Last update: 18.07.2019
Solar control
1.

Comfort

Super-cooled interiors and over-heated rooms are both uncomfortable environments to be in.

When rooms become overheated, it can be due to too much incoming solar energy. The floor, walls and furniture absorb solar energy and reflect it as long-wave heat radiation. For this reason, every effort should be made to keep this energy outside interior rooms to achieve an acceptable room climate – without air conditioning. This was previously achieved by constructing buildings using opaque building components that only had small openings in the walls.

Today‘s architecture, which strives to create living and working environments that are closer to nature with an open and spacious feel, has shifted away from this opaque manner of construction towards transparency. Therefore, it is essential to understand  the significant parameters involved in sun protection using glass in order to create a functional and comfortable interior, while also meeting other requirements such as building physical specifications and achieving energy efficiency.

Solar energy flow through glass

An interaction occurs whenever solar radiation strikes a window: one part of the radiation is reflected back into the environment; another part is allowed to pass through unhindered, and the rest is absorbed. The sum of all three parts is always 100 %: transmission + reflection + absorption = 100%

Solar energy flow through glass

 

Solar factor (g value)

The total energy transmittance degree (solar factor or g value) defines the permeability of insulating glass to solar radiation. Solar protection glass minimises the g value through the appropriate selection of glass and coatings. The g value of transparent thermal insulating glass is preferably high in order to use the passive solar energy gains for reducing heating costs during the cold seasons.

The solar factor (g) is calculated by adding the direct solar energy transmission (short wave radiation) τe and the re-radiated long wave radiation to the interior (caused by solar energy absorption) qin according to EN 410 (2011).

g = τe + qin

Shading coefficient (b factor)

This non-dimensional value helps to calculate the cooling load of a building and is also known as b factor. It describes the ratio of the g value of a particular glazing to 3 mm float glass with a g value of 87 %.
According to EN 410 (2011): b =gen410/0,87

Spectral selectivity 

Solar control glass minimises solar heat gain while maximising the amount of light transferred into the building. The “S” classification number represents the ratio of light transmittance (τv)  and total energy transmission (g value) of a glazing. The higher this value, the more efficient the glazing is.

S = light transmittance τv/g value

 

Since the visible light (VIS) is part of the solar energy and represents almost 50 % of the short wave energy, the gap between high visible light transmission on the one hand, and low solar energy transmission on the other, is limited.

Spectral selectivitySpectral selectivity

The latest generation of solar control glass from GUARDIAN, SunGuard SNX, already exceeds a ratio of 2:1, which has long been considered the maximum possible value.

Spectral selectivity