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UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION Thermal Insulation In order to maintain a constant temperature within a building we need to restrict the rate at which heat energy is exchanged with the surroundings. Keeping heat inside a building for as long as possible conserves energy and reduces heating costs. Good thermal insulation will also reduce the flow of heat into a building when the temperature outside is higher than the temperature inside. In other words a well-insulated structure will, if ventilation and direct solar gain are controlled, stay cooler in the summer than a poorly-insulated structure. Some people are reluctant to believe that insulation will help keep a building cooler in summer, but it may be helpful to imagine the discomforts of living in a tent or garden shed. This ‘building’ will be very hot in summer for the same reason that it will be very cold in winter: lack of insulation. In a large building good thermal insulation will give savings in the energy needed to run the cooling plant. Some current office buildings use more energy for summer cooling than for winter heating. Another benefit of good thermal insulation is that the risk of surface condensation is reduced because the internal surfaces of a room are kept at a temperature which is above the dew point of the air. Surface condensation is unsightly, unhealthy and damages decorations. Well-placed thermal insulation also reduces the time taken for a room to heat up to a comfortable temperature, such as in room that is unoccupied during the day. JESMOND AGIUS 1 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION Side Note What is the dewpoint temperature? The dewpoint temperature is the temperature at which the air can now longer hold all of its water vapour, and some of the water vapour must condense into liquid water. At 100% relative humidity, the dewpoint temperature and real temperature are the same, and clouds or fog can begin to form. While relative humidity is a relative measure of how humid it is, the dewpoint temperature is an absolute measure of how much water vapour is in the air (how humid it is). In very warm, humid conditions, the dewpoint temperature can reach 75 to 77 degrees F, but rarely exceeds 80 degrees. Interesting facts: SOUPY AIR: When the dewpoint approaches 75 degrees F, most people can "feel" the thickness of the air as they breathe, since the water vapour content is so high (about 20 grams of water vapour per kilogram of dry air, or 2% of the air's mass). 1 Insulating Materials A thermal insulator is a material which opposes the transfer of heat between areas at different temperatures. In present-day buildings the main method of heat transfer is by conduction, but the mechanisms of convection and radiation are also relevant. In some circumstances there is also a contribution from the process of condensation which releases heat energy when water vapour changes to water liquid. A vacuum provides perfect insulation against conduction but is not practical for everyday purposes. The best practical arrangement is to look for materials that have atoms spaced well apart; such materials will also have low densities. Gases have widely spaced atoms and gases therefore provide good insulation against conduction. Air, which is a mixture of gases, is commonly used as the ‘active ingredient’ for insulation in materials such as glass fibre and aerated concrete. Air is not, of course, the active construction material and the need for insulation must be balanced against other requirements such as strength and rigidity. For air to act as an insulator it must be held still, because if air is allowed to move it will transfer heat by convection. The primary purpose of JESMOND AGIUS 2 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION fibreglass or expanded plastic is therefore to trap and to hold air still. Also, any surface such as a wall or human skin holds a boundary layer of stationary air which provides a certain amount of thermal insulation against conduction. Heat transfer by radiation is restricted by using surfaces that do not readily absorb or emit radiant heat. Such surfaces, which look shiny, reflect the electromagnetic waves of heat radiation and this insulation against radiant heat depends only upon the surface appearance. When we use aluminium foil as an insulator, for example, it is the shiny surface of the material that is important. Aluminium is actually a good conductor of heat, but because the foil is thin the conduction effect is small. 2 Types of thermal insulator Thermal insulators used in construction are made from a wide variety of raw materials and marketed under numerous trade names. These insulation products can be grouped by form under the general headings given below. Rigid performed materials. Example: aerated concrete blocks. Flexible Materials. Example: fibreglass quilts. Loose fill materials. Example: expanded polystyrene granules. JESMOND AGIUS 3 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION Materials formed on site. Example: foamed polyurethane. Reflective materials. Example: quilts of fibreglass sheets of aluminium foil. The materials considered here are designed for the insulation of heat transfer at the relatively low temperatures of weather conditions and human comfort. There are other specialised materials for insulating against heat transfer under the high temperature conditions encountered in boilers, furnaces and flues. JESMOND AGIUS 4 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION 3 Properties of thermal insulator When choosing materials for the thermal insulation of buildings the physical properties of the material need to be considered. An aerated concrete block, for example, must be capable of carrying a load. The properties listed below are relevant to many situations, although different balances of these properties may be acceptable for different purposes: Thermal insulation suitable for the purpose Strength or rigidity suitable for the purpose Moisture resistance Fire resistance Resistance to pests and fungi Compatibility with adjacent materials Harmless to humans and the environment The measurement of thermal insulation is described in the following sections. As well as resisting the passage of moisture it is important that a material is able to regain its insulating properties after being made wet, perhaps during the construction of a building. The fire resistance of many plastic materials, such as ceiling tiles, is seriously altered by the use of certain types of paints and therefore manufacturers’ instructions must be followed. Some materials are incompatible with one another and this should be considered when installing the materials. For example, bituminous products may attack plastic-based products. Iron and concrete are mixed together to reinforce the structure. They have the same thermal expansion ratio, so it is a perfect combination to reinforce concrete. 4 Thermal conductivity, lambda value (). In order to calculate heat transfer and to compare different materials we need a measurement to quantify just how well a material conducts heat. Thermal conductivity () is a measure of the rate at which heat is conducted through a particular material under specified conditions. The unit is Watts per metre kelvin (W/m K) Note: k is a common UK symbol for coefficient of thermal conductivity. Lambda () is used worldwide as a symbol for coefficient of thermal conductivity. JESMOND AGIUS 5 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION Measurement of thermal conductivity The coefficient of thermal conductivity(lambda value or k-value), is measured as the heat flow in watts across a thickness of 1 m of material for a temperature difference of 1 degree K and a surface area of 1 m 2. Difference techniques of practical testing are needed for different types of material but the measurements required are shown schematically in the following figure. The following general formula is then used to calculate the value of thermal conductivity for the material tested. 𝐻= where 𝑄 𝜆𝐴(𝜃1 − 𝜃2 ) = 𝑡 𝑑 = coeff. of thermal conductivity for that material (W/m K) H = rate of heat flow (current) between the faces (J/s = W) t = time (s) Q = Heat (J) A = cross-sectional area of the sample (m2) 1 - 2 = temperature difference between the faces (oC or K) d = distance between the faces (m) 1 Heat Supply d Bar of material 2 Measured Heat flow Area A Insulation to prevent heat loss from sides of bar JESMOND AGIUS 6 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION Practical Example Heat current in a refrigerator wall Estimate the heat passed in one hour in the Styrofoam insulation in the walls of a kitchen refrigerator if the total wall area of a refrigerator is around 4 m2, the temperature outside is 25oC and the temperature inside is 5oC, the thickness of Styrofoam is 30 mm and the thermal conductivity of Styrofoam is 0.01Wm-1k-1. Answer: 𝑄 𝜆𝐴(𝜃1 − 𝜃2 ) = 𝑡 𝑑 = 0.01Wm-1k-1 t = 60 × 60 = 3600s Q=?J A = 4 m2 1 - 2 = 25oC – 5oC = 20oC d = 30 mm = 30 ÷ 1000 = 0.03 m 𝑄 0.01 × 4 × 20 = 3600 0.03 which implies 𝑄= 0.01 × 4 × 20 × 3600 = 96000J = 96KJ 0.03 Check Yourself Suppose that the refrigerator in the above example uses a 30mm thickness of fiberglass insulation of thermal conductivity of 0.04Wm-1k-1 instead of Styrofoam, what will be the rate of heat flow in the fibreglass? Which material renders the best performance? So, in your words, what is the meaning of thermal conductivity? JESMOND AGIUS 7 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION 5 Values of Thermal conductivity The thermal conductivities or lambda-values of some typical building materials are given in table 4.1. These values are a selection of measured values which can be used for standard calculations. It is important to remember that the exact thermal conductivity of practical building material can vary for the following reasons: Manufacturing variations in density and thickness Changes in moisture content Effects of time on insulating properties Table 1 Thermal conductivity of some common building materials Material Walls Lightweight aggregate concrete block Autoclaved aerated concrete block Concrete (medium density inner leaf) Concrete (high density) Mortar (protected) Mortar (exposed) Gypsum Gypsum plasterboard Sandstone Limestone (soft) Limestone (hard) Fibreboard Plasterboard Tiles (ceramic) Timber (softwood), plywood, chipboard Timber (hardwood) Wall ties (stainless steel) Surface finishes External rendering Plaster (dense) Plaster (lightweight) Roofs and roof finishes Density (kg/m3) K value or (W/m K) 1400 600 1800 2000 2200 2400 1750 1750 600 900 1200 900 2600 1700 2400 400 900 2300 500 700 7900 0.57 0.18 1.13 1.33 1.59 1.93 0.88 0.94 0.18 0.30 0.43 0.25 2.3 1.1 1.7 0.1 0.25 1.3 0.13 0.18 17.0 1300 1300 600 0.57 0.57 0.18 JESMOND AGIUS 8 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION Reinforced concrete (1% steel) Reinforced concrete (2% steel) Aerated concrete slab Asphalt Felt/bitumen layers Screed Stone chippings (“hardstone”) Limestone (“torba”) Tiles (clay) Tiles (concrete) Wood wool slab Floors Cast concrete Metal tray (steel) Screed Timber (softwood), plywood, chipboard Timber (hardwood) Insulation Expanded polystyrene (EPS) board Mineral wood quilt Mineral wood batt Phenolic foam board Polyurethane board 2300 2400 500 2100 1100 1200 2000 1300 2000 2100 500 2.3 2.5 0.16 0.70 0.23 0.41 2.0 0.8 1.0 1.5 0.10 2000 7800 1200 500 700 1.35 50.0 0.41 0.13 0.18 15 12 25 30 30 0.040 0.042 0.038 0.025 0.025 Note: Use manufacturers’ data for precise values. Source: Taken from Document F – Conservation of Fuel, Energy and Natural Resources. (Minimum requirements on the Energy Performance of Buildings Regulations, 2006) Thermal resistivity (r) is sometimes more convenient to use than conductivity for thermal conductivity of materials. It is the reciprocal of thermal conductivity, that is: 𝑟= 1 𝐾 The thermal resistivity can often be obtained from a manufacturer’s data sheet on the materials. JESMOND AGIUS 9 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION This thermal image shows heat loss from a building. The areas shaded in red are where most heat is escaping. Why do you think the roof is blue? For more info, go to Thermal Images PDF file. Go to the following site. http://www.thermoguy.com/globalwarming-heatgain.html JESMOND AGIUS 10 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION 6 Methods used to measure thermal factors A thermographic camera, sometimes called a FLIR (Forward Looking InfraRed) or infrared camera less specifically, is a device that forms an image using infrared radiation, similar to a common camera that forms an image using visible light. Instead of the 450–750 nanometer range of the visible light camera, infrared cameras operate in wavelengths as long as 14,000 nm (14 µm). JESMOND AGIUS 11 UNIT 10: BUILDING SERVICES DESIGN, INSTALLATION AND MAINTENANCE IN CONSTRUCTION Exercise 1 1. An aluminium pot contains water that is kept steadily boiling (100oC). The bottom surface of the pot, which is 12mm thick and 1.5 × 104 mm2 in area, is maintained at a temperature of 102oC by an electric heating unit. The remaining part of the surface is well insulated from the surroundings. Evaluate the heat current entering the water through the bottom surface. 2. A piece of wood, in the shape of a 350mm by 350mm slab of thickness 15mm, conducts heat through this thickness under steady-state conditions. The heat current in the slab is measured to be 14.3 W when a temperature difference of 25oC is maintained across the slab. 3. 4. a) Determine the thermal conductivity of this wood. b) Would this material be classified as a good heat conductor or as a good heat insulator? A 250mm long wooden (k = 0.19 Wm-1k-1) tube has circular cross section with inner radius a = 10mm and outer radius b = 20mm. Fitting snugly within the tube is a circular aluminium rod of the same length. A temperature difference of 150oC is maintained across the ends of this compound bar, and heat loss at the lateral surface is negligible. For steady-state conditions, a) what is the total heat current in the compound bar and b) how much energy is transferred through the bar in a hour? A double-glazed window consists of two sheets of 3mm thick glass separated by a 12 mm layer of air. Determine the heat current per unit area for such a window if the inside and outside temperatures differ by 20oC. The thermal conductivities are kglass = 1 Wm-1k-1, kair = 0.02 Wm-1k-1. Assume a steady-state heat flow due to conduction and neglect convection. JESMOND AGIUS 12