drawings, remakes the affected components and re-runs the sequence — a two-to-six-week hit on programme.
Because of this, sensible programmes book the mock-up test at least two months before the first panel is due to be installed on site, with a rebuild and re-test window built into the programme.
Frequently Asked Questions
Is CWCT testing a legal requirement?
No — it's a contractual requirement set by the specifier. But almost every meaningful UK commercial specification requires it, and refusing to test is not a viable path for a system on any credible project.
Do individual panels get tested on site?
Sequence B is a mock-up test at a laboratory. On-site testing is separate and usually consists of hose testing (BS 8000-16 or CWCT TN 41) on completed elevations to prove installed water tightness — a simpler test, but on the real building.
Can an installer be held responsible for a mock-up test failure?
Not for a system or design failure — those fall to the fabricator or designer. But if the installer builds the on-site work differently from the tested mock-up (wrong sealant, missed gasket step, incorrect fixing torque), the on-site hose test will find it and the responsibility sits with the installer.