Solar and heat protection techniques. Evaluation and design recommendations for different types of fabric awnings

Cloths and fabrics have always being used as solar protection. Interior and exterior curtains, urban and building awnings are common resources in Mediterranean architecture to create comfort in hot summers. Solar and heat control (for cooling) through external opaque protections present two main functions: a) On transparent elements of the envelope: the regulation of impinging radiation direct transmission to the interior space. b) On opaque elements of the envelope: the control of the impinging radiation on massive materials that would absorb and storage heat to release it to the interior space with a time delay (thermal inertia). In this paper, results of temperature monitoring of two different awnings (65% and 90% shading fabric / within and over vertical parapets) in similar courtyards in the city of Mendoza are presented. Mendoza, located in central western Argentina (32o40’ southern latitude, 68o51’ western longitude, 750 m above sea level), has a temperate continental climate with hot summers and cold winters. Therefore, the use of solar and heat protection techniques in the hot season are crucial. Results analyse comfort conditions that lead to design recommendations to achieve optimal responses with fabric awnings in courtyards. INTRODUCTION For many years now it is obvious that energy consumption of buildings has to be reduced in order to develop a sustainable architecture. Residential and tertiary buildings being responsible for around the third of energy consumption in western countries, their impact on the achievement on Kyoto protocol’s objectives is significant. To develop such energy-efficient buildings, designers may find themselves on two sources of knowledge: personal background acquired on former studies and experience from other buildings, description of technological choices, controls, and monitored results. As such publications are scarce, generic studies handling with a systematic approach of a particular or global design method are used as the second source of knowledge. (Van Moeseke et al., 2007). Therefore the great importance of evaluation and design recommendations for monitored real case studies. Bioclimatic conditioning of buildings relies on the design strategies applied, associated with formal and material definition of the project and its immediate surroundings. Solar and heat protection techniques are thus key strategies to ensure natural comfort, specially in high heliophany hot summer urban locations with slow air movement. It seems that a permanent yardstick would be established by relating their importance to human reactions, where a reasonable ratio would be 1 to 2. According to this, shading at overheating times is twice as important as heat gain during the unheated period. (Olgyay et al, 1957) On open space design, site specific characteristics are always determinant and, even though 1 architectural tradition and culture have always addressed urban open space issue, there are still scarce works that revise environmental quality funded in scientific studies and validation. (Sala et al, 2000) Open spaces un cities play a decisive role in their thermal behaviour. The courtyard, as a structuring element of the architectural project, is a significant open space through which heat exchanges between interior and exterior occur. (Canton et al., 2006) Improving microclimatic conditions in courtyards in summer depends on controlled dynamic shading. Shadow is the dark area behind an opaque matter that obstacles the radiant heat and direct light from radiant source. It will be possible to produce shadow if the location of sun as the light and solar energy source is known and determined. The effectiveness of a shading device depends on the proportionate success with which it covers a given surface during the overheated period without interception of the sun’s energy during unheated times. Operable external shading devices can be of many types, including shutters (hinged, sliding, etc.) routable fins horizontal plates, retractable venetian blinds or canvas awnings. They can be made of many materials, such as wood, metals, asbestos-cement, fabrics and so on. The common feature to all operable devices is that they can be adjusted at will to either exclude or admit solar radiation. As with fixed devices, they intercept the sun’s rays before the rays hit the envelope. (Givoni, 1994) Solar and heat control (for cooling) through external opaque protections present two main functions: a) On transparent elements of the envelope: the regulation of impinging radiation direct transmission to the interior space. Operable external shading devices can reduce solar heat gain through windows and other glazed areas down to about 10 to 15% of the radiation impinging on the wall. (Givoni, 1994). b) On opaque elements of the envelope: the control of the impinging radiation on massive materials that would absorb and storage heat to release it to the interior space with a time delay (thermal inertia). Cloths and fabrics have always being used as solar protection. Interior and exterior curtains, urban and building awnings are common resources in Mediterranean architecture to create comfort in hot summers. Fabrics, commonly used as sun screens, awnings and curtains, conform a varied world of materials. Their behaviour depends on their thickness and colour. A light fabric -in thickness and in colourwill allow an important amount of incident radiation and also it will reflect a part of it, absorbing little. A thick, dark canvas will have a low transmission but also a low reflection, therefore it will have an important absorption and will irradiate heat. (Paricio, 1997). In wet climates membranes made of impermeable fabric are usually used, while in hot dry climates perforated membranes are preferable, to make the screen airily and light and thus to function as solar and heat protection. (Sala et al, 2000) Besides the material properties of the shading device it is also important its design as it can be related to the space in a variety of solutions that may change the microclimatic situation of the protected exterior. Awnings can be attached to a window or to the entire courtyard, they can also go from one wall to the other, acting as a roof or be suspended over the walls acting as a ventilated roof. If the space between the exterior protection and the glass is perfectly ventilated, the shadow coefficient and thermal transmittance respect to a single glass can be around 0.16 (Paricio, 1997). In this paper two different awnings (65% and 90% shading fabric material / within and over vertical parapets) that protect similar “box type” courtyards belonging to single family houses in the city of Mendoza are studied.