Photos would be good (or a link to a listing, or both).
See what I can do but Tuesday
Found the advert.
Mmmm, that looks to be pure snake oil.
It seems to be some sort of semi permeable membrane with the argument that water vapour will move will spontaneously move from inside (high vapour pressure/RH) to outside (low vapour pressure/RH).
Yeah but … no.
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Agreed, I did not pay much for it so thought I would just have a play.
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They are definitely a thing, but I can’t see any review or online proof that they are effective - and I note (cynically) that he manufacturer says they work best installed alongside a fan system.
Hmmm… 
Well osmotic membrane dehumidification is quite well known so probably like peltier dehumidifiers this unit may do very very little but as its hopefully working all the time maybe it might do something. I have been intending to try and create a positive pressue inside the building to see if it would also discourage the inward passage of moisture. Just some fun little experiments.
We have been researching PIV units to try and take care of the bit of mould that we get in the bathroom even with the bathroom fan on for long periods.
Where is the mold Peter, as with the reply to Billy, have you treated the mold? Even running the extract fan for long periods may not bring down the humidity sufficiently and that is 1 key point. I noticed that in France it seems common to connect extract fans with that concertina pipe rather than a smooth solid walled pipe, that can reduced the airflow quite a bit as can the choice of flue terminal.
The ribbed effect of the concertina tube is external, the inside of the tube is smooth.
Not from what I see in the shops, its wire wound so unless you get it stretched tightly it can be very bad. Stretching it often splits. Rigid PVC duct is far better. Bring mine over with me as could not find it in Brico’s
It appears on the walls round the windows - i am guessing its a thermal joint type thing causing it to occur there. I clean it off with vinegar/water mix and let it dry.
I’ve had a mould problem in both a bathroom and a bedroom. I washed the affected ceiling/wall down with a strong bleach solution, let it dry well, then painted with an anti-mould paint. That was 3 years ago and it hasn’t come back. Can’t remember the brand . . . .
Quite often the window reveals are the cold spot with little air movement to dry the surfaces so yes thats where mold often starts. Chlorine bleach can work, I used chlorohexadine mouth wash as its much longer lasting and kills just about everything (would never use it as a mouth wash). If you have enough room you can break the cold spot using a thin piece of Wedi panel (6mm) which can also be plaster skimmed. People often insulate the walls but seldom the reveals as it impinges on the windows but the thin board can work. As to the anti mold paints there are several on the market often containg nano silver so are self sterilising. Used in hospital operating theatres as well. Zinser Perma White being one. You can also buy the silver mold additive to mix in with your own paint.
Air movent so the surface dries is equally important. In my shower room the new fan and humidity controller have made a huge difference over the old, the walls are no longer getting damp high up as they used to. Leaving the door open after use helps the fan draw air across the room which is a cross flow which is ideal. Hope you can get things sorted.
For that reason, I put an louvred air vent in the bottom of the bathroom door so that there is a crossflow of air across the bathroom and out through the extractor fan. One thing I had to get others to do was to keep the bathroom window (which is beside the extractor fan) closed while the extractor fan was running. This is because I noticed the air tended to ‘short-circuit’ with air being pulled in the window and out through the fan, leaving all the damp air hanging in the bathroom. With the vent in the bottom of the door, and the window closed, the air is drawn across the room so that the damp air is extracted and replaced by warm air from the landing. No more wet ceiling or tiles . . .
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