With cavity wall insulation, the difference in temperature between the surfaces inside and outside can cause a dew point to form, where water either condenses on the insulation material inside the wall, or on internal walls. If it stays there and permeates the building fabric, it's easy for damp to set in.

Can having cavity wall insulation cause damp?

If installed incorrectly, or in unsuitable properties, cavity wall insulation (CWI) can lead to damp.

What are the disadvantages of cavity wall insulation?

General Cavity Insulation Disadvantages:

If moisture gets into your cavity insulation, its ability to keep your home warm will diminish. Cavity fill insulation means you'll no longer have a void in the cavity to prevent moisture getting across to the inner leaf of the wall. If poorly fitted it won't be as effective.