1 Internal environment in ETFE foil covered building enclosures James Ward and John Chilton School...

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Internal environment in ETFE foil covered building enclosures

James Ward and John ChiltonSchool of Architecture, Design and Built Environment

Nottingham Trent University

Alex Heslop and Lance RowellArchiten Landrell Associates

16 September 2010 TensiNet 2010 Sofia

Outline

• Introduction• Thermal/optical properties of

ETFE foil• Design considerations for

ETFE-covered enclosures– Visible light transmission and

quality– Internal environment

• Overheating• Ventilation• Glare• Noise levels

• Conclusions

16 September 2010 TensiNet 2010 Sofia 2

Photo: Alastair Gardiner

Introduction


A recent conference paper highlighted:

“Limited research regarding the modelling of ETFE in building applications and limited availability of information on material properties...”

It also pointed out that:

Previous ETFE studies have focused mainly on structural properties and related issues, while little research has been carried out in order to determine energy transmission properties and characteristics in terms of environmental building design.

Source: Poirazis, Kragh and Hogg (2009)

Thermal/optical properties of ETFE foil


Source: Dyneon™ Fluorothermoplastics Product Comparison Guide (2009)


The properties of ETFE 80 µm clear 100 µm clear 150 µm clear 200 µm clear 100 µm white 200 µm white

light transmittance

0.93 0.93 0.91 0.9 0.45 0.37

light reflectance 0.07 0.07 0.08 0.09 0.5 0.59

solar direct transmittance

0.94 0.93 0.92 0.91 0.55 0.47

solar reflectance 0.06 0.07 0.07 0.08 0.39 0.47solar absorbence 0 0 0.01 0.01 0.06 0.06total solar transmittance

0.94 0.93 0.92 0.91 0.57 0.49

short-wave shading coefficient

1.08 1.07 1.06 1.05 0.63 0.54

long-wave shading coefficient

0 0 0 0 0.03 0.02

total shading coefficient

1.08 1.07 1.06 1.05 0.66 0.56

UV transmittance to 380 nm

0.88 0.86 0.83 0.76 0.06 0.01

UV transmittance to 400 nm

0.89 0.87 0.83 0.79 0.13 0.06

mean solar gain factors

0.86 0.86 0.85 0.84 0.52 0.45

sample thickness (mm)

0.082 0.105 0.155 0.206 0.097 0.205

Source: ETFE Foil Cushions as an Alternative to Glass for Roofs and Atria

Source: http://www.building.co.uk/clear-choices/1600.article (2000)

http://www.building.co.uk/clear-choices/1600.article

Design Considerations

• ETFE foil is increasingly seen as the ‘wonder’ material for lightweight structural enclosures.

• Some issues that affect architectural design decisions are:

– Visible light transmission and quality

– Internal environment• Overheating• Ventilation• Glare• Noise levels


Photo: Alastair Gardiner

Light transmission and quality

• Transmittance depends on the wavelength of incident radiation but for ETFE foil is fairly consistent over the visible spectrum 380-780nm.

• This provides interior colour rendering close to that of natural daylight.

• Ultraviolet light transmission (below 380nm) is higher than for glass and polycarbonate.

• This has benefits for solaria and swimming pools and also for greenhouses.

• Infra-red transmittance is also high for wavelengths up to about 7000nm.


Measured transmittance ETFE 200μm samples


0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500

Tran

smis

sion

(%

)

Wavelength (nm)

Clear

70% FrittedVisible Light

2500 25000100007000

Measured long-wave infrared transmittance of 200μm ETFE sample


Courtesy: Professor Wayne Cranton, NTU

Internal Environment

• ETFE cushions are still a relatively new solution for covering courtyards and atria.

• Poor performance is sometimes inappropriately ascribed to the system rather than other aspects of the design.

• Basic properties of the ETFE foil can be modified by fritting and low-emissivity coatings.


Fritting to modify transmittance


Frit Ref # Description Ink Trans % Coverage % Trans %

n/a Clear 200μ ETFE n/a 0 871 Large dots 23 20 74.22 Static lines 28 30 69.33 Camo 23 35 64.64 Clouds 32 46 61.75 Med dots 32 50 59.56 Med dots 23 50 557 Large dots 23 62 47.328 Hex 28 70 45.79 Inverse Camo 23 70 42.2

10 Chequered 23 75 39

‘Intelligent’ fritting


Lancaster University, ISS Building, 2009 Architect/Designer: Wilson Mason Main Contractor/Customer: John Turner and Sons

Overheating

• The U-value and solar heat gain coefficient, or g-value, for glass and ETFE are often quoted as being quite similar.


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 DGU 2.0 0.35

2 Layer ETFE Cushion 2.9 0.71-0.22(with frit)



Glass and ETFE cushion thermal/solar data as cited in Poirazis, H., Kragh, M. And Hogg, C. (2009)


ETFE Atrium RoofThickness (mm)

Transmittance (%)Layer Description

Clear Float 6 0.78

Cavity 12

Clear Float 6 0.78

Shading Device None

EN-ISO U-value: glass only (Wm-2K-1) 1.9

EN-ISO U-value: inc. frame (Wm-2K-1) 1.9

G-value (BS EN 410): 0.30/0.74

Example of design data assumed by building services consultant for ETFE covered new-build atrium in the UK, May 2010.

• Values are often taken to be interchangeable even though the physical characteristics may be different.


Kingsdale Comprehensive School, London, where, with the use of intelligently fritted three-layer cushions designed to reduced transmittance to as low as 5%, when necessary, and the provision of extensive high-level cross-ventilation, summer temperatures at high level were kept below 35°C, compared to up to 45°C predicted by initial thermal modelling.

[Source: Building Services Journal, 09/04, CIBSE, London, 09/2004, pp. 28-32]


Nottingham Boys High School2009Architects: Maber Architects, NottinghamETFE cushion system: Architen Landrell Associates Ltd


Ventilation

• As with glazed atria, appropriate ventilation is essential.

• Also, care must be taken in planning of occupied spaces to avoid the potential stratified layer of hot air immediately under the roof.


Glare

• Although translucent, ETFE foil does not have the transparency of glass.

• In the following example, due to the orientation and the configuration of the enclosure (enveloped with ETFE foils) under strong sunlight it created an internal environment that was uncomfortable to remain in for any extended period of time.

• This is chiefly due to light levels far in excess of any of the adjoining rooms or corridors (glare) and, in summer, the elevated internal temperature.



William Smith Building, British Geological Survey2008/9Architects: Maber Architects, NottinghamETFE cushion system: Vector Foiltec

Noise levels

• Due to the taut drum-like nature of tensioned ETFE and ETFE cushions; in heavy rain there is a chance that a level of noise is reached that could be considered unacceptable in occupied areas under the cushion array.

• This may be addressed with the application of an external mesh to attenuate the noise created by rain droplet impacts.


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Conclusions

• Unlike for glazing, there are few readily identifiable sources available for universally applicable information pertaining to ETFE foil cushions.

• Given the bespoke nature of most ETFE enclosures it is difficult to designate standard values

• This leads to three possibilities:– Test each configuration for each enclosure.– Designate a standard cushion profile and for it construct ISO

testing specifications based on existing specifications for physical and theoretical determination of attributes.

– Increase knowledge of the properties of ETFE cushions to ensure ‘thermal models’ take into account the varying profiles and account for them.


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Conclusions

• The current understanding of the thermal/optical properties of ETFE foil is somewhat limited.

• The environmental performance of spaces such as atria enclosed by ETFE cladding is also sometimes poorly understood.

• There is a clear need for further research in both of these areas.


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