by Magic Door Industries
External motorised operable and external retractable blinds or external window louvres have been globally tested and universally found to be the best method of reflecting solar heat, while at the same time, allowing natural light, through the glazing(windows).
The percentage of solar heat reflected by external motorised operable and retractable aluminium louvres is dependent upon the reflectivity of the slat colour with the colours most reflective, silver and white, resulting in an 87% decrease of solar heat entering through the glazing.
External motorised operable and retractable aluminium louvres allow natural light to penetrate through the glazing while removing the solar heat and solar glare and are, in this regard, unique to all other shade building products.
Other methods of solar heat minimisation used in or on glazing ; reflective film, e glass, nano particle surfacing, single tint and double tint etc. It can only achieve partial success to the detriment other aspects of glazing. All such methods permanently degrade light transmission, diminish the natural internal light levels (lux measurement) inside the building and fail to overcome the central issue, that of removing the solar heat before the glazing and frames.
Similarly external shade products such as window roller shutters, fabric roller blinds on wire guide, shade cloth, shutters etc. are either highly wind unstable and/or permanently degrade light transmission, diminish the natural internal light levels (lux measurement) inside the building and fail the European norm for the requirement of 40% vision out with shade on the glazing.
In excess of over 2 million external retractable blinds or external venetian blinds are sold annually in Europe, and to date these models have either been fully imported or copied and assembled in Australia. However the European market has accepted that blinds or louvres should be retracted in moderate winds as Europe does not have the experience of high wind and summer heat and the European market is also most interested in heat lose rather than heat gain.
Current Australian Standard for Wind Actions
Currently AS/NZS 1170.2-2011. There is no specific criterion for louvres other than the general criteria for all structures exposed to wind actions. The louvres when fixed to a building would fall under the Building Code of Australia.
Louvres however are not specifically covered under the BCA. The BCA requires that a building structure must:1/ Remain stable and not collapse2/ Minimise local damage and loss of amenity due to excessive deformation, vibrant degradation.3/ Avoid causing damage to others
Double Skin glazing structures
European methodology for coping with wind conditions has been to create building designs with double encased glazing structures and associated thermal ventilation. These structures may have minimal cavity between the glazing (approximately 400mm or provide 900 -1000mm + between the glazing profiles) The resultant building design of a “glazing skin” does provide a banal wind environment for the venetian blind but the modelling of the heat gain that results from this design is most discouraging. Testing has shown that a 30 degree temperature delta is required between the building structure glazing and associated “glazing skin” to achieve a thermal air mass movement to negate heat entrapment and associated temperature increases. A temperature delta of such magnitude has been observed only in countries and cities of the Middle East such as Qatar and Dubai.
For European countries, with heat loss and associated costs of heating the major objective this issue the lack of neutral heat gain is not considered insurmountable, however in Australia for this design to be appropriate significant research must be undertaken to study any result shade availability from surrounding structures to make such a design appropriate.
Thus in all cases where the dual glazing structure design is undertaken the venetian blind functionality is that of glare control and architectural design rather than minimising solar radiation and associated heat gain.
The study of atmospheric wind modelling is the study of chaos. There are many elements that effect the wind loading, wind flow around a building is primarily governed by the size and mass of the building and the local building surrounds. The geometry of the building will determine the local wind speed and direction. The wind speed and direction varies considerably over the surface of a building. Adjacent to the building facades, the direction of the wind flow will be parallel to the facade. Further from the solid building facade there will be a component of the flow normal to the facade. The proximity of the porous louvres to the building facade has a significant effect on the flow direction and hence wind direction relative to the louvre blades.
The wind direction on the windward face will radiate from the stagnation point towards the roof an upward corners. The wind accelerates towards the windward edges typically reaching twice the approach wind speed and separates from the windward corners creating a wake region. Wind in the shear layer is highly turbulent, fluctuating considerably from the solid facade and direction. Louvres wrapping around a corner some distance from the solid facade could experience fast highly turbulent rear wind flow.
The study of water flow ideally matches that of wind and to have a visual representation of wind flow and replication of the above can easily be replicated by standing in the surf and observing the water flows and eddies around your legs.
Wind test results and in situ reality
To date wind testing of external retractable blinds and external venetian blinds has been done in two ways. There is standardised European testing to the Swiss procedure or individual wind velocity testing with direct wind flow.
Under the heading above Wind Loading you will recall the vagaries associated with the science of the study of wind. All testing to date of external venetian blinds or external retractable blinds does not take these vagaries into account but replicate only one aspect of the very complex nature of wind.
Tests to date extrapolate results from a sample within a niche being closed in on either side by mullions that project beyond the plane of the external blinds and a solid panel behind and when in situ are expected to also be protected by a horizontal projection at the top and bottom of each unit. Such construction creates static air movement once the initial air pressure is realised and therefore provides a banal wind environment.
If these idealistic conditions cannot be obtained in reality then the following rules apply. Ideally the product is installed 100mm from the facade, if products are installed between 100mm and 300mm from the facade wind class value should be reduced by 1 class, if the distance extends between 300mm and 500mm wind class value should be reduced by 2 classes and so on.
Accordingly, to date, all external venetian blinds or external retractable blinds are fitted with wind velocity sensors which cause the blind or louvre to be retracted in moderate winds.
Globally the external venetian blind industry has evolved slowly from internal venetian blind designs. Today some methodologies are identical with the craft industry design of internal venetian blinds including the use of a simple convex crowned slat and ladder string for slat tilt. The use of the basic external venetian blind is widespread in Europe where the external venetian blind is installed within the glazing (window) revel and next to the glazing (glass) and therefore the venetian blind is not subject oscillating or gusting wind pressure (kPa).
Wind tests undertaken under such conditions will show a wind stable product even with such basic and primitive engineering using ladder string for the venetian slat control. However when such a “boxed” installation scenario is not present these venetian blinds are highly wind unstable and must be retracted under light wind loads of 10 metres/sec.
n summary, the global product range can be divided into three (3) categories of innovation to reflect the capacity of the blind to cope with Australian winding loadings, from the least capacity Category 1 to the most capable Category 3.
Category 1 Basic wind loading models
(internal venetian blind methodology)
Ladder braid slat control & wire or extruded side guides & Zamac slat side pins
Category 2 Superior wind loading models
(progressive engineered designs)
Hagofix® slat control, extruded side guides with plastic rails & Zamac slat side pins
Category 3 Ultimate wind loading models
(total product engineering including slat, fittings, pelmet and tracking)
Wind stable aerodynamic slat profile, integrated sealing keder for maximum light minimisation and improved sound insulation, Hagofix® slat control, extruded side guides with plastic rails, Zamac side pins, 316 stainless steel end caps, grub screws & eye pin.