Of course will help.
Air-con's sole use is just to bring down the temperature of the room coz the temp would rise with people inside the room. Typically we release about 100-150W of heat per pax. A LED TV would also release about 80-100W of heat, a 50" PlasmaTV about 200-300W. Etc....
This is why it gets so damn hot during a CNY get-together or customary wedding tea ceremony with 20 pax in the living room. Each pax is all psyched up + physical activity, 150W per pax is v possible. 20 pax = 3kW of heat. A 9000btu/hr aircon can't cope coz its cooling capacity via the FCU is typically about 2.7kW. (cooling capacity is NOT the aircon's power consumption). That is why without ventilation through the windows, the room would get v hot. Can you imagine operating 2 pcs of big 1600W hair dryer in the room?
When you close the room and windows, the hot air cannot be blown out. Thus bringing the room's temperature up. As the temperature gets higher and higher and a bigger temperature gradient exists, it'd tend to be dissipated "out" via the walls and floors and ceiling. But that's a much much slower process compared to airflow through the windows. You'd sweat like a pig over here in Singapore.
