How fast it loses heat to the outside depends on several factors such as how much insulation is in the walls and ceiling, how cold it is outside as compared to inside, and how hard the wind is blowing. Conduction is the transmission of heat through a medium without perceptible motion of the medium itself. Put your hand on the hood of your car after the car has been out in the sun for a while. Your hand gets hot from direct contact with the hot metal. Convection is heat transfer in a gas or liquid by the circulation of currents from one region to another.
Radiation is the emission or propagation of energy in the form of rays, waves or particles.
On a clear day in mid-winter, step out from the shadow of a building into the bright sunlight. You are being warmed by solar radiation. Another thing to think about is that heat always moves from a warmer object to a colder one. As the house loses heat to the cold winter air, you lose heat to the house and the colder objects in it such as walls, windows, etc. To be comfortable, your heating system needs to replace the cold being lost to the outdoors.