I have recently moved into a small house built out of concrete blocks in a very humid part of Spain and I am looking at how best to insulate it. The winter temperatures rarely fall below 0 degrees celcius but due to the humidity it feels colder. Summer temperatures go up to about 40 degrees. Winter rainfall can be very heavy but in the summer there's practically no rain. Humidity is high all year round. I am looking at insulating the external walls and ceiling with rockwool (on the inside). My question is, do I need an impermeable layer or a semipermeable layer or nothing on the inside wall underneath the insulation? I've been advised that an impermeable layer directly on the interior wall under the insulation will prevent humidity and rain water from reaching the insulation, while allowing the walls to dry out from the outside, but then I read that an impermeable layer on the inside can attract humidity on the inside in the summer when it is hotter outside than inside. In terms of heating I plan on just having a pellet stove (the house is only 72m2 and mostly open plan). I do not plan on having A/C, but instead on installing a couple of ceiling fans. If I do need an impermeable/semipermeable layer(a vapour barrier or retardant) what material do you advise for the job? Also the builder suggested that in stead of using normal plasterboard over the insulation he would use a special plasterboard that is normally used in bathrooms and is good with humidity. Is this a good suggestion? Any insight much appreciated!

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Rockwool is good.
The system you describe for the wall is good (membrane directly on the brick interior with a new plaster finish)
The ceiling maybe.... Do you have an attic or will you be insulating between rafters? Is the roof shingles, tile, membrane, or metal? There must be a way for moisture to escape/evaporate (ventilation) on both sides of the water and air barrier ...... You need to understand the differences between; water barrier, air barrier, moisture barrier, thermal barrier. And you need to understand how they work. Some materials can serve as only one function, or one can serve all of these functions.

Happy New Year!

Yes, you always want to provide a means for the wall assembly to dry. You may find this calculator to be useful in sorting out if there is a moisture issue within the wall assembly:

https://www.borstengineeringconstruction.com/Building_Assembly_Moisture_Analysis_Calculator.html

Thank you so much for taking the time to reply AWWarn! The roof is ceramic tiled with no attic so we are planning on insulating under a fake ceiling. Do you have any suggestions for what sort of material to use for the membrane on the interior walls? Our builder recommended a thin layer of polystyrene but I don't want to use that as it takes ions to decompose. Can I just use a waterproof paint? Thanks again and Happy New Year!

Thank you so much for the calculator sailawayrb! I'll have a look at it now. And Happy New Year!