a) If walls, windows, ceiling and floor are warmed, and if the outside temperature is lower than the inside, heat is conducted through the material of the shell where it escapes partly by radiation and partly by convection through where it escapes partly by radiation and partly by convection through warming the air in contact with it. On occasions when the sun is shining, the outer surface may get warmer than the inside of the house and heat may be gained from outside. Since heat lost by conduction from the outer shell is the more important factor, it is known as 'conduction heat loss'.
b) Air in the house is warmed and drawn up the chimney, through windows, doors, or roofs. This loss of heat, through the displacement of heated air inside the building by cold air from outside, is known as the 'convection heat loss'.