All furnaces, even hi-efficiency, have a make-up air duct. An uncontrolled air duct is worse than an open window, as it draws in cold air to your home all winter. This makes your heating system inefficient. Next to a mid-efficiency furnaces is a combustion air duct, which may deteriorate into an ineffective bucket. Cold air pours into your basement continuously. Shutting this duct manually is unsafe; your furnace needs this air, but only when is firing.
A combustion air damper or a fresh air damper (gas code and CSA approved) will solve the problem. The damper is built to shut off the cold air automatically, but guaranteed to open before the furnace fires. In the end you have an efficient, automatic and safe way to prevent cold air from coming in your home; you burn less fuel--reducing your carbon footprint--and you save money.