This thread is making me very interested in a PIV unit. We get a musty smell in some of the upstairs rooms and a bit of mold on one outside wall in the lounge behind furniture where there is limited air circulation.
One thing that worries me though is that we definitely get condensation at times in the loft on the felting on the underside of the roof. I can't see that pumping air from the loft in this situation is going to be a good idea as it obviously isn't that dry. In the loft with the light off I can see slivers of daylight entering under the eaves so there is some kind of ventilation up there.
Maybe condensation in the loft is a sign something is not right with the insulation/ventilation up there? The musty rooms are under the loft space thinking about it.
Any advice?