you need a differential pressure transducer which means you will need an outside reference point. You will have to run a tube outside and the transducer will give you the differential on the display if it has a display. If it is hooked up to a DDC program you need to make sure the program is set up correctly to read the transducer but you definitely need an outside reference to determine whether building pressure is positive or negative as compared to the outside pressure. It should be a slight positive. Negative pressure inside the building is bad. If the building has an economizer you can set up a DDC program to maintain a slight positive using the economizer.