A draughty room is uncomfortable. It may be difficult to believe that a little air coming in through gaps in the windows, doors or ductwork can make such a difference, but drafts make a room chilly and damp--not to mention the added expense of heating a draughty room. If a room is small it's generally pretty obvious where the draft is coming from, but finding one in a big room may take some detective work.
Turn off the furnace. Turn off all the exhaust fans in the house, close all the vents, then shut all the doors and windows.
Light a stick of incense and stand in the centre of the room. Watch the smoke. Move slowly in the opposite direction from the way the smoke is blowing until you are near a possible draft source.
Light the candle and move it slowly around the perimeter of the suspected source of the breeze. Watch the flame; whenever it flickers or bends, you're near a draft.
Draft detection is best attempted on a cold, windy day; the higher the wind, the easier the drafts will be to find. Common draft sources are doors, windows, mail slots, utility entrances (electrical sockets, TV/Internet/phone line cables, water or gas pipes), vents, fans, air conditioners and outside structural gaps, including gaps between bricks, window frames or layers of siding. Some drafts are most easily located at night. Stand in a darkened room with a flashlight and slowly play the beam over possible draft locations. Have a partner stand outside and watch the building's wall. If the draft is caused by a large crack, a shaft of light will be seen from outside when the flashlight beam touches the crack.
Don't seal a room completely. Air must pass through a building's foundation to protect the timber from dampness, while rooms with fireplaces, heaters and other appliances must be adequately ventilated to prevent the build-up of lethal gases.