we have what I believe is negative pressure in our home. when we moved in years ago I caulked all the gaps, etc to tighten up all drafts and then yrs later had it sided with vinyl with foamboard underneath the siding. now when the a/c or heat is on it wants to draw air down the chimney and even after 15-20 hours push the water out of the upstairs trap in the bathtub and we wake up to sewer gas in the morning. If I run some water in the tub before bed, then it is usually ok unless hvac has been running alot. HELP - do I need a ventilator system to let more fresh air into the system?