Please bear in mind that we're not professional builders or renovators, so take this with a pinch of salt... but here are some notes about damp in old buildings.
Old buildings were designed to absorb water from rain, the air and the ground, and breathe it out through its porous walls. Unfortunately, that design is at odds with modern methods of managing damp by sealing it out. If you use modern materials on old houses, you often end up sealing the water IN to the house.
We've observed damp in several ways throughout our old house, and here are some causes and effects:
Peeling wallpaper 1
The wallpaper had been painted with vinyl emulsion paint, which formed a waterproof barrier and stopped moisture coming out through the wall. As a result the paper itself peeled off as moisture built up behind it.
Remedy - remove the painted wallpaper and let the underlying lime-render breathe again.
Peeling wallpaper 2
The wallpaper had been laid over the top of a paint-on sealant which had trapped water behind it. Over the years, this water had forced its way through the sealant and damaged the wallpaper where it had broken through.
Remedy - remove the wallpaper and the underlying sealant (using paint stripper) and let the lime-render breathe.
Peeling wallpaper 3
The wallpaper was peeling off in small areas. The wallpaper itself had plastic content which made it an effective barrier for moisture trying to evaporate from the lime-rendered walls. The same paper, where it went over gyspum (pink) plastered areas was not peeling. Proof, perhaps, that gypsum plaster does not allow breathing and just shunts the moisture along the fabric of the building to its nearest possible exit.
Remedy - Strip the wallpaper. Use breathable coverings like chalk-paint or
Stained carpets near chimney breast
The carpet was stained dark in the corners near the chimney breast. The breast had been painted with gloss, which was trapping moisture in the brickwork and forcing it out through the nearest unpainted area - the floor. The discolouration was the salts and creosote from the years of fires coming out with the moisture.
Remedy - remove the paint from the chimney breast and let the brickwork breathe.
Condensation on lower walls
The lower walls had been re-plastered using gypsum (pink) plaster, which does not breathe. This traps water in the walls and makes them stay very cold. When warm air hits the cold walls, the moisture in it condenses.
Remedy - this is a long-term solution, but remove the gypsum plaster and re-cover in lime-render or breathable boards. It will take months or years for the walls to warm up.
Wallpaper stained and peeling on chimney breasts
The chimney stacks had lost their pointing, had broken bricks and rotten flashing. Rain water was getting in in various ways and not able to escape due to the wallpaper and paint.
Remedy - chimney stacks need re-pointing and fixing, and then after the walls have dried out replace the wallpaper with breathable paints (chalk paint or similar).
The solid concrete floors and tiles on concrete do not allow the moisture in the ground to escape, so they trap cold, sodden ground underneath them and rob the house of heat. This moisture will eventually find its way outwards and up your walls, where it will try to evaporate.
Remedy - if you can remove the concrete, do so, and replace with a breathable flooring like limecrete, or a raised wooden floor.
Concrete render (external)
This is murderous on old walls. It doesn't breathe at all, and completely stops the natural evaporation process. Furthermore, any hairline cracks in the concrete render will let water IN, but not out again, which will soak your underlying brick or stone structure.
Remedy - remove the concrete render and replace with a thermal lime-render, or remove it completely (note: removal of large portions of render will require Building Control liaison)
Concrete pointing on stone or old brick walls
Again, this is murderous to the fabric of the walls. Old lime mortar was designed to actively transport moisture out through itself, very gradually wearing away in the process. If it has been replaced or re-pointed with concrete or cement pointing it cannot breathe and all that moisture is either held in the wall with nowhere to go (except through your wallpaper and in to your woodwork) or forced in to your brick/stone work.
Remedy - scrape out and replace the concrete mortar with lime mortar.
Woodlice, worms, slugs, frogs etc. in walls
You have damp problems!
Remedy - try to work out why the house isn't able to get rid of its damp on its own.
Concrete floors, especially cold ones, can attract condensation or even in some cases attract water from the air due to their salt content (Hygroscopic). If you have non-breathable underlay or carpet, this will turn to chronic damp and attract mould.
Remedy - very carefully remove the carpets. Use masks - mould is nasty stuff. Replace with breathable carpets or alternative flooring.
Brickwork on a chimney breast is staying wet
Burning wood and coal in a real fire or woodburner releases salty, acidic gases which soak in to the bricks. This makes the bricks become hygroscopic, so they attract water out of the air itself and always appear wet.
Remedy - clean the chimney to stop any further absorption of gases. It will take months or years for the bricks to lose their hygroscopic nature. Seek specialist help if you want to cover them.