Tanking fairly dry rendered and artexed cellar

[starboya]

We've decided to convert our cellar in to a usable space and have had quotes and varying recommendations from builders and a specialist cellar conversion company. The cellar itself is fairly dry - its built in to chalk which I think must help. It has been rendered at some point and a coating of Artex applied to the walls / ceiling with fetching swirl patterns! When we moved in to the house 15 years ago the render was okay down there but over the last few years it has started to blow in a few places and the room smells a little musty. Yesterday I bought a moisture meter from Maplin and it is showing damp where there render has blown but fairly benign readings elsewhere.
The cellar steps are brick with wooden noses. Until a couple of weeks ago they had been covered with hardboard with sheet plastic under. This has had the effect of rotting the wooden noses. Tests with the moisture meter show the bottom steps to be damp and those higher up less so. We've put a heater and dehumidifier down there to see if they will dry out. Hopefully, if they dry out and can be renovated and protected from dampness they'll make a nice feature.
We're at a loss as to what method of damp proofing to use. The specialist company is suggesting completely lining the cellar with delta membrane and digging a channel around the walls just in case any water appears and (optionally) fitting a sump pump. This was the most expensive quote and seems overkill to install a system that is designed to cope with severe dampness and running water.
Another chap recommended removing paint / lose material and applying 2 coats of bitumen paint, fixing foil backed plasterboard on tannised batters. I'm not what effect that would have on the existing render (I suspect it may continue to blow) or how effective it would be. That was the cheapest quote at about 1/2 of the specialist company who would offer a guarantee.
Any suggestions / advice / recommendations would be very much appreciated.

 

[1john]

You may as well go with the bitumen route, if it fails, you can afford to do it again. In my opinion the only way to keep it dry would be to do it from the other side of the wall, the part thats buried. I dont see the bitumen idea as bad, I see the complete lining of it bad if they think that even after they have done the job that water might get in and you might want to pump it out, seeing as you haven't mentioned the need to pump your basement out to date.

 

[wedginald]

It depends what you want to use the cellar for, personally I always go down the CDM tanking route when doing a cellar as there is less to worry about with the existing substrate. Its easy enough to install and it shouldnt fail if you take your time and do it properly. Bitumen is useless and will fail in a matter of years if the rendered surface is failing.

Technically this is controlled work so as a minimum you will need to put in a building notice for change of a thermal element. There are specific u values that you should reach when doing this kind of refurb, this will mean insulating the floor and the walls. In my experience my local BC have just been happy with 25mm polystyrene on the walls between the battens and 50mm on the floor as its an inprovement on whats there but that will depend on your own BC as those thicknesses of poly dont meet the required u values .

Vapour check plasterboard is required but I would reconmend using a vapour barrier as well since the joints in the board wont be vapour tight and you will end up with interstitial condensation. If you remove the existing cieling the bc will also want to see rockwool installed for noise reduction.

You could also consider removing the existing render and coatings and using a cement based system such as Vandex and then drylinging with insuulated plasterboard, I do however feel it is always better to control the damp in the building rather than trying to stop it getting in, theres an awful lot to consider lol. If you have any futher questions then bont hesitate to ask, alternatively have a look at www.twistfix.co.uk or www.safeguardeurope.co.uk