I have been advised to lower the external ground level to below 150mm below the DPC, to do this would mean lowering the drains as well as ground level, is a chemical DPC above the current physical DPC a work around?
haven't taken readings but we tend to get a lot of humidity building up in the winter on the double glazing on a cold night. Also the wall closest to the area we're worried about looks like it has a penetrating damp (but not rising damp) problem. Thechap that installed the chemical DPC suggested the damp problem we have on this wall in the kitchen could be due to a build up of condensation on the wall just because it is cold and we're producing a lot of moist air in the kitchen. We recently discovered the drain just close to this wall has been leaking causing very damp ground around this affected wall, I strongly suspect this may be the root cause, should the chemical DPC protect vs this?
If the ground level outside is at least 150mm below the damp proof course and the drain is unblocked there may be no need for a DPC to be injected at all. Its actually quite rare for rising damp to be the cause of internal damp. If however removing any likely causes fails to stop the damp, see our projects on DIY damp course injection, rising damp, damp or mould and various other references we have there to Damp.
Before you start spending on this you need to do more research. Beg steal or borrow a damp meter and do some tests all over the walls, if the readings are higher around the suspect area and then get even higher the closer to the floor you get then it could be rising damp. The high humidity in the room could just be a culmination of poor insulation, and porous walls.
Don't know what it's like where you live but here in Wales, damp proofing companies do a free damp checks. Might be worth a look.
Thanks for the advice mikester, thedoctor. I'll try and get hold of a damp meter or get someone in. The chemical DPC is installed already so the cost issue isn't top of my consideration at present. Inspite of this I still appear to have a damp problem that appears in winter (some paint blistering of paintwork 5ft up and signs of flakey plasterwork lower down where dampness appears to be drying out), I'd thought if this were rising damp the chemical DPC would resolve this but with the saturated ground outside had second thoughts. Reading a little more I also wonder whether the rendering on the wall outside might also be a factor (hairline fractures), also someone in the past relocated the back door which was close to where the damp issue is. Too many variables for me to work it out.
as an answer to your question, the need for the ground levels to be 150mm below the dpc is to prevent the risk of rainwater splashing fron the ground and penertrating into the wall above the dpc. Building regs state that a minimum gap of 150mm should exist and there is a british standard for this too.
There are tests that may help determine the cause of dampness to. for example an Ashworths Speedy Test (calcium carbide test) is for testing for dampness within a wall and not on the surface such as condensation, and dampness above 1.5m max is almost certainly not rising damp. so that leaves you with a penertrating dampness problems caused by a leak or external defect such as pointing, pourus wall masonry. An electronic conductivity moisture meter probe tests will only tell you that electric is being conducted, and this maybe due to moisture. Therefore this is only an aid and will not diagnose a problem.