We have a basement apartment that has just been moved into. All summer there has not been a problem with any odors. It has a toilet, bathtub, sink, floor drain and stacked washer and dryer. We have checked to be sure all of the P-traps have water, and have even taped off the tub drain and overflow. The weather has now turned cold and we get quite bad methane odors. We have checked the roof vent for hoar frost and ice closing off the vent, but it is clear. What would you suggest?