Radiation
Objects release heat energy as infrared radiation. The hotter an object is, the more infrared radiation it emits or radiates. No particles are involved in radiation, unlike conduction and convection, so radiation can even work through the of space. This is why we can still feel the heat of the sun although it is 150 million km away from the Earth.
Where is thermal insulation used?
If you have an object or area that is at a certain temperature, you may want to prevent that material from becoming the same temperature as neighbouring materials. This is usually done by employing a thermal insulation barrier.
For example:
- If the air outside is cold, you may want to protect your skin by wearing clothes that keep the cold out and the body warmth in.
- If your house has cool air inside during the summer, you may want to prevent the temperature from becoming the same as the hot air outside by having the house well insulated.
- If you have a hot drink, you may want to prevent it from becoming room temperature by putting it in a thermos bottle.
How insulation works?
Insulation is a barrier that minimizes the transfer of heat energy from one material to another by reducing the conduction, convection and/or radiation effects.
Insulation from conduction
Conduction occurs when materials--especially solids--are in direct contact with each other. High kinetic energy atoms and molecules bump into their neighbours, increasing the neighbour’s energy. This increase in energy can flow through materials and from one material to another.
Solid to solid
To slow down the transfer of heat by conduction from one solid to another, materials that are poor conductors are placed in between the solids. Examples include:
- Fibreglass is not a good conductor nor is air. That is why bundles of loosely packed fibreglass strands are often used as insulation between the outer and inner walls of a house.
- Heat cannot travel though a vacuum. That is why a thermos bottle has an evacuated lining. Heat cannot be transferred from one layer to the other through the thermos bottle vacuum.
Gas to solid
To slow down the heat transfer between air and a solid, a poor conductor of heat is placed in between.
A good example of this is placing a layer of clothing between you and the cold outside air in the winter. If the cold air was in contact with your skin, it would lower the skin's temperature. The clothing slows down that heat loss. Also, the clothing prevents body heat from leaving and being lost to the cold air.
Liquid to solid
Likewise, when you swim in water, cold water can lower your body temperature through conduction. That is why some swimmers wear rubber wet suits to insulate them from the cold water.
Insulation from convection
Convection is transfer of heat when a fluid is in motion. Since air and water do not readily conduct heat, they often transfer heat (or cold) through their motion. A fan-driven furnace is an example of this.
Insulation from heat transfer by convection is usually done by either preventing the motion of the fluid or protecting from the convection. Wearing protective clothing on a cold, windy day will inhibit the loss of heat due to convection.
Insulation from radiation
Hot and even warm objects radiate infra-red electromagnetic waves, which can heat up objects at a distance, as well as lose energy themselves. Insulation against heat transfer by radiation is usually done by using reflective materials.
A thermos bottle not only has an evacuated lining to prevent heat transfer by conduction, but it also is made of shiny material to prevent radiation heat transfer. Radiation from warm food inside the thermos bottle is reflected back to itself. Radiation from warm outside material is reflected to prevent heating cold liquids inside the bottle.
Reducing heat loss
Heat energy is transferred from homes by through the walls, floor, roof and windows. It is also transferred from homes by . For example, cold air can enter the house through gaps in doors and windows, and convection currents can transfer heat energy in the loft to the roof tiles. Heat energy also leaves the house by radiation through the walls, roof and windows.
Ways to reduce heat loss.
There are some simple ways to reduce heat loss, including fitting carpets, curtains and draught excluders.
Heat loss through windows can be reduced using double glazing. There may be air or a between the two panes of glass. Air is a poor conductor of heat, while a vacuum can only transfer heat energy by radiation.
Heat loss through walls can be reduced using cavity wall insulation. This involves blowing insulating material into the gap between the brick and the inside wall, which reduces the heat loss by conduction. The material also prevents air circulating inside the cavity, therefore reducing heat loss by convection.
Heat loss through the roof can be reduced by laying loft insulation. This works in a similar way to cavity wall insulation.
