Can External Wall Insulation Cause damp?

There is potential for moisture to enter a wall from both sides: rain on the external skin and moisture from people and the activities they undertake (cooking, drying clothes, washing, breathing, etc.) on the internal skin.

Prior to the widespread introduction of the cavity wall, the housebuilder generally had a choice. To build a solid brick wall that is  impermeable and stops moisture penetration to both surfaces, or to build a stone wall which is a breathable wall that allows moisture to penetrate (to a degree) and be evaporated away.

In comparison the cavity wall is an impermeable wall in that the cavity is intended to form a ‘barrier’ to prevent moisture penetration; any rainwater entering the wall is evaporated away by the air movement in the cavity.

When adding modern insulation materials these are usually non-permeable so need to be rendered or clad to prevent the weather penetrating through.  However, this does not stop moisture from inside reaching the walls and causing issues.  It may mean further ventilation is required to reduce the moisture getting to the walls.

To explain further the dew point (where air meets a temperature that turns moisture to water) is moved further away from the internal walls when external insulation is fitted.  

The effect the insulation has is to warm the wall moving the dew point further out and therefore reducing the risk of condensation on the internal walls.