Thanks interesting reference. I have recently been told that builders now will not work with lime mortar because of problems using lime base.

Although it can be recommended - if well maintained cement mortar is in place is it cost effective to replace ?

Until the 20th century masonry buildings in Europe and North America were generally constructed from highly permeable materials such as stone and lime-based mortars and renders covered with soft water-based paints which all allowed any damp to diffuse into the air without damage. The later application of impermeable materials which prevent the natural dispersion of damp, such as tiles, linoleum, cement and gypsum-based materials and synthetic paints is thought by some to be the most significant cause of damp problems in older buildings.

The Telegraph's property expert Jeff Howell has recommended lime mortar for pointing/rendering old buildings rather than cement.

Most tradional construction materials will allow the flow of moisture vapour through. Bricks, gypsum cement based mortar, timber, plasterboard will all allow moisture through. If they did not they would be permanently wet in some cases. If they get wet either through rainfall or condensation, as soon as it stops raining or you finish boiling the potatoes the materials will start to naturally dry out if there is adequate airflow. This applies to inside and outside of structures. The OPs' problem seems to be the structure is retaining the moisture. If this is a sudden occurance after 20 years then it points to a recent problem arising. This could be roofing and flashing failing or gutters and downpipes overflowing or rising damp caused by settlement breaching the damp course ( if there is one in a Victorian house). Look to the obvious first. These are easy to check with a ladder or from the loft or careful examination of the first few brick courses as they come above ground level. Also moisture usually penetrates where the problem is, so if the interior wall is damp the chances are the problem is the other side of the wall allowing penetration. Fix the problem and the structure will dry out naturally, especially over the summer. It really isn't rocket science.

For years I worked especially on below ground structures. People assume that concrete is waterproof; wrong. But a concrete structure designed with reinforcement and constructed of high quality concrete with an addmix or tanking will probably be so. Even then, although water does not penetrate the structure, moisture vapour does and this is dealt with via forced ventilation. Airflow is one of the major keys to moisture control. Just stand in a telephone box in winter with the door closed and see how fast the windows steam up, open the door for a while and it all disappears. QED.

Cheers Concrete