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From this post we are going to define the essential points or bases that we should take into account in every building envelope in order to identify the keys in energy efficient buildings.
After previous study, and taking care of the envelope in the Building, we can determine three points.
- Attenuation of solar heat loads.
- Use of natural ventilation.
- Control of natural lighting.
These strategies will serve as a guide to be applied to each of the different architectural components and the facilities, equipment and furniture.
ATTENUATION OF SOLAR HEAT LOADS
First we must determine the sources through which heat penetrates the Buildings:
- The Sun: direct and diffuse solar radiation reaches the building from the sun and the sky, as well as by reflection from nearby surfaces (albedo).
- The air: during the day the sun increases the temperature of the outside air through the soil and the particles contained in it. At night, in the absence of the sun, the air, due to the accumulation of heat, maintains an outside temperature level that in the tropics does not present a large thermal jump between day and night.
- Other sources of heat: users, according to their metabolism and activity, emit heat to the environment. Likewise, the facilities, equipment and electrical appliances generate heat to a greater or lesser extent according to their purpose and their efficiency.
The most important causes of heating inside buildings is the sun, which acts in essentially two ways
• Direct penetration through openings and glazed surfaces.
• Heating of opaque exterior enclosures, and subsequent transmission to the interior.
If we analyze the outside environment, both solar radiation and air temperature obey 24-hour cycles that are constantly repeated. Outside, the temperature of the air and the external surfaces of the building envelope is at its lowest level before dawn. As the sun rises in the sky, the temperature of the outside air increases until it reaches its maximum value, and at the same time a heat flux caused by direct, diffuse or reflected solar radiation is stored in the envelope. The envelope stores heat to a greater or lesser extent and then transmits it to the interior; This process depends on the thermophysical properties and surface characteristics of the constructive components. The heat transmission mechanism is associated with two very important concepts:
-. Damping: represented by the difference between the maximum indoor temperature and the maximum outdoor temperature.
-. Lag or lag: represented by the difference, in time units, between the maximum outside and inside temperature.
The concept of thermal mass or thermal inertia of a building refers to the characteristic that the building as a whole has of cushioning the heat that falls on it and transmitting it to the interior with a delay.
• If the thermal inertia is strong, the delay time and damping are large and the building is said to be heavy.
• If the thermal inertia is weak, the delay time and damping are small and the building is said to be light.
Strong thermal inertia is suitable for buildings designed for daytime operation with air conditioning systems, for example for government and office buildings. Weak and medium inertia are more suitable for buildings for day and night use conditioned with natural ventilation. The buildings, according to the needs of use and the climatic characteristics, can be environmentally conditioned in an active or passive way. In any case, an adequate design strategy must follow the following guidelines:
- Adequate implementation, shape and orientation of the building.
- Taking advantage of the urban context and landscaping for shading.
- Use of sun protection and other sun blocking techniques.
- Selection of opaque construction components based on their thermal inertia and surface characteristics.
- Adequate selection of window and glass façade technologies.
TAKING ADVANTAGE OF NATURAL VENTILATION
Natural ventilation is designated as the process of exchanging air from inside a building for fresh air from outside, without the use of energy-consuming mechanical equipment such as air conditioners or fans. Air movement is caused by the pressure difference, which has two sources: temperature gradient or dynamic effect of the wind when it hits the building.
Natural ventilation, used in combination with insulation, thermal mass, and sun protection, can reduce or eliminate the need for indoor air conditioning. To maximize the opportunities to naturally ventilate a building, unrestricted access to outside winds must be ensured. The air speed in an environment is conditioned by the speed of the incident wind and the pressure fields that are generated around the building, which are determined by the layout and shape of the building, the permeability of the facades and the distribution. interior of the environments.
The behavior of the air around and within the building is governed by the following principles:
• Air movement within buildings is based on the basic principle of "pressure balance" between environments. As long as a pressure difference is maintained, a continuous process of air circulation occurs.
• When colliding with the building, the wind causes pressure differences between the sides. In this way, the air moves from the windward zone (pressure +) to the leeward zone (pressure -), through the openings.
• A form of building that produces greater disturbances in the movement of the wind will create greater pressure differences.
• Air tends to enter through the openings facing the incidence of the wind and to exit through the remaining openings, depending on the dimensions, location and type of window.
• If an environment has only one hole to the outside, a neutral zone is created there where the air enters from above and leaves below, with little renewal of the same.
To take advantage of natural ventilation efficiently, the building and the construction components must be properly oriented; There should also be openings and windows that promote cross ventilation inside the rooms. An appropriate architectural response must also take into account the characteristics of the plot and the urban context. The design strategies can then be summarized in the following recommendations:
- Adequate layout and shape of the building to produce greater air movement around and within the buildings.
- Use of landscaping to channel air movement within the plot.
- Location and sizes of windows and / or openings that stimulate air circulation and renewal.
- High permeability in the facades and interior walls.
LIGHTING CONTROL
The sun is the natural source of daytime lighting, and its effect depends on the geographical location, so the light characteristics of the sky are determined by the latitude, altitude and climatic conditions of each region. What we perceive as light is the visible spectrum of electromagnetic radiation from the sun, between 380 and 780 nm. This light is received directly on the facades oriented in the east-west axis, and diffusely, due to the multiple reflections of the light in the celestial vault in the other orientations.
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An adequate use of natural light requires a knowledge of its fundamental properties, transmission and reflection:
• Transmission: the so-called opaque bodies, when exposed to solar radiation, block the passage of light, thus producing shadows behind them. Other bodies transmit much of the incident light, which is why they are called transparent or translucent. The incident light is distributed in three ways: reflectance (r), absorbance (a) and transmittance (t), which define the properties of the bodies, through the relationship:
r + a + t = 1
In the case of opaque bodies
t = 0 and so r + a = 1
Translucent materials transmit much of the incident light, but by interrupting its straight path, it is scattered in all directions and results in diffuse light.
• Reflection: is a property associated with the behavior of light when reflected by a surface. If the parallel rays of the incident light when reflected by a surface continue to be parallel, it is called specular reflection, and the surface in this case is a plane mirror. The basic rules of geometric optics apply to this type of surface.
On a matte surface, incident light is reflected in all directions and produces diffuse light. Often, and depending on the material and the color of the surface, a mixture of specular and diffuse reflections will be produced, therefore two types of reflections called semi-diffuse and scattered are generated. Materials and colors with high transmittance and / or reflectance are determining design factors for taking advantage of natural lighting and for rationalizing energy consumption. The reflection property of mirrors allows their practical use in architecture for the conduction or redistribution of natural light, as in the case of lighting ducts and solar trays.
In summary, an adequate strategy for the controlled use of natural light should be based on the following recommendations:
• Orientation and protection of windows and other openings, with sunshades, eaves, lattices, blinds or other means of blocking solar gains.
• Use of high-tech crystals that allow an appropriate transmission of natural light with a controlled gain in solar heat.
• Location and appropriate sizes of windows and other openings depending on the use and volumetric proportions of the environment.
• Use of interior finishes in light and reflective colors.
• Use of reflective surfaces to redirect light, and provide environments with greater and better natural lighting.
• Control of exterior and interior glare of buildings.
Article stolen from the University of Venezuela (Faculty of Architecture and Urbanism)
