vents will definitely help in winter because without them, despite humidity, warm air from the house has nowhere to circulate to in the outside world in most houses that don't have ventilation tiles. The air that circulates may be humid but it is still circulating, exact same principle as positive input ventilation systems stopping condensation on windows in houses.
Our heating has hardly been on and they solved the problem for us this year, installed in winter, after having the issue last year.
Dehumidifiers aren't that cheap to run imo esp compared to PIV systems. Most are 300-400w which adds up with regular use
You’re possibly right for your instance but may not be for others, same as me etc.
Depends how leaky the house is for heat, into the loft, and how leaky that is to the outside world. This dictates temp, and temp curve meets dew point, for any given humidity. It’s effectively an equitation, with a few different variables and switching one variable likely won’t fix the issue, if there are other issues also.
Adding a vent might decrease humidity, but only if the air coming in replacing it is less humid, which isn’t easy to do when humidity outside is 80-90%.
Source control is the best way, vent fans to outside in showers/ hobs etc, this is also quite good as you’re often swapping out heat where you don’t need it. In a loft you could be swapping like for like and making it colder, which doesn’t
do anything for humidity, and will make it colder, which will make any humidity more of a problem. Also means your house will cost more to heat up, especially if insulation is crap.
Dehumidifiers are expensive, but running one for half an hour a day, with a humidity cut off at say 70% can be efficient in most cases, certainly not all though. Depends on the house type too etc, and how old it is, how many people in there etc.