The Building research Establishment (BRE) book "Understanding Dampness" has this to say:
"There are two types: active and passive; neither has been approved by a recognised laboratory. By far the greater number of systems are of the passive kind, where there is no external source of electricity. They have always been something of a controversial issue. On theoretical grounds, it remains a mystery as to how they can work; their effectiveness has not been demonstrated in the laboratory and field evidence is disappointing.
Active electro-osmotic systems use an external source of electricity. BRE has no evidence to suggest that the two types behave differently in practice, though some of the active systems may be rather susceptible to the effects of mechanical damage and electrochemical corrosion.
Installation is quick and relatively simple in practical terms and involves, in principle, the installation of a continuous electrode in walls at DPC level. the system is either earthed through earthing rods set in the ground (passive types) or a potential is applied between the DPC and earth or between DPC and another set of wall electrodes set at a different level (active types).
The claim for passive types is that a damp wall contains an electrical potential and the earthing of this potential causes the dampness to fall. It is true that the existence of electrical potentials in a damp wall can be demonstrated. However, where such potentials are caused by the movement of moisture and salts in the first place, earthing the potentials might be expected to increase rather than reduce the upward flow of moisture and salts.”
The installations inspected by BRE were coupled with a replastering system which provided a good barrier to moisture; it is suspected that claimed successes for the system relied heavily on the render and plaster system. As far as is known the passive system is no longer available, though many thousands of installations still exist. Active systems do attempt to make use of true electro-osmosis: the movement of moisture through finely pored materials under the influence of an electrical field. Site experience is not encouraging and, again, the systems rely on the assistance of plaster to contain moisture.
Of the complaints about electro-osmotic damp-proofing that BRE has investigated, some have involved condensation problems that the installation could not be expected to cure; in others there appeared to be at least partial failure of the system, suggesting that electro-osmotic systems are not effective in preventing rising damp in walls in all conditions."
