Notes to accompany 1014 insulationDocument Transcript
Notes to accompany 1014 “Insulation and thermal comfort”Slides 1-2 Warmth and comfort are note the same. Warmth is a measure of absolute temperature, comfort is what it says, feeling comfortable. If you are active you will want a different environmental temperature than if you are sedentary. This difference needs to be considered all the time. You are aiming to design a house which, form the users view point, is thermally comfortable, whilst at the same time, from a global point of view, consumes as little energy resource as possible and produces as few environmental pollutants as possible. All things have an intrinsic quantity of heat. If one with more heat (at hot body) is in contact with one with less heat (a cold body) then heat energy will move from the hotter body to the colder body until the heat of each is equal. Then the heat stops moving, it is at equilibrium., In the far far distant future, all heat will be in equilibrium, there will be no transfer of energy and the universe as we know it will cease to exist. This cheery thought is known as the heat death of the universe. Well beyond out time.Slide 3 We have hot bodies. When our immediate environment is less hot than us, heat travels for us to it and we feel cold (which just means less hot, there is no such thing as cold.) If we want to feel less cold (even though there is no such thing as cold) we can do two things. Raise the temperature of the environment until it is closer to our own (turning up the radiators for example) or surround our bodies with an insulating layer which makes our very immediate environment warmer (putting a jumper on). The latter consumes less fuel, but we so often do the former, that we have to design buildings to reduce the flow of heat form the building to the external environment, for all the reasons of fuel conservation, reducing greenhouse gas emissions etc.Slides 4-6 Controlling heat flow is central to managing the energy use of a building and achieving internal thermal comfort. Heat flows in one or more of three ways, conduction, convection, and radiation. These three are very different. All hot bodies (i.e. everything) loose heat by radiation. Some also loose heat by conduction. Heat is only lost by convection if there is a fluid (gas or liquid) involved, which is free to move. But this fluid in turn gains and looses heat primarily by conduction. Conduction occurs when two bodies, of whatever form (solid, liquid or gas) are at different temperatures and are in direct physical contact. Energy from the fast vibrating molecules of the hot body is transferred directly to those of the cold body, so that the former slow down (get cooler) and the latter speed up (get warmer) until eventually both are vibrating at the same speed (reach the same temperature)
Some materials are very good conductors of heat (metals, glass, ceramics) and make poor insulators. Others are very bad conductors of heat (wood, fabrics, foam plastics, still air) and these are exploited as thermal insulators. A wool jumper keeps your warm, but chain mail doesn’t.Slides 7-8 In nearly all building products, insulation is achieved by incorporation of large volumes of still air. The base material itself is not so important. The glass in fibre glass quilts is a bad insulant, but it traps lots of air and keeps it still, which is a good insulant. Moving air is not a good insulant (see convection below).The best insulant is a pure vacuum. This is for practical purposes not usable in a conventional building, but some materials are being developed which have vacuum filled cells, which potentially will have very good heat retention properties, so long as the vacuum is not broken.Slide 9 Measuring heat loss is complex and arithmetic heavy. The important figure used in the Building Industry is called the “U” value of a building element, be it wall, floor, roof or window. The U- value is the measure of how much heat (in Watts) will pass through one square metre of the element when there is a temperature difference of 1oC between the two sides of the element (WoC/m2). Acceptable figures for each major element (walls, floors and roofs) are given in the Approved Documents of the Building Regulations. For walls it is currently 0.35WoC/m2This means if there is a temperature difference of 1oC between inside and outside, the maximum allowable loss of heat is 0.35W for each square meter of wall. If there is a 10oC difference it is 3.5W. Heat loss is always directly proportional to temperature difference. This is why your mother would rather you put on a jumper rather than turn up the radiator. The warmer the house is, the more heat that is lost. Detailed U-value calculations will be explored in a later lecture.Slides 10-12 When fluids (liquids and gases) get warmer they expand and this makes them less dense. The less dense (warmer fluid) will rise as gravity pulls the colder (denser) fluid downwards. This means that fluid which has been warmed by conduction from a hot body which it is touching, will move upwards and take that extra heat with it. Colder fluid will now touch the hot body and take more heat from it. This removal of heat by a fluid under the influence of gravity is called convection. There is no heat loss in space due to convection as it only happens under the influence of gravity. Heat can be lost through cavity walls by convection currents in the air in the cavity. This is why the cavity in a wall is not there to improve thermal performance. It may help a bit, but it is very insignificant. To insulate a wall, the air inside the wall, which in absolute terms is a very good
thermal insulator if you can stop convection currents, must be kept still, even if it is warmed. This is done by trapping it in a very lightweight matrix, either of bubbles in a plastic or glass, or between the fine fibres of a quilt. This still air is a poor conductor of heat and its propensity to convection is prevented, largely, by the matrix.Slides 13-14 All bodies emit infra-red radiation (IRR) when they are above absolute zero (−273.15°C or 0°K). All bodies in the universe are above 0°K so they all emit IRR. Hot bodies emit more and cold bodies less (although of course, cold doesn’t exist). This radiation carries energy away from the emitting body so it gets colder. If this emitted IRR hits another body, that will absorb the radiation and get hotter. Heat has been transferred from one body to another, even across a perfect vacuum. When we stand in a room, we emit IRR which is absorbed by the walls, floor, furniture and everything else around us. These things in turn emit IRR which we pick up. If we go into a room with a “comfortable” air temperature, say around 23°C, it will still seem chilly if the walls and floor are much colder, as we will not be receiving much IRR back from them. This is why air temperature alone is not a good measure of thermal comfort. We have all sat in lecture rooms where the thermometer says it is nice and warm, and we feel cold. It is because the solid surroundings have not yet warmed up and we are losing a lot of IRR. IRR cannot simply be stopped, but it can be controlled. Shiny reflective surfaces will reflect some of the IRR, just as a polished mirror will reflect some visible light. So aluminium foil can be very effective in walls and roofs. It is not an insulator (it is one of the best heat conductors there is) but it will reflect some IRR back into the room. If this is coupled with a good insulator such as expanded polystyrene, the two materials together are much more effective in containing heat than either by itself.Slide 15 Thermal comfort depends on what you are doing, what you are wearing, what the air temperature is and what the surface temperature is. You feel most comfortable in an environment where you are where overall you are losing a small amount of heat. Being in absolute heat balance is actually very uncomfortable. Can you think why?