seal damp or not, ventilate or not?

I am currently renovating my under dwelling, I would call it a basement, but as all the walls are above ground level it aint! (I think?).

I have got my head round most of it. There are however some issues I am still struggling with.

The skirting boards and plaster was in terrible condition around the bottom of the walls. I have removed the damaged plaster - back to stone around the base of these walls. I have found the walls having been injected with a chemical dpm in the past. The problem arose due to the walls being plastered from the base across this dpm..... Allowing the damp to soak through and up the plaster past the dpm and some of the way up the walls.

I intend to lay joists on the floor and studs around the walls, insulate and board with plasterboard & floorboards. As I have decided to isolate the new structure, leaving a 10mm or so gap between the structure and walls, do I need to treat this damp? The the floor is also convinently dampproofed by what looks like a bitumen based floor leveling compound

If so what is the best product to use? I've looked on-line and found a national building supplier (sounds a bit like sticks) has a 2.5kg tub of waterproofing mortar for £3.50 where as a specialist supplier had a similar product (cementatious slurry) for nearly double the price, in a far greater volume than I would need. I know which I would go for, but would there be a vast difference in their performance? (not entirely sure about the exact prices/volumes. Looked them up for the first, better flowing and written version of this until I deleted just before posting it a few hours ago! Doh. Couldn't sleep so here is my 2nd go)



My other issue is do I need to put a membrane on stud walls/floor before boarding and/or ventilate behind the walls?

I left a small hole on the inside of the stone wall, when I removed a toilet. Quite a draft seems to flow through. I think through gaps in the stone wall cavity - from a nearby airbrick on the exterior. There are a couple that dont come through to the inside. I think they are remnants of when the fireplace was in use. I am still thinking of filling this hole though.

I have read a bit about it all and spoke to some building suppliers and still cant resolve these issues. I am hope that one of you will be able to help.

Thanks

Andy

Chemical dpms in stone walls are rarely effective. Better solution - as your walls are above ground level - is to install a French drain to the outside. Make sure the external face of the wall is as breathable as possible - no sealants, no cement pointing and no cement render.

Your new floor must be well ventilated, otherwise severe dampness is inevitable. It's difficult to see how you can cross ventilate in your situation. If I were you I would consider an insulated concrete floor with a membrane.

For the walls I would use an insulated plasterboard - e.g. Lafarge Thermalcheck-K board. This is moisture proof and vapour resistant and very effective in your situation. I have specified this many times over the years with no problems. I would fix it mechanically rather than with adhesive.

Thanks John,

you have just confirmed my niggling doubts, I had considered concrete or screed at the start. Then ditched it, as timber seemed the easier option to carry down stairs.

Whats the point though, when I would just be ripping it all out again in 5 or so years when its rotted away.

The insulated board would work well. However, as the room is to be a music studio, one of my priorities is to insulate for sound. For this reason I like the room within a room principle. I may still use vapourcheck board as the first layer when I come to double board.

Now I have to weigh up the pros and cons of concrete vs fibre screed. More importantly - I am on track to do a proper job.

Thanks again for your much needed advice.

Andy