new damp course? dry line the walls?

[loupeake]

Hi, We are gutting our house (built in 1890) and are having an extention at the same time. It is currently being underpinned, had no footings or damp course and we are going to render the outside. Prior to us moving in last year the old owners had a damp course injected (using Rentokil). Since we have lived here we have had mushrooms growing out of the top of the skirting board in 1 room, wet carpet in 2 corners of another and damp around skirting board level in others. At the moment all downstairs floors have been taken up and a new membrane and concrete poured. We intend to put in under floor heating.
My questions are - 1. Do we need to put any other type of damp course into the original part of the house and 2. We are trying to suss out whether it's a good idea to dry line the walls. Obviously we want to insulate the house but don't want any damp returning later so are not sure what to use on the original cottage. So many websites/builders have conflicting advice that we don't know what the best thing to do is.
Any advice gratefully received

[welsh brickie]

I suggest painting the walls with a buitumin paint, like sythapruff to a height of 1 metre from the floor,you need at least 3 coats, and with the last coat throw sand on it so when it dries the drywall adhesive will have something to grip.
You must ensure all the surfaces are well painted and all holes are filled for it to work,basically if in doubt paint it.