Energy is money and the cost of it is going up all the time. It is desirable therefore to minimise the amount of energy wasted from our homes. Even more important perhaps, our use of energy contributes to global warming, so we can reduce the extent of global warming by using energy in useful ways and wasting as little as possible.
The three major methods of heat transfer are conduction, convection and radiation. Being energy efficient means minimising these in the home so that heat is retained within the house, though in practice losses from radiation are much smaller than from conduction and convection.
Energy losses by conduction through the walls are minimised by making the walls out of poorly conducting brick and breeze blocks. The walls of many houses have a cavity in the middle, and energy losses can occur there by convection. To minimise these, the cavity can be filled with insulating material. This is called 'cavity wall insulation'. Convection can be eliminated using cavity wall insulation so that the only energy losses are by conduction.
Convection and conduction losses may still occur through the windows. To reduce these, the windows may be double glazed. Losses due to convection may be further reduced by putting the panes of glass close together so that convection currents are not easily formed.
Probably the most cost effective method of insulation is loft insulation. Heat passes through the roof by conduction through the plasterboard ceiling, by convection in the loft space, then by conduction through the roof tiles. If thick insulation is placed over the base of the loft, then the rate of heat conduction is much reduced. Convection currents cannot then easily form in the loft and heat loss is much reduced.
In addition, putting rugs on the floor reduces heat lost by conduction, closing internal doors, using draught excluders around the doors and closing the curtains reduces heat loss by convection.