Robinson-Gayle (2003), carried out an investigation on the normal and angular transmittance of ETFE foil samples. A calibrated Perkin Elmer PE883 spectrometer was used to measure the spectral transmittance of ETFE-foils for wavelength between 295nm and 2500nm. The results obtained from the spectrometer were then used to calculate solar and light transmittance of ETFE foils by using methods described in ISO 9050:2003 (ISO, 2003). The results obtained on solar and light transmittance of ETFE foils showed that clear foils had high solar (91%-94%) and light transmission (90%-93%). Robinson-Gayle (2003) also, investigated angular transmittance of ETFE foil samples, which was carried out in the National Physical Laboratory (NPL). The results (see Figure 4-2) revealed that transmittance through the ETFE sample was
4-71 insensitive to the angle of incidence. It was found during the experiment that, 80% of the light transmitted through ETFE-foil sample was at 180° to the angle of incidence, which was similar to the total transmittance through the sample. The study also stated that even at low solar angles the majority of sunlight would be transmitted through ETFE foil into the space enclosed with it.
Figure 4-2: Angular transmittance of 100 µm ETFE sheet at various detector and sample angles (Robinson-Gayle, 2003)
Thus, it can be assumed that normal, as well as the angular transmittance of transparent ETFE foil samples for the entire spectrum, would be similar. The reason behind this can be the thinness and transparency of ETFE foils which makes it different from fabric membranes, those were investigated by Harvie (1996) and Devulder (2004). However, for ETFE-foils of different colour and fritting, this phenomenon could be different because of the scattering of light on ETFE-foil surface.
Dimitriadou (2015), carried out transmittance test of different types of ETFE-foil using a Fourier Transform Infra-Red Perkin Elmer Spectrum 100 spectrometer. The ETFE-foil samples were examined between wavelength 2500nm and 16,667nm. The study discussed and compared transmittance of different types of ETFE-foil samples at different wavelengths where light and solar transmittance were based on average transmittance between wavelength 2500nm and 16,667nm. BS EN 410: 2011 (updated version of ISO 9050:2003) specified methods to calculate thermal optical
4-72 properties of single, two- and three-layer glazing systems. However, Dimitriadou (2015), did not consider any particular method to determine transmittance of ETFE-foils. Information provided on the transmittance of ETFE-foil samples seemed unreliable. Therefore results determined by Dimitriadou (2015) was not considered in this study while comparing measured results with previous research.
Mainini et al. (2014), measured spectral transmittance and reflectance of ETFE-foil samples with a UV-Vis-NIR Spectrophotometer Perkin Elmer Lambda 950. On the basis of result acquired from the spectrophotometer, actual transmittance and reflectance of single layer foil were calculated according to ISO 9050:2003.
Methods adopted by Robinson-Gayle (2003) and Mainini et al. (2014), to determine spectral properties of ETFE-foils were similar. Both used a spectrometer to measure spectral transmittance and reflectance, while overall transmittance of ETFE foils were calculated based on the method stated in ISO 9050:2003. These methods applied in previous studies are found suitable to determine solar and light transmittance of ETFE-foil samples for this study.
4.4 Justification
The thermal optical properties of transparent/ translucent building envelopes impact the thermal environment of enclosed spaces by allowing access of external environmental stimuli such as solar radiation, outdoor temperature, etc. This phenomenon was evident in the previous chapter, where the results of the pilot study demonstrated that an ETFE foil cushion transmits a significant amount of solar radiation affecting the thermal performance of the space it encloses. Therefore, it was thought to be necessary to obtain the overall solar transmittance and light transmittance of a selection of ETFE-foils. Collectively these properties are known as thermal optical properties (Bauer, 1965).
On the other hand, the knowledge of the thermal transmittance (U-value) of building envelope is necessary for the quantification of heat losses and (or) gain through it.
Thus, it was necessary to determine thermal transmittance of the ETFE-foil cushion envelope also.
According to Harvie (2015), it can be difficult to obtain thermal optical properties of architectural fabrics and foils (e.g. PVC coated polyester, PTFE coated glass, ETFE foil, etc.), because manufacturers are more used to supplying information about structural performance. Moreover, the specification sheet of manufacturers reflects the priorities of the designers which tend to be as follows:
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• Structural properties
• Behaviour in fire
• Colour and degradation
• Transmittance of visual light
However, an attempt was made to compile data available from the manufacturers on the thermal optical properties of ETFE-foils. From the acquired information, it was also evident that the available information tended to concentrate on structural rather than environmental properties. The standard information on the thermal properties included solar transmittance and reflectance, and U-value of single layer foil etc. The limited available information on the thermal properties of ETFE-foils might be due to that, in most countries, there are regulatory requirements for the maximum thermal transmittance or a minimum thermal resistance, while no regulations impose any strict value for optical properties of glazing material to limit solar gain. Thus, the information on material properties tended to fulfil regulatory requirements and priorities of the designers. As a result, the environmental properties obtained from the manufacturer’s information were incomplete, unsubstantiated and also entirely inadequate.
It also proved difficult while collecting information on these properties from previous research work carried by Robinson-Gayle (2003), Mainini et al. (2014), DETR (2003), Dimitriadou (2015). Poirazis et al. (2009), also found difficulties while obtaining information on the thermal optical properties of ETFE foils, particularly in the longwave spectrum. His research suggested that detailed knowledge on this area is essential to predict the impact of longwave radiation on the thermal performance of space enclosed by an ETFE-foil cushion envelope. He also stated that as ETFE-foil is not entirely opaque to longwave radiation, considering ETFE-foil as glass while simulating building’s performance might result in significant error. Besides Harvie (2015), suggested that accurate modelling of architectural fabrics requires specification on optical thermal properties which can be developed based on either a specific request to the manufacturers, measurements or assumption made on the known properties of similar materials. Therefore, measurement and documentation of thermal optical properties of different types of ETFE-foils are important to determine their impact on the thermal performance of spaces.
It was apparent from the above discussion that the most appropriate way to acquire the appropriate information necessary for this research would be to measure the thermal optical properties of ETFE foils. The following sections describe the experimental techniques that were carried out to measure the thermal optical
4-74 properties of different types of ETFE foils. Information obtained by these procedures was used to develop the simulation model used to predict the thermal performance of ETFE foils and spaces enclosed with it, which is presented in Chapter 7.