Negative pressurization
 

I am trying to simulate a room which is to be maintained at a negative pressure with respect to the outside conditions i.e. supply<extract to ensure inward leakage through the building fabric. Can you please suggest any way of achieving this? I am aware that mass has to be balanced within the simulation.
So if I lower the extracts to the supply levels, assuming that supply is not less than extract by a few percentage points, will it achieve a fairly similar effect.