Get yourself an laser/infared surface temperature reader, condensation needs cold surfaces.
At 70% RH surfaces need to be about 14°C for condensation. This is a bit of a high humidity, but if you have trickle ventillation and intermittent heating not unreasonable to expect during autum/spring (or mildly cold humid weather). If it is new build and of decent construction, then it is not unreasonable to expect wall surface temps of +16°C if the air temp is 20°C, but some areas of poor insulation may be colder than that.
Can you ensure that the insulation is not wet for any reason, that will significantly affect it's performance.#
Can you eliminate interstitial condensation as a cause
https://en.wikipedia.org/wiki/Interstitial_condensation
At 20°C air temperature, the following approximate humidity / surface condensation figures apply
60% RH needs 12°C for condensation
50% RH 9°C
40% RH 6°C
You can see lowering humidity significantly effects how cold surfaces need to be.
With low heating and more air changes (like the PIV system) you won't get near 40% RH, maybe 60% RH. Again, not unreasonable to expect well insulated surfaces to be only a few degree's lower than the internal air temp.
Mechanical heat extraction needs a well insulated AND AIRTIGHT house to work effectivly, the kind of house with 200mm of insulation minimun, so it is not really the solution for you.
