Mildew spray migh help, but a fan is better. Bath fans wouldn't cost much to install. They aren't hard either. First calculate the square feet of the room by measuring the lenghthand width and multiply. Buy the fan that will ventilate a room of square footage. Also se if you want the fan to have a built in light, heater, etc.
Branch off off an existing light and cut the hole for the fan. Follow the instructions with the unit. Ducting needs to be installed, but we usually don't vent for a roof unless it is for a kitchen vent hood. Run the duct to the soffit vent screen where the air can get outside. If there is no soffit vent screen install a damper similar to that of a clothes dryer vent, except under the soffit. You may want to hire an electrician to wire if ou aren't sure. ALWAYS TURN OFF breaker first.