On many mid- and high-rise residential buildings, the glass facade is not a curtain wall at all — it is a window wall that sits between the floor slabs, a different system with its own strengths and its own well-known problems.
Many people call every glass facade a curtain wall, but on a great deal of residential high-rise construction the system is actually a window wall — framed glazing units that sit on top of one floor slab and stop beneath the next, filling the space between slabs. It looks similar from outside but behaves differently from a curtain wall, which hangs continuously in front of the slab edges. Window wall is economical and lets units be replaced floor by floor, but its stacked joint at each slab, its slab-edge detailing, and its thermal and acoustic behavior are distinct considerations — and in some markets, systemic window wall problems have driven major replacement programs.
The value is in understanding the system. The sections below cover the four aspects that matter most — window wall versus curtain wall, how the units sit on the slab, the slab-edge and fire detail, and replacement and performance — and how ACE approaches window wall systems.
These challenges rarely stay in one silo: the same discipline runs through the same playbook behind insulated metal panels, lessons from architectural metal cladding, and our guidance on fire-rated glazing, and it all ladders up to ACE’s broader remedial waterproofing and repair.
Window wall versus curtain wall is the distinction that defines window wall systems, because a curtain wall hangs continuously in front of the floor slabs as one unbroken skin, while a window wall is made of units that sit on each slab and stop beneath the next, so the slab interrupts the facade at every floor. Understanding that difference is the key to the whole system.
The two look alike from the street but behave differently: the curtain wall is continuous past the structure, whereas the window wall is a stack of floor-height units bearing on the slabs, which changes the joints, the thermal behavior, and the water paths.
Because window wall is often chosen for its economy and simpler floor-by-floor installation, it is very common on residential towers, which is exactly where its distinct behavior most needs to be understood.
Representative of the window wall factors ACE assesses. Illustrative distribution, not a published statistic.
How the units sit on the slab is central to window wall systems, because each unit bears on the top of a slab and terminates below the next slab, creating a horizontal joint at every floor where one unit meets the next. That stacked joint at each slab is the system’s most important and most vulnerable detail.
The head-to-sill joint between stacked units has to manage water, air, and movement at every floor line, and because it recurs at each slab around the building, its detailing has an outsized effect on the whole facade’s performance.
Because the slab interrupts the facade, the interior face of the slab edge is typically finished or covered, and the interface of the window wall units with that slab edge is where much of the water and thermal detailing lives.
A window wall is treated like a curtain wall, so its stacked joints and slab-edge details are misunderstood and leak, bridge heat, or age poorly.
Recognize window wall as its own system and design or assess the stacked joints, slab-edge, fire, and thermal details for the window wall condition.
The recurring joints and slab edges perform, the system’s advantages in replaceability are kept, and its known vulnerabilities are managed.
The slab-edge and fire detail differ from a curtain wall’s, because in a window wall the slab comes out to the facade at each floor, so the perimeter fire condition is different from a curtain wall’s continuous slab-edge gap, and the slab edge itself is a thermal and water detail. How the system meets the slab edge shapes its fire, thermal, and water performance.
The slab-edge interface is a thermal bridge and a potential water path if not detailed well, and the way the window wall meets the slab affects both the interior finish and the building’s fire compartmentation at the perimeter.
Because the slab-to-facade relationship is fundamentally different from a curtain wall, the fire, thermal, and water details at the slab edge have to be designed for the window wall condition specifically rather than borrowed from curtain wall practice.
Get the stacked joints, slab-edge, and thermal details reviewed for the window wall system specifically.
Schedule a consultationCall (866) 389-8883Replacement and performance are where window wall systems show both an advantage and a caution, because units can often be replaced floor by floor without a continuous hung system, which simplifies future replacement, yet window wall’s thermal performance, condensation behavior, and the durability of its stacked joints have been real problems on some buildings. Weighing the replaceability against the performance history is part of understanding the system.
In some markets, systemic window wall issues — leakage, thermal performance, and aging seals at the stacked joints — have driven major facade replacement programs, so the system’s long-term behavior is a known concern worth assessing.
The most valuable point overall is that window wall is a distinct, common system with real advantages in cost and replaceability but specific vulnerabilities at its stacked joints, slab edges, and thermal details — and it should be designed, assessed, and maintained as the particular system it is.
In ACE’s field work, most window wall systems problems trace back to a few recurring locations — Stacked joint at each slab, Slab-edge thermal & water, and Perimeter fire condition — rather than the open field of the wall or roof. The cost of resolving them climbs by roughly an order of magnitude at each stage: a detail corrected during design costs a fraction of the same fix during construction, which is itself far cheaper than a repair once the facility is operating. That economics is why we push envelope decisions as early in the project as possible.
Determine whether the facade is a window wall sitting between slabs or a curtain wall hanging past them, since the details differ fundamentally.
Detail the recurring head-to-sill joint and the slab-edge thermal, water, and fire conditions for the window wall arrangement.
Assess thermal performance, condensation, and joint-seal aging, and use the floor-by-floor replaceability to plan maintenance or renewal.
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