Damp Walls are Thermal Bridges
If the pores of a wall collect moisture, more heat is transferred than it would be possible if the cavities were filled with air. In conclusion, the thermal conductivity of the wall decreases enormously with dampness.
Tests have shown that a damp content of 5% in a plain brick wall, for example, can lower the insulation performance by up to 50%, especially where cavity wall insulation has been installed.
Consider that porous building materials, such as natural stone, brick and solid brick, to name a few, normally have good insulating properties. They have air-filled cavities, which offer low thermal conductivity but only when they are dry.