Hot gases rise because they are lighter then air. Because smoke is usually much hotter than the surrounding air it tends to rise. If it can move through open doors it will move to the top of the house, and gradually fill it As smoke rises away from a fire it tends to draw in cool air that mixes with the smoke. Eventually there is more air in the smoke than products of combustion. This makes the smoke less hot, but there is much more smoke to deal with.
Smoke is dangerous.
When moving in smoke stay low - remember the smoke tends to rise. Fires burn fast in rooms. This is because the smoke in the room radiates more heat to the fire. This means that adjacent items can ignite quickly as well. A fire cannot burn without air.