The question is in the subject heading. Top floor flat on exposed hill with flat roof. High levels of condensation. What plaster is best suited for flat or rendering with sand or cement and any suggestions for stone cladding around window recesses ?
Let's start with the condensation.
Condensation is man and woman made.
It is caused by boiling kettles, washing, breathing, cooking, drying things on radiators.
We all add about 1.5 litres of water vapour into the air every 24 hours, old people a bit less children and animals a bit more.
This normally disappears out the windows and doors during the summer but, when winter comes we tend to keep windows and doors closed as much as possible to conserve heat.
Warm air can hold more water vapour than cold, so when you turn the heating off, or down, the air can no longer hold the same amount of water vapour and it turns into visible condensation on cold windows and mirrors and invisible condensation where it soaks into our beds, bed cloths walls and soft furnishings.
Having heat exchange extractor fans, and using them, in the kitchen and bathroom will help solve the problem. Having the window open for ten minutes in the morning will help, having and using a de- humidifier will also solve the problem. Keeping the kitchen door closed when cooking will help.
All these solutions are basically life style things.
Now if you are on the round floor and have an old building that has rising damp and you are not causing the water vapour there are special lime based plasters, suitable for that situation.
You can find them on the net.
Fix one inch thick polystyrene round the reveals to stop the cold and plaster over.