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Insulation and thermal comfort Keeping the warm in and feeling comfortable
Some basic physics• All about heat: – There is no such thing as cold, just differences in levels of heat – Heat energy moves from bodies with higher heat levels to bodies with lower heat levels (which we inevitably describe as “cold”) =
Heat in the house• Heat in buildings – If our environment is at a lower temperature than our bodies, heat flows from us to it and we feel “cold” – We heat the environment to restore a comfortable balance – The whole building now becomes warmer than the external environment and heat starts to flow out of the building – Modern buildings are designed to minimise this heat flow.
Two crucial considerations• When considering heating and insulating buildings, two points must be addressed: 1. How does heat flow from one body to another? 2. What is thermal comfort?
Heat flow• Heat flows from one body to another through three physical processes – Conduction – Convection – Radiation• Insulation addresses conduction and convection• Radiation is more difficult to control
Conduction• Heat conduction occurs when a hot body is in direct contact with a cold body• Energy is transferred until both are at the same level of heat (temperature)• Materials with high conductivity – Metal, glass, ceramics, stone=bad insulants• Materials with low conductivity – Still air, wood, fabrics= good insulants
Insulating buildings materials• Special materials are incorporated in the building enclosure which are good insulators (bad conductors of heat energy)• Invariably the fundamental insulating material is stationary air• Fibreglass quilts, expanded plastics, foamed paints all rely on the air contained in them to provide the insulation
Measuring heat loss: the “U” value Inside temp 10 OC For a difference in 1m2 of wall temperature between inside and outside of 1OC The amount of heat travelling through one m2 of wall is the U valueOutside temp 9 OC Acceptable U values: Walls - 0.35 W/m2 OC Roofs - 0.16 W/m2 OC
Convection• Fluids (liquids and gases) can move internally within the body of the fluid.• Heating fluids makes them expand and their density lowers• Under influence of gravity, denser fluid falls and less dense rises. This movement is convection• Air is a fluid…
Convection inside a wall cavityWarm inner leaf causesair to riseCold air drawn in frombelowWarm air drawn over toouter leaf at top andcools as it descendsHeat is transferred fromthe warm inner leaf tothe cold external leaf
Convection inside a wall cavityWarm inner leaf causesair to riseCold air drawn in frombelowWarm air drawn over toouter leaf at top andcools as it descendsHeat is transferred fromthe warm inner leaf tothe cold external leaf Cavity insulation stops convection currents as well as reducing conduction
Radiation• Heat energy is carried away from a hot body by infra red (IR) radiation• All bodies above 0oK emit IR• Smooth, light coloured surfaces emit, and absorb, least IR• Dark, matt surfaces emit, and absorb, most IR• Smooth, shiny surfaces may reflect IR
Aluminium foil radiation control layers • Foil layer on warm side of insulation acts as reflector and vapour barrier
Thermal comfortAir temp (conduction)Drafts (convection)Wall, ceiling and floor temp (radiation) Body temp 37oC Environmental temp slightly below body temp most comfortable. Why?
References• Hall, F E. ; Greeno, R. (2010). Building services handbook. Butterworth- Heinemann• Roaf, S. ; Fuentes, M. ; Thomas, S. (2007). Ecohouse : a design guide. 3rd ed. Architectural Press.