Thermal performance of building sections
A thermal performance of building sections depends upon thermal properties, outside surface finish, orientation and climatic conditions. These properties are represented in two categories.
a)Thermal characteristics of the building section
b)Thermal performance index.
The U-value of the building sections can be computed with a knowledge of thermal conductivities of building sections, their thickness and surface resistance. Although there will be slight variation in the U-values for difference cities due to variation in surface resistance, for all practical proposes in building design certain constant values of surface resistance are assumed in U-value calculation . The average value of inside surface conductance (fi) for building material surface is about 9.3 W/ m2 deg C for still air and for wind velocity of 8.0 km/h, outside surface conductance (fo) is 19.90 W / m2 degC.
The values of surface conductance and the surface resistance are given in the appendix.
Transmittance of composite wall
In the composite wall construction, the U-value of the wall of that particular composition would not be available in the given table and computation of the U-values should be processed following the formula: U=1/Ra
Here the summation of resistance (Ra) of all materials used in the wall or in any structural member is necessary. The resistance can be worked out with the values of conductivity (k) for different materials and their thickness.
The values of the conductivity (k) are given in the appendix.
Taking an example of a composite wall, that consists of 110 mm brick wall (1/2 brick) as a outer leaf, cavity of 50 mm then after, 110 mm inner brick wall, 2.5mm insulating layer of glass wool on the inner leaf of brick wall. There is a plaster of 12 mm after the insulation. The wall is facing west.
As a first steps to find the resistance (U) value of this wall, the values of the conductivities of the different wall section to be found referring table in the appendix;
In some cases (e.g. for the prediction of condensation) it will be necessary to know the temperature at any point with in the wall, i.e. the thermal gradient through the wall, or other constructional elements. This can be established quite simply by a graphic method .
The thermal gradient can be clearly explained with the help of the cross section of the wall considered in the above example. Assuming the temperature of inside and outside of the room to be 20ºC and 0ºC respectively.
Now, drawing a cross section of wall to a scale representing the resistance of the individual layers, instead of thickness. Say, a scale of 1mm=0.01 m2ºC/W, thus the external surface resistance is represented by 7.6 mm and the resistance of the brickwork is shown as 9.5 mm, etc. along cross-section of the wall can be drawn to a scale, say 1:10.
Establishing the temperature of outside and inside 20ºC and 0ºC points at the faces of the resistance section, and connecting them with the straight line, the intersecting points of this line with the various layers can now be projected across horizontally to the corresponding layers of the physical wall section. A line connecting the points thus derived in the wall will represent the thermal through the wall.
Types of insulation
The practical ideal insulation is a vacuum or air when kept completely motionless in a space separating two solid components. Air, however can be kept motionless even in a narrow vertical cavity (in a wall assembly). As a result of warm and colder sides of the cavity, heat transfers from warm side to cold side by radiation . The air in the cavity coming in contact with the warm side is heated by convection and this will give rise of convective currents. These convective currents can minimized by breaking up horizontally the air in the space in to tiny compartments. Thus the air will remain still and exhibits excellent insulating properties. The principal way in which thermal insulation works is the capacity of that material; to resist heat flow by forming the tortuous path through the material around voids. The more efficient insulation types are also made from materials with poor thermal conductance. i.e, Mass type insulation, Reflective insulation
Mass type insulation
Reduces the flow of heat by preventing convection in entrapped air and also by forming a barrier to radiation. Mass type insulation may be;
1. Slab and blocks- cork board, mineral wool slabs, cellular glass or rubber slabs saw dust or cement boards.
2. Blanket insulation- flexible fibrous materials such as; glass fibers, cotton and animal hairs.
Reflective insulation
Reflective insulation is also called the radiant barriers. These types of insulation reduces the transfer of heat through air space by minimizing radiation of energy from the warmer or emitting surface of one of the components which enclose and air space to a colder or receiving surface of the other component. Emissivity of various building materials at the same surface temperature vary. The radiation across an air space between two polished aluminum surfaces will be only about 3% of that between two black surfaces . Reflective materials act as insulation, because of their low surface emissivity by reflecting incident radiation energy. It is estimated that in a cavity wall, up to 60% of heat transfer takes place by radiation.
Types of insulating materials
Organic: fibers, including cellulose fibers, thatch and wood fibers.
Inorganic: foams, including foamed glass, cellular concrete, hollow glass bead concrete, perlite and vermiculite, expanded polystyrene bead concrete etc.
Metal foils and metal foil laminated on other materials: one form combines shiny aluminum foil with flexible polyethylene foam and its manufacturer reports very favorable insulation values especially, where radiant heat is the predominant mode of heat transfer.
Insulation of building components
Organic foams including expanded polystyrene, polyurethane foam, polyisocyanurate foam and cork etc.
Composite materials combining several of the materials listed above.
Natural materials of low thermal resistance, which nevertheless, act as thermal insulators and also provide thermal storage including earth, masonry and turf. 61
Insulation of building components
1. Perimeter and foundation insulation
2. Basement insulation
3. Wall insulation •Masonry wall insulation •Framed wall insulation
4. Roof and attic insulation
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