Facade Glazing Calculator: Vision Glass, Spandrel and WWR

What it calculates

A curtain wall is built from one module repeated across the facade and up the building. Once the module width and the split between vision glass and spandrel are fixed, most of the quantities follow from simple geometry. For one rectangular elevation the calculator gives:

• the number of modules on each floor;
• the gross facade area, the vision glass area and the spandrel area;
• the window-to-wall ratio (WWR) of that elevation, with a warning if it is above the check value you set;
• the number of vision glass units and spandrel panels, including wastage and attic stock;
• the perimeter sealant run around the vision units.

It is a take-off tool for early planning and for checking quantities in a tender, not a facade design.

Inputs

Facade width is the length of the elevation in metres, measured along the glazing line.

Number of floors is how many storeys the curtain wall covers on that elevation.

Floor-to-floor height is the distance from one slab level to the next. The calculator treats every floor as the same height; enter a podium or a double-height lobby as a separate run.

Module width is the centre-to-centre spacing of the vertical mullions. Common modules are set by the architect's grid, the glass sizes that can be made and handled, and the internal partition layout.

Vision glass height per floor is the height of the see-through band on each floor. The rest of the floor-to-floor height is treated as spandrel.

In the assumptions panel are the wastage and attic stock percentage and the WWR check value. The check value defaults to 40% and is labelled as the ECBC 2017 prescriptive maximum; confirm the value that applies to your building with the energy consultant before relying on it.

Method

The vision width is capped at the facade width so that a narrow last module does not add glass that is not there. If the vision height entered is more than the floor-to-floor height, it is capped and the spandrel is zero. The sealant run is measured on the net number of units, without wastage, because spares are not installed.

Assumptions

Every one of these can be edited. The calculator holds no glass prices or rates.

Limitations

• One flat elevation at a time. Corners, curved facades, sloped glazing and setbacks need to be measured separately and added.
• No openings. Entrance doors, louvres, vents and openable windows replace some vision units and need their own counts.
• One vision band per floor. Some designs add a transom, a clerestory light or a fritted band; split the vision height accordingly or measure from the elevation drawing.
• Frame area is ignored. The vision area is the module grid, not the visible glass inside the frames. Energy calculations and daylight studies use the real glass and frame dimensions.
• No structural or glass specification. Glass make-up, thickness, the mullion and transom sections, brackets and anchors are designed by the facade engineer for the building's actual loads and conditions.
• No compliance verdict. The WWR here is for one elevation and the check value is an input. Compliance is assessed by the energy consultant for the whole building.

Worked example

A 30 m wide elevation, six floors at 3.6 m floor-to-floor, a 1.5 m module and 2.4 m of vision glass on each floor, with 5% wastage and the default 40% check value:

The facade divides into exactly 20 modules, so there is no narrow module. The WWR of 66.7% is above the 40% check value, and the calculator shows a warning. That does not mean the building fails: it means this elevation is heavily glazed and the energy consultant should look at it early. Options they may explore include more spandrel, glass with a lower solar heat gain, shading, or demonstrating compliance by the whole-building performance route.

To see what a taller spandrel does, change the vision height to 2.1 m. The vision area falls to 378 m², the WWR to 58.3%, and the spandrel panels become 1.5 m high.

How engineers use it

A facade consultant or architect uses this kind of take-off at concept and schematic stage, before the elevation is fully drawn. It answers quick questions: how many units of each size, how much spandrel to price, and roughly where the WWR lands compared with the energy brief.

Once the module and the vision band are agreed, the take-off moves to the elevation drawings and the panel schedule. Each unit gets a mark, a size and a glass type, corner and special units are listed separately, and the sealant and gasket lengths are taken from the system's own details. The vision and spandrel areas also feed the energy model, where the glass properties such as solar heat gain coefficient and visible light transmission are tested against the code and the cooling load.

The calculator is also useful for checking a tender. If a bid lists far fewer vision units than the module grid produces, or no attic stock at all, it is worth asking why.

When a professional design must replace it

Use the calculator to estimate quantities, compare module options and see how the vision height changes the window-to-wall ratio. Replace it with a facade engineer's design before any glass or framing is ordered. That design sets the glass make-up and thickness, the frame sections, the brackets and anchors, the movement joints and the fire-stopping at each slab edge, all for the building's actual site and height. The window-to-wall ratio and glass performance are confirmed by the energy consultant for the building as a whole.