Cooling of buildings

Efficient heating of buildings is important in reducing the amount of energy used, This is because the amount of energy required for heating can be very large. Thermal energy will transfer from inside warm buildings to the cooler surroundings outside, so reducing this thermal energy transfer is important.

When trying to keep houses warm, the choice is between materials that are poor conductors such as brick, wood, plastic and glass. A house built of conducting materials like copper would be very cold to live in, as energy would be able to leave the house easily.

curriculum-key-fact
Thermal conductivity is a measure of how well a material conducts energy when it is heated.

Some typical values of conductivities are:

MaterialThermal conductivity (watts per metre per degree Celsius (W/m/°C))
Copper385
Glass0.170
Brick0.150
Air0.024

This means that 385 joules (J) of energy will flow per second through a cubic block of copper (1 m × 1 m × 1 m) when the temperature difference between its sides is 10°C.

To reduce thermal energy transferred from a warm house, the walls can be built thicker, so the energy must travel further before it is transferred to the outside. Thermal energy transfers can be reduced further if there are two walls with an air gap between them, as air has a lower thermal conductivity than brick. This is known as a 'cavity wall'.

Question

Referring to the table of conductivities, why is it better to have a window made of two layers of glass with a layer of air trapped between them?

Both glass and air are insulators because they have low thermal conductivities. The layer of air has the lowest thermal conductivity and reduces the overall conductivity of the window unit. Since air and glass are both transparent people can still see through the window.

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