Interstitial condensation happens within a building element — inside a wall build-up, in a roof void, or beneath a floor — rather than on the room-facing surface. Warm, moist indoor air diffuses into the structure and, when it reaches a layer that is below its dew point, condenses there out of sight.
It is a particular risk when insulation is added incorrectly. Internal wall insulation, for example, keeps the original masonry colder than before; if warm indoor air can still reach that masonry — through gaps, service penetrations, or a missing vapour control layer — it can condense within the wall, causing hidden damp, timber decay and insulation failure.
This is why insulation, especially solid-wall insulation, needs to be designed rather than just installed: the build-up has to manage vapour as well as heat. A property showing damp that does not match surface condensation patterns, particularly after insulation work, should be assessed for interstitial condensation.