Giving this a bump for you - as interested in the possible answers myself too !

:-)

Believe read somewhere that using different paint types can cause peeling ... oil based and water based ... hopefully someone else here could comment if that's possible x

Maybe, we have replastered a few years ago after taking old, wet plaster off. This was painted on in early November.

That is almost certainly damp. The problem is usually caused by the outside being rendered, no DPC and the wall not being able to breathe. The damp gets trapped indoors as it can't escape out because of the render on the outside. Try this product after thoroughly brushing and sanding:- Ronseal Anti-Damp Paint

I would get a qualified builder in to take a look with a damp monitor. Absolutely no to panelling that will just cover it over not sort out the problem.

Man About the House is exactly right. If your house is 300 years old, the walls will be stone, probably with a rubble core. It would have been constructed using lime mortar. This allows the structure to breath. It does not require a damp proof course. Injected DPC's do not work and are a complete con. (See Peter Ward on Youtube)

The render on the outside will be the likely cause of your problem, especially if it is cement based. It will be even worse if it is painted with "masonary" paint.

Also check that the window is sound as water can run into the wall at the bottom of the frame if not in good condition.

Damp in houses is usually caused by either water penetrating the wall from the outside via leaking gutters, windows or drainage, or the outside ground level being too high and surface water coming into contact with the walls. The other big problem is condensation caused by little or no ventilation. (usual sign for this is black mould)

On a house of this age, do not use cement based products. Do not use gypsum plaster and do not use modern wall paint. Use only lime or earth based paint. Hope this helps.

Thanks for all advice so far. Last paint tried was earth based, that peeled. Plaster is sound and dry, window in good condition.

I agree injected DPCs useless, just saying someone tried that before :-)

In time we will strip external render, needing a stopgap.

The plaster underneath the paint won't be dry otherwise this paint would not be bubbling like this.
Don't cover it up with paneling as this could cause additional work later.
If it were mine I would try to identify the source and see if it could be easily removed- you show just one area on your pictures, assuming other parts of the house are not affected concentrate on trying to identify the cause there as the solution may not be as expensive as replacing all the render- it could be that the garden level and soil are too high outside the window and need digging out, it may be that the window cill does not protrude enough and collected rainwater is running down this part of the house. It might be a faulty gutter and rainwater is washing down the house rather than being piped away, there could be a leaking pipe washing water down the outside, or there could be a blocked drain outside the window and therefore standing water, or the render could have been cracked and a repair could be inexpensive compared to full replacement.
It might still be rising damp but check it's not something else first.
Eventually you will have to have the plaster off inside and the wall injected and replastered and the skirting and floor replaced. You should check at that time the water hasn't rotted joists but until you can afford that you can regularly scrape of the paint and repaint with an oil based paint which will still likely cost less than the wood paneling.

Try checking that the window cill has a groove cut along the length of it on the underside (known as a "drip")
This "drip" is designed to stop water running off of the cill and running back onto the wall.

Is the plaster modern gypsum or lime based?

There is a drip, but it is modern plaster.

The walls need to breath inside and out, so if cement based render or plaster have been used the inner wall can't breath.

If your plaster internally is lime based you need to strip all layers of paint & repaint using a breathable paint.

We had similar in a commercial shop we leased, outside cement render inside cement plaster - walls when we moved in were terrible. Stripped off internal plaster back to brick, replastered with lime plaster & painted with Jonmatt paint & 2 years later all is ok.

Hi Victoria. I don't have the answer :( but I'm just curious as to whether you have any form of ventilation as this is a 2nd home isn't it so not frequently "manually" aired? Has this also been an ongoing issue since you replastered? How long was it left to dry before painting as it can take weeks in winter months or damp homes? Are there any other signs of damp with the dehumidifier been on 24/7? I totally sympathise. As you may recall I was looking to paint the exterior and now since storm Angus (?) a large damp patch appeared overnight in a spare room and I'm looking at my cement render with dismay : (

If you cannot see any defects in the windows or the render, try looking at the house facade in the rain. Look for leaking guttering or down pipes, blocked gullies or water collecting on the ground nearby.
Undoubtedly the gypsum plastering is a problem. It tends to host condensation and stops the wall behind from breathing.

Looking at your photos, salts are being extruded from the plaster and these should be regularly brushed off but the removal of the gypsum and it's replacement with lime plaster would be the only way to eliminate your problem, but only if you are certain that no water is entering to wall from any source.

Hi there, if you're using a dehumidifier 24/7 you will be drawing the moisture internally all the time from the full thickness of the wall. SWITCH IT OFF!!!!! Old buildings with no damp course and lime mortar,release the moisture slowly back into the air and allows the walls to breathe, almost a harmonious balance. Period properties need open fire places and drafts to create good air flow, this air flow is what picks up the moisture and naturally disperses it into atmosphere. Modern boilers take the air directly from outside now, making them more efficient but reducing air circulation on the houses. Only use super Matt paints on the problem walls as this allow them to breathe, vinyls create a barrier, sealing moisture in the walls and increasing Visible problems. Air circulation will be key, maybe a mechanical air exchange system. Drying wet clothes inside during winter months doesn't help. Hope this helps as it's not all bad, just living a little differently in a period property.