Skylights, in the form of domes and a variety of rooflights (monitor or saw tooth) are particularly useful to bring daylight into interior areas of deep plan buildings, top floors of multi-storey buildings or where perimeter windows are not possible. Indeed, skylights can be used to bring light to lower floors through the use of reflective devices. Since overcast skies
advanced facade systems with highly reflective surfaces that are designed to deviate diffuse daylight deep into the building and that their performance depends on maintenance and durability factors due to dust, condensation and surface deterioration reducing optical efficiency by more than 50%.
Atria use a range of roofing and structural systems including internal and external shading devices. For any situation, a system that effectively introduces daylight and controls heat transfer, and works for both day and night conditions is very useful. Maintenance, cleaning and heat loss/gain issues also have to be carefully resolved when using the different roofing systems.
In an atrium building, the atrium roof determines the quantity of light that is admitted and its distribution on entering the atrium space; this is discussed in a greater depth in Chapter Three. In addition to the external illuminance conditions, daylight will be affected by the roof structure and geometry, its orientation and type and the type of glazing or cover incorporated and its transmittance properties, and indeed the shading system it uses. Although, to understand the impact of atrium wall reflectances, the experiments undertaken in this thesis do not include the atrium roof, the results take into consideration the possible losses associated with the incorporation of atrium roofs.
Glazing Systems
When windows were first filled with glass, the panes were small and were secured by lead beading or leaded lights. Developments in glazing allowed for larger panes and today include a range of glazing systems that transmit diffuse skylight and control sunlight. Whilst clear single glass transmits light well, it transmits noise and heat too, and will result in heat loss from the interior space to the outside in winter. The cold glass surface will cause cool downdraught of air and condensation. These problems are reduced with double or triple glazing and can be enhanced even more if a double glazed unit incorporates a heat reflecting coating (CIBSE, 1999).
The use of clear glass is preferred as it admits more natural light. Whilst, tinted glass reduces thermal transmission to some extent, it conducts heat to the inside space after it absorbs it and also reduces the daylight significantly. On the other hand, solar-reflective glass reduces solar penetration without affecting the view. However, it also reduces light transmission (Al-Sallal, 2004). Low emissivity glass is highly recommended as part of a green strategy as it has the appearance of clear glass and it reduces direct heat gain by transmitting a greater proportion of light than heat allowing larger glazed areas in building. Recent intelligent glazing systems include photo-chromatics, phase-change materials, holographic and electrically responsive glass. Philips (2000) categorised glazing in four groups:
1. Systems used for daylight and views and at the same time control temperature and external noise
Single glazing
Double glazing – two layers of glass with air gaps with the possibility of placing acoustic absorption material at the reveals. Electrically controlled blinds between the two panes to control solar heat gain and glare.
Triple glazing – similar to double glazing but with three panes and increased thermal and acoustic qualities
2. Special coatings to reduce solar gain into interior spaces but result also in reduced light transmittance and colour distortion of the view
Glass coatings, usually dark, to reflect sun’s rays and control solar gain and provide privacy to the interiors and alter the colour appearance of the exterior and interior, therefore diminishing the impression of daylight. An alternative to this is to use a glass that gives an impression of sunlight even on a dull day.
3. Intelligent systems that reduce solar gain but rely on a range of means of control that result in reduced daylight and views out. When electric controls are used, energy savings
Light activated or photo-chromic glass- Due to receiving ultraviolet light from changing exterior conditions, light transmission to the interior is altered
Heat-activated or thermo-chromic glass- change in exterior temperature alters the optical properties of glass and thus the daylight admission
Electrically controlled or electro-chromic glass – formed of a series of glass layers and other elements where optical properties are altered by electric current
4. Shading systems, internal and external
Simple internal or electrically controlled blinds are less successful in controlling solar gain due the fact that ultra violet rays have already entered the building. But this system can easily be controlled by the building occupants.
Slatted or venetian blinds between two layers of glass – most appropriate for sun or sky glare but have issues of long-term maintenance and reduced view to the outside
External shading – significantly reduce solar gains as they stop sun rays to enter the building, however they affect external building appearance, need to be weatherproof and robust in structure and finish, and can be prone to long-term maintenance issues.
ETFE (ethylene tetrafluoroethylene)
ETFE (ethylene tetrafluoroethylene) is a lightweight material; it takes the form of inflatable cushions comprising two or more sheets of foil that are laid on top of each other and joined at the edges with a constantly maintained air pressure between them. Due to its light transmittance (95%) and potential to improve energy performance by providing thermal insulation at reduced costs and structural support in comparison with glazed roof (Robinson, 2005), this material has been increasingly used in the roofing of courtyards and atria (Poiraziz et. al, 2009). However, it does not provide clear visibility typically expected in a clear glazed roof. While glazing is almost opaque to long wave radiation, ETFE transmits part of it (Salz and Schepers 2006). “The visual light transmittance of ETFE is 94-97% with ultraviolet transmittance being in the 83-88% range” (Poiraziz et. al, 2009). Salz and
Schepers (2006) compared the performance of insulated glazing units and ETFE cushions in terms of thermal transmittance (U value) and total solar energy transmittance (g value) as shown in Table 2:10. Both, the optical and thermal properties of an ETFE roof can be altered by the use of coatings, print, geometry etc. Frit is often introduced for shading and reducing the transmitted solar energy. The use of this material for roofing of atria is particularly suitable due to the daylighting and thermal benefits it offers over glazed units.
Table 2:10 Comparison between the performance of glazed units and ETFE cushions (Schepers, 2006)
Glazing and ETFE Cushions U value
(W/m2k)
g- value
6mm monolithic glass 5.9 0.95
6-12-6 Double Glazing Unit (DGU) 2.8 0.83
6-12-6 High Performance Double Glazing Unit (DGU) 2.0 0.35
2 Layer ETFE Cushion 2.9 0.71-0.22 (with frit)
3 Layer ETFE Cushion 1.9 0.71-0.22 (with frit)
4 Layer ETFE Cushion 1.4 0.71-0.22 (with frit)
Polycarbonate
Polycarbonate is essentially a transparent thermoplastic which is known for its exceptional strength under impact, its lightweight, high transparency, high light transmittance (0.7 - 0.8), durability, excellent fire performance, recyclability, good dimensional stability and heat resistance. This inexpensive material can be used for domed, flat, curved or pitched roofs including replacing vertical cladding and glazing. Its multiwall (approximately 4mm thick for twin wall to 55mm thick for ten wall) and corrugated constructions (0.8 mm - 2.0 mm) can be either transparent or translucent and come in different tints and colours to address issues of glare. Additionally, they may incorporate UV protection, an anti-drip layer to reduce condensation and solar heat reflection technology (solar inserts and laminates) to reduce
Daylight will be reduced due to the incorporation of the atrium roof and glazing in the atrium’s facades in addition to the losses due to the roof structure, window frames and the dirt factor. While developing an understanding of the effects of these elements is vital, they have been eliminated in the experiments in order to focus primarily on the assessment of the effects of different surface reflectance distributions in the atrium facades and the composition of the facades as a result of the disposition of the fenestration and opaque areas on the daylight availability in the atria and their adjoining spaces.