How the two differ in kind
A curtain wall has two distinct zones and they are usually discussed as one. The VISION area is the glazing people look through, and it is what a quoted U-factor describes. The SPANDREL is the opaque band at each floor level, concealing the floor edge, the perimeter services zone and often a fire-stopping detail — and it is where the thermal problems occur.
The vision glass's performance is a well-understood product property: the glazing make-up, the coating, the gas fill, the spacer and the frame together give a U-factor, and improving it is a matter of specification. It is the number that gets quoted for the facade.
The SPANDREL is a different construction. Behind an outer glass or panel face there is a shallow cavity, then insulation, then a back-pan closing it off from the building's interior, all of it sitting directly against a concrete floor edge and a perimeter framing zone. Three things make it the worst-performing part of the wall: the framing and the slab bridge the insulation, the cavity depth available for insulation is limited by the facade's own thickness, and the outer face is a cold surface with a shallow air space behind it.
So the back of that outer pane is cold. Any moist air that reaches the cavity condenses on it, and the condensate runs down into the spandrel's base — where it stains the panel, corrodes framing and fixings, wets the insulation and, over time, appears as a visible mark on the facade.
And the air reaching it is usually the mechanism rather than the insulation being inadequate. A spandrel that is not AIR SEALED from the interior — an imperfect back-pan, an unsealed junction at the slab edge, a services penetration — is being ventilated with warm humid room air, which condenses in quantity. Sealing it is the fix that matters; adding insulation to a leaking spandrel does not address the cause.
The factors that actually differ
Show
| Spandrel condensation risk | Vision glass U-factor | |
|---|---|---|
| What it describes | The opaque zone at the floor edge — a shallow cavity, insulation, and a back-pan against the slab. | The glazing people look through, and the number the facade is quoted by. |
| Thermal performance | Usually the WORST part of the wall, bridged by framing and slab with limited depth for insulation. | The best-understood part, and a product specification. |
| Where condensation occurs | On the back of the outer pane, running down into the spandrel's base. | On the room-side surface if the U-factor is poor, which is visible and gets fixed. |
| The usual mechanism | Warm humid indoor air leaking into the cavity, not inadequate insulation. | A cold inner surface below the room's dew point. |
| Visibility | Invisible. The damage happens inside a sealed cavity and appears years later as staining. | Immediate — the occupant sees water on the glass and reports it. |
| The fix | Air seal the back-pan and every junction and penetration; then consider insulation and venting. | A better glazing make-up, a warm-edge spacer, or a better-insulated frame. |
| What makes it worse | A poorly sealed slab-edge detail, service penetrations, and a humidified interior. | A thermally poor frame, which is a large proportion of a small unit's area. |
| Consequence of neglect | Corroded framing and fixings, wet insulation, staining, and eventual panel failure. | Discomfort and, in a humid interior, mould at the frame. |
| Who checks it | A condensation risk assessment of the spandrel build-up at design stage — frequently not done. | The glazing specification, which is always done. |
| Which drives the complaint | The long-term damage, discovered during maintenance or when a panel is opened. | The immediate one, from whoever sits beside the window. |
Which one, and when
Choose spandrel condensation risk when…
- Designing or reviewing the spandrel build-up, which is the part usually not assessed.
- Where staining, corrosion or a persistent mark has appeared at a floor line on a facade.
- Where the interior is humidified — a pool, a laboratory, a residential building in a cold climate.
- At the slab-edge detail, where the air seal and the fire-stopping have to coexist.
Choose vision glass u-factor when…
- Specifying or comparing glazing, which is what the U-factor describes.
- Where occupants report cold surfaces, draughts or condensation on the glass itself.
- Assessing the facade against an energy target, where the vision area dominates the calculation.
- Choosing the frame and spacer, which are a large share of a small unit's performance.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does condensation happen at the spandrel and not the glass?
- Because the spandrel's outer face is colder and the moist air reaching it is less noticed. Vision glazing is a designed thermal product with a coating, a gas fill and a warm-edge spacer, and its room-side surface is kept above the dew point by that specification — so condensation on it is unusual and, when it happens, somebody sees it and reports it. A spandrel is a shallow cavity behind an outer pane, bridged by framing and by the concrete slab edge, with limited depth for insulation. The back of that outer pane is therefore cold, and any warm moist air in the cavity condenses on it. Nobody sees it, because the cavity is sealed and opaque, so the water accumulates, runs down, and does its damage quietly for years.
- Where does the moist air come from?
- From inside the building, through the back-pan, and it is the mechanism rather than a side issue. The spandrel cavity is supposed to be separated from the interior by an air-sealed back-pan, so that room air cannot reach the cold outer face. In practice that seal is defeated by the things that pass through it: service penetrations, the junction at the slab edge where the fire-stopping also has to be made, fixings, and the joints of the back-pan itself. Warm humid room air then enters the cavity continuously — driven by stack effect and by the building's pressurisation — and condenses on the cold pane. That is why a spandrel with excellent insulation and a poor air seal performs badly, and why sealing is the first intervention rather than the last.
- Why is the spandrel the worst-performing part of the wall?
- Three reasons at once. The framing that carries the facade passes through the insulation zone and bridges it, as do the brackets and the slab edge itself — so the assembly is full of thermal paths that the insulation does not interrupt. The depth available is limited by the facade's own thickness, which is set by the mullion depth rather than by what the insulation would like, so there is less of it than in a comparable opaque wall. And the assembly is thin and complex, with a shallow cavity and several layers in a small depth, so a detail that would be straightforward in a wall becomes constrained. The result is a band at every floor level whose performance is well below the vision glazing that gets quoted for the facade.
- Should the spandrel cavity be vented?
- It depends on the system, and it is a design decision with arguments both ways rather than a default. Venting the cavity to the outside lets any moisture that does get in dry outward and equalises the pressure across the outer pane, which reduces the condensation risk from small leaks — and it is what many systems do. Against that, a vented cavity admits outdoor air and, in a humid climate, that air can itself condense on a cavity face cooled by the air conditioning, so venting is not universally beneficial. Sealed spandrels rely instead on the cavity being genuinely sealed on both sides, which returns the question to the quality of the air seal. Whichever the system uses, the arrangement is a designed one and mixing the two — a nominally sealed cavity with accidental openings — is the worst case.
- What does the damage actually look like?
- Staining first, then corrosion, and the timescale is years. Condensate running down the inside of the outer pane collects at the base of the spandrel, where it wets the insulation, sits against aluminium framing and steel fixings, and leaches whatever it dissolves. The visible symptom from outside is a mark at the bottom of the spandrel panel — a tide line, a discoloration, or on a glass spandrel a visible pool of dirt in the condensate path. Inside the cavity, which nobody sees until a panel is removed, the insulation is wet and compressed, the fixings are corroding, and the back-pan may be staining through to the interior finish. By the time it is visible from outside, it has been happening for a long time.
- How is the risk assessed at design stage?
- By calculating the temperature of the surfaces within the assembly under design conditions and comparing them with the dew point of the air that could reach them — which requires modelling the build-up including its thermal bridges rather than taking a nominal insulation value. Two-dimensional thermal modelling of the spandrel detail, including the framing, the bracket and the slab edge, gives the surface temperatures the simple calculation misses, and it is the tool the assessment needs. The other half is the air: the assessment has to state what air is assumed to reach the cavity, because the answer for a sealed spandrel and a leaking one is completely different. An assessment that assumes a perfect seal and a building that does not achieve one is a calculation about a different wall.
- Does a humidified interior change things?
- Substantially, and it is the case where spandrel condensation moves from a risk to a certainty if the detail is not right. Raising the indoor humidity raises the dew point of the air inside, so surfaces that were comfortably above it are no longer — and the spandrel's cold faces are the first to fall below. Buildings with humidification, swimming pools, laboratories, some manufacturing, and residential buildings in cold climates with high occupancy all push in this direction. Where the interior is humidified, the air seal becomes critical rather than merely good practice, the thermal modelling is done at the actual design humidity rather than a default, and the spandrel build-up is frequently upgraded beyond what the energy target alone would require.
- Can an existing spandrel be improved?
- Yes, and the interventions are ranked by how well they address the cause. Air sealing the back-pan and its penetrations from the interior is the first and usually the most effective, because it removes the moisture supply — and it can often be done from inside during a refurbishment without touching the facade. Adding insulation helps the surface temperatures and does nothing about a leak, so it is second. Replacing the spandrel unit with a better-performing one is a facade intervention with a facade cost. And reducing the indoor humidity, where that is controllable, addresses the dew point directly. The diagnosis matters because the four have very different costs: sealing a leaking back-pan is cheap, and replacing spandrel panels because the insulation was assumed to be the problem is not.