Curtain Walling Maintenance, Refurbishment and Replacement: What Fails and What to Do

Most problems with existing curtain walling start in the parts that wear, not in the aluminium frame: seals, gaskets and silicone, the insulated glass units, blocked drainage, tired finishes and vent hardware. These can usually be repaired or replaced in the existing frame, and full replacement is normally worth it only when the frame itself performs badly or the building is being upgraded. BDF Architectural is a curtain walling contractor and wrote this guide, so it is a contractor's view. Regulatory points apply to England, with sources at the end.

How curtain walling ages and what fails first

The mullions and transoms are rarely the first thing to go. The parts that wear are the ones that are exposed, soft or moving:

• Seals, gaskets and silicone: the outer seal takes the weather. CWCT notes that a single outer seal is exposed to weathering and prone to failure through accelerated ageing or poor workmanship. Gaskets shrink and harden and open up at corners, and silicone joints at interfaces split or lose adhesion
• Insulated glass units: when the edge seal of a double or triple glazed unit fails, moisture gets into the cavity and the unit mists between the panes. It cannot be cleaned out, so the unit is replaced
• Drainage paths: in a drained system, water that gets past the outer seal should be caught and drained back to the outside. Dirt, debris or stray sealant in the drainage slots stops that, and the water finds its way in instead
• Finishes: powder coated and anodised faces fade, chalk or mark over time, and damage to the coating exposes the metal
• Vent hardware: hinges, stays, locks and vent gaskets wear with use. A vent that drops or no longer closes tight lets in air and water

Design matters too. CWCT describes drained envelopes as far more robust than face-sealed ones, where failure of the single outer seal always lets water in.

Signs of trouble with existing curtain walling

• Water staining or drips at transoms, sills or the slab after rain
• Misting between the panes of a glass unit
• Draughts at joints and vents, or condensation on inner frames
• Gaskets shrunk back from corners, cracked glass, or vents that stick

Curtain walling maintenance: a planned regime

Curtain walling needs planned maintenance, not just a response to leaks. Start from the building's operation and maintenance information, then build a regular check into the façade cleaning cycle, when access is already in place:

• Clean the frames as well as the glass, and check that drainage slots are clear
• Inspect external gaskets, corner joints and perimeter silicone for shrinkage, splits and loss of adhesion
• Open and close every vent, and check the hardware, fixings and vent gaskets
• Log misted units and cracked glass by elevation, floor and bay
• Photograph what you find, so a pattern of local faults or a wider problem shows up over time

Diagnosing a curtain wall leak

Find the entry point before resealing anything. Water seen inside may have travelled along a transom or down a mullion from somewhere else, so sealing where it appears often fails. A controlled water test traces it.

CWCT says workmanship is checked using a hose test, while design is tested with a cabinet test or, for patent glazing and rainscreens, a spray bar test. All of these are described in CWCT Technical Note TN41, and CWCT has published a further technical update on site hose testing. Site testing is no substitute for laboratory testing of a design, but it is the practical way to trace a leak. Ask for the method to be named in the report, test one area at a time, and repeat the test after the repair.

Curtain wall refurbishment options: repair, re-glaze, re-gasket or replace

Local repair

Clearing drainage, re-sealing corners and interfaces, and adjusting or replacing vent hardware. It costs least and disturbs least, but it does not change the thermal performance of the wall.

Replacing double glazed units and gaskets in curtain walling

Glass units and gaskets can usually be replaced in the existing frame. On a capped system the glass is clamped by a pressure plate and cover cap, which come off to release the unit. On a structurally glazed system the glass is bonded to the frame with structural silicone, so a new unit has to be bonded in again. Replacement units must suit the frame's glazing depth, and new gaskets must fit the existing profiles, so identify the system first and check that parts are still available. New units can use better glass than the originals, but the frame still limits how much the whole wall improves.

Partial replacement

Replacing one elevation, a stair core or the worst bays deals with the problem where it is. The joints between new and old need careful detailing, and matching sightlines and finish is harder with a different system.

Over-cladding or full replacement

Over-cladding adds a new skin in front of the existing wall. It avoids stripping out the old wall, but it adds load to the structure and fixings and changes the line of the façade. Full replacement costs most and disrupts most, but it is the way to fix failing frames or poor thermal performance, for example with thermally broken curtain walling. Our curtain walling specification guide covers system choice.

Building Regulations when replacing curtain walling

In England, Approved Document L Volume 2 covers buildings other than dwellings. Its Table 4.1 sets a limiting U-value of 1.6 W/m²K for new or replacement curtain walling in existing buildings. Where whole window or door units are replaced (controlled fittings), paragraph 10.3 says they should be draught-proofed, meet Table 4.1 and have insulated cavity closers where appropriate. A 2026 edition takes effect on 24 March 2027, or 24 September 2027 for higher-risk building work, and keeps the 1.6 W/m²K figure, so check which edition applies to your project. Where the curtain walling serves dwellings, Approved Document L Volume 1 applies instead, and it sets 1.4 W/m²K or Window Energy Rating band B for windows, including curtain walling.

The new work must itself meet the applicable requirements, including B4 on external fire spread (regulation 4(1)). Under regulation 3, an alteration is material if it would leave the building not complying with requirements such as B3 (internal fire spread) or B4 (external fire spread) where it did before, or more unsatisfactory where it already fell short. Regulation 4(3) requires the building to comply after the work, or be no more unsatisfactory than before. Slab-edge fire stopping and cavity barriers disturbed by the work need to be reinstated.

Regulation 7(2) applies to relevant buildings: those with a storey at least 18 metres above ground that contain dwellings, an institution or a room for residential purposes. Materials that become part of the external wall must be class A2-s1, d0 or A1. Regulation 7(3) exempts some items, including window frames and glass, and seals, gaskets, fixings, sealants and backer rods, but Approved Document B Volume 2 says spandrel and infill panels must comply. In other buildings with a storey 18 metres or more above ground, Approved Document B Volume 2 (paragraph 12.6) says insulation and filler materials in the external wall, including spandrel panel cores, should be class A2-s3, d2 or better. In buildings with a residential purpose (purpose groups 1 and 2, such as flats, hotels and hospitals) with a storey 11 metres or more above ground, paragraph 12.7 raises this to class A2-s1, d0 or better. Where the façade must resist fire, see fire-rated curtain walling.

Refurbishing curtain walling on an occupied building

Most office refurbishment happens with the building in use. We usually phase the work by elevation and floor, so each area is handed back before the next starts. Agree these points before the programme is fixed:

• Phasing: which elevations, floors or zones go in which order, and how tenants move around the work
• Weathering: how each opening is made weathertight at the end of every shift
• Access: scaffold, mobile platforms, mast climbers or the building's cleaning access
• Working hours, noise, security and deliveries on an occupied site
• Programme: order dates for glass and coatings, and when each phase starts and finishes. See our guide to choosing a curtain walling contractor