'Interstitial' means 'in the spaces between' — in this case, within the layers of a construction. Interstitial condensation occurs when moisture-laden air or diffusing vapour penetrates into a build-up and reaches a point cold enough for its dew point to be met, depositing liquid water inside the wall or roof. Because it happens out of sight, it can wet timber, sheathing and insulation for years before any symptom appears at the surface.
How it forms — the condensation plane
Picture a wall in winter: warm and humid inside, cold outside. There is a temperature gradient through the wall, from ~20 °C internally to near-outdoor temperature externally. There is also a vapour-pressure gradient driving moisture outward. As the warm, moist air or vapour moves toward the cold side and cools, it eventually reaches a layer cold enough that the local temperature equals the dew point of that air — the 'condensation plane'. There, moisture condenses inside the construction.
- Warm, moist internal air/vapour enters the build-up (by diffusion through materials, or far more powerfully by air leakage through gaps).
- It travels toward the cold outer side, cooling as it goes.
- At the point where the temperature falls to its dew point, vapour condenses into liquid water — typically on the cold face of a layer (e.g. behind sheathing, against cold masonry, on the underside of a cold roof deck).
- Repeated through the heating season, this accumulates faster than it can dry, wetting the surrounding materials.
Where it typically forms
| Location | Why it's at risk | Primary defence |
|---|---|---|
| Cold face of internal wall insulation (IWI) | Original masonry now colder/wetter; interface hits dew point | Air barrier + vapour strategy + hygrothermal check |
| Cold roof — underside of deck/felt | Moist air leaking into the loft condenses on cold surface | Ceiling air barrier + loft ventilation |
| Timber-frame sheathing (cold face) | Warm-side air/vapour control compromised | Continuous AVCL on warm side |
| At and around thermal bridges | Localised cold spot pulls a layer below dew point | Thermal-bridge-free detailing |
| Flat roof (wrong vapour profile) | Moisture trapped under the waterproofing | Correct warm-deck build-up |
- On the cold side of internal wall insulation (IWI) — the original masonry, now outside the new insulation, runs colder and wetter, and the interface can hit the dew point. IWI is the highest-risk common retrofit for exactly this reason.
- In cold roofs — moist air leaking into the loft condensing on the cold underside of the roof deck or felt.
- Within timber-frame walls — at the cold face of the sheathing board if the warm-side air/vapour control is compromised.
- At and around thermal bridges, where a localised cold spot pulls a layer below the dew point.
- In flat (warm-deck/cold-deck) roofs, where the wrong vapour profile traps moisture under the waterproofing.
Why it's so damaging
Hidden, persistent moisture inside a construction does slow, serious harm: it decays structural timber (joist ends built into walls, rafters, sheathing, wall plates), corrodes embedded metals and fixings, soaks insulation so it slumps and loses performance, and feeds mould and fungal growth (including wet and dry rot) in concealed voids. By the time it shows as a stain, a soft timber or a smell, significant deterioration may already have happened — and remediation means opening up the construction.
How it's assessed
Interstitial condensation risk is predicted before building, by hygrothermal calculation. The traditional method is the steady-state Glaser method (BS EN ISO 13788), which checks month by month whether a condensation plane forms and whether it dries again in summer. For higher-risk, moisture-sensitive or heritage build-ups, a transient simulation (WUFI) accounts for liquid transport, rain, solar drying, sorption and real material data — giving a far more realistic picture. We cover both in the dedicated condensation-risk-analysis article in this guide.