How the two differ in kind
Both hold glass in a curtain wall, and they differ in what is actually carrying the load.
CAPTURED glazing retains the glass mechanically. The pane sits against gaskets in the mullion, a pressure plate is bolted through to clamp it, and a cap covers the plate. The glass is held by metal, and the gaskets and sealants keep water and air out. If a seal fails, the glass is still held; the failure is a leak rather than a loss.
STRUCTURAL SILICONE GLAZING dispenses with the cap on one or more edges and bonds the glass to the frame with a bead of structural silicone. That bead is not a seal that also happens to hold — it is the STRUCTURE. It transfers the wind load on the pane into the framing, in tension when the wind pulls and in shear under the glass's own weight if the design does not include a setting block arrangement to take it, and it does so for the life of the building.
That is why the bead's width — the BITE — is a calculated dimension. It follows from the design wind pressure on the pane, the pane's short dimension, and the sealant's allowable design stress, which manufacturers publish at a conservative value with a substantial factor against the tested strength. The thickness of the joint matters too, since the sealant has to accommodate differential movement between glass and metal without exceeding its movement capability.
And it is why SSG is a FACTORY operation on most projects. A structural bond depends on the substrates being clean, correctly prepared and primed where the sealant requires it, on the temperature and humidity during application, and on a full cure before the unit is loaded. None of that is reliably achievable on a facade in the weather, which is why units are glazed in a controlled shop and delivered as assemblies.
The last requirement is the one that catches projects out: adhesion depends on the exact combination of sealant, substrate and coating, so compatibility and adhesion testing on the ACTUAL materials is part of the process.
The factors that actually differ
Show
| Structural silicone glazing | Captured (mechanically retained) | |
|---|---|---|
| What holds the glass | The silicone bead. It is the structure. | A mechanical pressure plate and cap. The sealant weatherproofs. |
| What sets the dimension | Bite width, calculated from wind pressure, pane short dimension and the sealant's design stress. | The system's gasket and plate arrangement, from the manufacturer's details. |
| Where it is done | In a factory, under controlled temperature, humidity and cleanliness, with a full cure before loading. | On site, which is one of its main practical advantages. |
| If the seal fails | A structural failure. The glass is no longer attached. | A leak. The glass is still mechanically held. |
| Appearance | Flush glass with no external cap — the reason it is specified on most projects. | A visible cap line on the glazed face, which is part of the architecture or is not. |
| Testing required | Compatibility and adhesion testing on the actual glass, coating, metal, finish and any accessory in contact — not a formality. | Standard system testing; far less sensitive to material combinations. |
| Glass replacement | Specialist. The unit is usually removed and re-glazed in a shop, or a tested field re-glazing procedure is used. | Straightforward — unbolt the plate, replace the pane, re-fit. |
| Movement | The joint accommodates differential movement between glass and metal, within the sealant's movement capability. | Gaskets and the glazing pocket accommodate it. |
| Self-weight | Carried by setting blocks in most designs rather than by the bead in shear — a design decision, not an assumption. | Carried by setting blocks. |
| Sensitivity to workmanship | Very high, and the workmanship is invisible afterwards — which is why it is a controlled factory process. | Moderate, and inspectable. |
Which one, and when
Choose structural silicone glazing when…
- A flush glazed appearance with no external cap is required, which is the usual driver.
- The units can be shop-glazed and delivered as assemblies, which the programme and logistics must allow.
- There is a specialist facade contractor and a testing regime, which the system requires rather than prefers.
- The design has been engineered for it — bite, joint thickness, movement and self-weight support all resolved.
Choose captured (mechanically retained) when…
- On-site glazing is required by the programme, the logistics or the building's access.
- Glass may need replacing over the building's life without specialist re-glazing.
- A visible cap is acceptable or wanted architecturally.
- The project has no facade specialist and no testing regime, in which case a mechanical system is the responsible choice.
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
- What is bite and how is it determined?
- The width of the structural silicone bead measured across the joint — the dimension over which the sealant is bonded to both the glass and the metal — and it is what carries the wind load into the frame. It is calculated rather than chosen: the wind pressure on the pane, acting over the pane's area, has to be transferred through the perimeter bead, and the governing geometry is the pane's SHORT dimension, since that sets how much load each unit length of bead carries. Dividing the resulting force per unit length by the sealant's allowable design stress — a conservative published value with a substantial factor against the tested strength — gives the required bite. The joint's THICKNESS is a separate dimension governed by movement rather than by load, since the sealant has to accommodate differential expansion between glass and aluminium.
- Why does it have to be done in a factory?
- Because a structural bond is created by conditions that cannot be controlled on a facade. The substrates have to be genuinely clean — a two-cloth solvent wipe procedure rather than a wipe over — primed where the sealant system requires it, and the sealant applied within a temperature and humidity range and tooled to fill the joint without voids. Then it has to cure fully before the unit is loaded, which takes time. On a building elevation in wind, dust, rain and whatever temperature the day provides, none of that is reliable, and the result is invisible: a joint with poor adhesion looks exactly like a good one. Shop glazing puts the operation in a controlled environment with documented procedures and quality control, and delivers cured assemblies to site — which is why SSG and unitised curtain wall go together.
- What is adhesion testing and why is it not a formality?
- Testing the actual sealant against the actual substrates from the actual project, because silicone adhesion is specific to the combination rather than a general property. Glass coatings, the metal's anodising or powder coating, the spacer and edge seal of the insulating unit, gaskets, setting blocks and even the tape used in fabrication can all affect whether the sealant bonds and whether the materials are chemically compatible over time. Sealant manufacturers run compatibility and adhesion testing on submitted samples as standard practice before a project proceeds, and they issue a report specifying the preparation and any primer required. The reason it is not a formality: a change of glass supplier, a change of coating, or a substitution of a gasket after testing invalidates it, and that kind of substitution happens on projects routinely.
- What happens if the bond fails?
- The glass is no longer held, which is why this is treated as a structural matter rather than a sealing one. In a captured system a failed seal produces a leak and the pane stays where it is, mechanically retained. In SSG the bead is the attachment, so a loss of adhesion over a length of joint transfers its load to the remainder and can progress. That is the reason for the conservative design stress, the factory process, the adhesion testing, and — in many jurisdictions and on many specifications — for secondary mechanical retention on units above a certain height, so that a failed bond cannot drop glass onto a public area. Some codes and standards require that retention explicitly. It is also why inspection and maintenance regimes for SSG facades include checking the joints rather than only the weather seals.
- Can a pane be replaced?
- Yes, and it is a specialist operation rather than a maintenance task, which is a genuine consideration over a building's life. The usual approach is to remove the affected unit and return it to a shop for re-glazing, or to replace the whole unitised panel — which on a tall building means access equipment and a sequence rather than a person with a ladder. Field re-glazing procedures do exist, using structural sealants formulated and tested for site application with a defined procedure and quality control, and they are used where removal is impractical; they are a controlled process with their own requirements rather than a repair. By contrast, a captured system's pane is replaced by unbolting the pressure plate, swapping the glass and re-fitting, which is why buildings expecting glass breakage sometimes prefer it.
- What carries the glass's own weight?
- Setting blocks, in almost every design, rather than the structural bead in shear — and it is a design decision that should be explicit rather than assumed. The structural silicone is engineered to carry wind load in tension and compression across the joint; asking it to carry the pane's dead weight in shear continuously for decades is a different and more demanding duty, and standard practice is to support the glass on setting blocks at the sill so the weight goes into the frame directly. Four-sided SSG systems still use them. Where a design does rely on the sealant for dead load, that is a specific engineered case with its own analysis. The blocks' material and position matter too, since they are in contact with the glass and the sealant and form part of the compatibility question.
- Is two-sided SSG different from four-sided?
- Yes, and two-sided is the common compromise. In a two-sided system the glass is structurally bonded on two opposite edges — usually the verticals — and mechanically captured on the other two, which gives a flush appearance in one direction with a visible cap in the other, and leaves mechanical retention on half the perimeter. Four-sided SSG bonds all four edges and produces the fully flush, capless facade, with correspondingly greater reliance on the sealant. The engineering is the same in kind and the consequences of failure differ, which is why four-sided systems attract more attention from codes and specifications regarding secondary retention. Which is used is usually an architectural decision about the facade's appearance, taken with the facade engineer rather than in isolation.
- How long does the sealant last?
- Structural silicones are chosen for durability and their record in this application is long, which is why silicone rather than another chemistry is used — it resists ultraviolet, ozone and temperature extremes far better than organic sealants, and a correctly bonded joint is expected to last the facade's design life. That expectation rests on the bond having been made correctly in the first place, on the materials having been compatible, and on the joint not being loaded beyond its design. Failures in service are overwhelmingly traced to preparation, incompatibility, or a substitution after testing rather than to the silicone degrading. What does have a shorter life is the weather sealant at the joints, which is a maintenance item on any facade and is replaced without disturbing the structural bond.