Boy, I have a Money Pit. One of my problems is that we have a dirty or musty smell in our master bath. The master bath was remodeled about a year ago. New walls, paint, floors, shower, sink, etc.
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A few things come to mind. If you have a tub and separate shower, is the tub used at all? If not the smell can be coming from the trap of the tub. If its not used odors can result out of the plumbing as the trap dries out over time. That would be my first guess.
The next thing I would look into is the venting system. Does the sinks make any gurgling noise when you flush the toilet? Do they drain fast or are they a bit sluggish? Once again if this is the case the trap(s) under the sink can become sucked dry when a toilet is flushed as the drain is looking for vent air to allow the volume of water from the toilet to go down the drain. The air is pulled through and down the outside vent, but if its plugged, undersized, frozen over the air needs to pulled in from somewhere and the area most likely is the sink. So when it happens it pulls the water out of the trap in an effort to bring air into the system to allow the water to go down the drain.
The blown in insulation and sheetrock is sealing the bath tight. The results is the air is not leaking out of the bathroom as it once was before when you did not have smells, or so you think. The tighter you make the room the less fresh air is supplied to it. The result is smells that were always being vented out are now trapped and appear to amplify themselves.
So check your plumbing system. Vent issues can cause all sorts of issues. Check around the toilet to make sure its secured tightly to the floor, a leaking wax ring may not show up for a long time, but the odor can come out right away.
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