Page 181 - AGC_EN_iBook

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Disregarding special applications, the choice of glazing is always dictated by the need to optimize the following four properties:
– light transmittance (LT)1 and colour rendering index (Ra)2 for visual comfort;
– thermal insulation (Ug)3 and solar factor (g)4 for thermal comfort. Generally speaking, ‘ideal’ glazing provides light transmittance, thermal insulation and high colour rendering, in combination with a low solar factor to prevent overheating. In practice, however, there is no glazing on the market offering optimal performance on all these accounts at once, as the low-e and/or selective coatings applied to the glass to improve thermal insulation and/or the solar factor themselves reduce light transmittance and/or colour render- ing. A compromise is therefore necessary, even if the research con- ducted by the glass industry is doing everything to reduce its size. A different approach involves adding mobile solar protection sys- tems to the outside of a building glazed with non-selective clear insulating glass.This external solar protection system can take the form of a semi-opaque screen or opaque movable louvres, ena- bling a low solar factor to be achieved, though signi cantly reduc- ing light transmittance and outside views. Such an approach leads to the paradox of having to use arti cial lighting inside the build- ing in the middle of a summer day.
The Building Division of AGC Glass Europe goes one step further towards achieving this impossible marriage of solar protection and transparency, proposing movable screen-printed striped glass louvres applied to the external facades. Always positioned at right angles to the sun, they are an effective way of blocking direct sun- light while at the same time letting through diffuse light, thereby achieving very good light transmittance. In addition, the gaps between the screen-printed stripes enable outside views.
With the louvres providing solar protection (the solar factor of the glazing + louvres was measured at a very good g = 0.148 ± 0.02), clear glass can be used, thereby optimizing thermal insulation, light transmittance and colour rendering.
The original plan had been to use a ‘tri’ layer normally reserved for triple glazing on top of Clearvision extra-clear double glazing, but this would have meant a poor Ug value for the double glazing. It is given in Figure 1 by way of comparison.
1 - Light transmittance (Lt; dimensionless; eN 410): the fraction of incident light passing through the glazing.
2 - Colour rendering index (Ra; dimensionless; eN 410): the ability of glazing to faithfully render the different colours of the visible spectrum.
Its ‘challenger’ was ‘EnergyN on Clearvision’ (LT = 0.75, Ug =
1.0 W/m2K, Ra = 97%), a glazing with similar properties except
for its thermal insulation which has an excellent value for a
double glazing.
Nevertheless, a third glazing system was ultimately chosen: ‘TOPN+ on Clearvision’ (LT = 0.81, Ug = 1.1 W/m2K, Ra = 98%), despite its lesser thermal insulation. Dynamic thermal simulations show that the additional heat loss is more than offset by the electricity sav- ings resulting from its better light transmittance, reducing the need for arti cial lighting.This very high light transmittance is due to the use of extra-clear Clearvision glass (FeO3 < 0.02%), a type of glass that also has the advantage of faithfully rendering colours, in con- trast to traditional oat glass with its greenish tint.
All the above refers to the building’s external facades.These have to be xed and airtight for acoustic reasons. Maintenance walk- ways circling the building support the louvres.
The facades overlooking the patios are designed differently.The patios are designed as easily accessible outside spaces offering a calm and convivial atmosphere. As a result, the facades surround- ing them do not have to act as sound barriers as is the case with the external facades. Instead they are ‘permeable’, interspersed with doors.The glazed frames, tted with opening windows, are made of wood, a material pleasant to the touch and to the eyes and which will acquire a patina with time. In a similar way, the outside blinds are made of bamboo, thereby avoiding excessive light re ection.They are automatically raised or lowered to opti- mize daylight availability. In the same vein, EnergyN on Clearvision glazing has been selected for these facades on account of its better solar factor and its high light transmittance.
Although Philippe Samyn had strongly recommended executing the whole facade in wood and with a wooden frame, as the physi- cal performance of wood is better than that of aluminum, the latter material was nevertheless ultimately used - for compelling reasons escaping technical logic.
Even the wooden frames of the patios are supported by an inside aluminum structure.
3 - thermal transmittance (ug; W/m2k; eN 673): the heat ow through 1 m2 of glazing for a temperature difference of 1 kelvin.
4 - Solar factor (g; dimensionless; eN 410): the fraction of incident solar energy passing through the glazing (direct+indirect).
ThE ChOICE OF GLaZING