Procedimiento del proceso de gestión de mejoramiento conductual

Nowadays, there is a growing interest in using highly glazed facades in commercial buildings. The trend of covering large portions of the façade or even the entire façade by glass has its origin in Europe and is expanding to other regions. As with many other architectural trends, understanding and improving the building performance of highly glazed buildings is very important. Prior simulation studies have shown that it should be technically possible to produce an allglass façade with suffi cient energy and indoor climate performance although it is not a simple challenge (Lee, et al., 2002).

This Subsection gives a brief background to the problems often met and the solutions given, in order to improve the building’s performance.

When glazed façades are designed, several devices are often imple-mented, in order to keep the heat losses low and to avoid undesired heat gains through solar radiation (during summer). According to Compagno (2002) the two main criteria when designing a fully glazed façade are the number of glazing skins incorporated in the design (single or multiple skin façades) and the positioning of shading devices.

2.4.2.1 Glazing

In energy effi cient design the proper selection of glazing elements is prob-ably the most complex task. Glazing and window design are two areas in which great technical developments have occurred over the last years. In order to achieve good window design, it is essential to fi nd the balance between demands which are often confl icting such as passive heating and cooling functions, e.g. allow solar gains but avoid excessive solar heat, provide suffi cient daylight without causing glare, allow controllable ven-tilation into the building but keep out the noise, allow visual contact with the surroundings but ensure acceptable privacy levels (A Green Vitruvious, 1999). This Subchapter focuses on the thermal insulation that glazing can provide and suggests a number of ways to decrease heat loss through it.

Single glazing provides relatively little resistance to loss of heat, since the glass is a poor insulator. To decrease the thermal transmittance, a second pane of glass separated from the fi rst pane by an air space can be added.

This layer of enclosed air provides extra thermal resistance to long wave radiation exchange.

The incorporation of an air space provides several opportunities for increasing the thermal resistance of glazing:

• increasing the width of the air space: by increasing the width of the air space, extra resistance is provided. There is a limit due to

convec-tion within the air space, which occurs at about 15mm width, after which little extra thermal benefi t is obtained. Adding a third pane of glass to give a second air space provides further improvement.

• Incorporating low emissivity coatings: The use of a low emissivity (low E) coating on the glass makes it possible to reduce the long wave radiation exchange between the panes. The higher insulating effect (lower U value) provided by a Low E coating in a double glazed unit is due to the high refl ectance of long wavelength radiation. In cold climates the higher temperature of the inner glass surface of double glazed units using Low E coating diminishes the effect of long wave radiation, which causes discomfort near the window.

• Using gases of lower conductivity: Sealed Low E double glazed units may contain gases with lower thermal conductivity than air such as argon, providing further decrease in U value.

• Evacuating the air space: the air space may be fully or partially evacuated.

The properties of glass, such as solar shading and emissivity infl uence the transmission through the glass (Carlsson, 2005). Drastic changes can be obtained by applying a coating on the glass. Coatings can infl uence the range of transmitted radiation and its absolute level. The coatings can be refl ective and selective.

Effi cient solar shading can be obtained by refl ective coatings. Increased refl ection results in reduced total transmission. Currently, the total solar energy transmittance (g value) for a sealed double glazed unit can be varied between 0.2 and 0.7 with a daylight transmittance between 0.3 and 0.8 W/m²K.

Lower U values can be obtained with coatings of low emissivity. The emissivity can be reduced from 0.87 to 0.04. The infrared radiation can be reduced to 20 %, without lowering daylight transmittance below 0.75.

This type of coating is selective, as it allows transmittance of the main part of daylight, but has a high refl ectivity of the infrared radiation. Currently the U value (middle of the glass) for a sealed double glazed unit can be varied between 2.8 and 1.1 W/m²K. In modern offi ce buildings sealed double glazed units are preferably used, often with a refl ective coating.

2.4.2.2 Shading devices

In order to achieve a certain level of solar transmittance through the single skin façade, solar control glass is often used. However, since the properties of this glass are fi xed, they restrict useful solar gains during cold months and they can reduce daylight levels. Thus, by providing additional

adjust-able shading devices the building performance can be further improved.

Some of these devices are:

• exterior shading devices: The main advantage of these devices is that the heat resulting from the radiation from the device itself remains out of the building, keeping the cooling load levels lower during summer months. The main disadvantage, however, is that they are exposed to the effects of weather, often resulting in high mainte-nance and cleaning costs. If the exterior shading is movable, then the low solar transmittance effect could be limited to the summer months, when cooling is needed; if they are fi xed, they have similar effect as the solar control glass.

• interior shading devices: This type of shading device is less effective, since the radiation absorbed by the devices stays in the room raising the cooling demand. However, the cleaning and maintenance of these devices is much simpler than with exterior ones. Additionally, internal shading can provide the “clean façade” look, which is quite often required by architects.

• intermediate shading devices: Shading devices placed in between the panes of glass are less common in offi ce buildings. Costs associated with cleaning are lower than with the exterior ones but maintenance may be more expensive, mostly when the electric motors are also incorporated inside the cavity (Compagno, 2002). The increase in temperature between the panes due to absorption by the shading should be considered in order to avoid cracking of the glass due to the dramatic temperature increase.

A comprehensive study of several aspects related to solar shading devices has been carried out within the “Solar Shading” project at the Division of Energy and Building Design, Department of Architecture and Built Envi-ronment, Lund University. The project included the following tasks:

• determination of the primary and total solar energy transmittance (g value) of shading devices through measurements;

• development of an advanced computer program (Derob-LTH) and a user-friendly design tool (ParaSol) to predict the impact of shading devices on energy use in buildings;

• parametric studies as a basis for the development of design guidelines aimed at architects, engineers and consultants;

• measurement of the daylight transmittance and interior illumi-nance/luminance conditions in rooms equipped with shading devices.