Interstitial Condensation: Moisture Inside the Wall or Roof

Interstitial condensation is water that forms inside a wall, roof or floor build-up when water vapour moving through it meets a layer colder than its dew point. It is hidden, so it is usually found only after insulation has lost performance, metal has corroded or mould has appeared.

By Dhruv Agarwal · · 1 min read

Why it matters on a project

Surface condensation is visible: water on a window or a sweating pipe. Interstitial condensation is not. It wets insulation from the inside, which lowers its thermal performance, and it can corrode fixings and sheets and grow mould behind a finish long before anyone sees a sign.

The mechanism

StepWhat happens
1Humid air on one side of the build-up carries water vapour
2Vapour diffuses or leaks through the layers towards the drier side
3Temperature falls through the insulation towards the cool side
4Where a layer is below the vapour's dew point, liquid water forms
5If it cannot dry out between seasons, it accumulates

Air leakage often carries far more moisture than diffusion, which is why an airtight layer matters as much as the vapour retarder.

The Indian climate twist

Much building science literature comes from cold climates, where the inside is the warm, humid side. In hot and humid Indian regions, with air-conditioned interiors, the outside is often the warm, humid side for much of the year. The safe position for a vapour retarder can therefore be the opposite of what a cold-climate detail shows. Envelope requirements under ECBC 2017 address thermal performance; the condensation check is a separate step.

Common mistakes

  • Copying a cold-climate detail with the vapour retarder on the inside.
  • Two vapour-tight layers that trap moisture between them with no way to dry.
  • Unsealed laps and penetrations in the vapour retarder.
  • Broken vapour seal on chilled-water and duct insulation, leading to wet insulation and drips.
  • No calculation for unusual build-ups such as cold rooms or server rooms inside a warmer building.

Standards referenced

Calculation under ISO 13788; dynamic simulation under EN 15026; envelope thermal requirements under ECBC 2017. The layer sequence and vapour control for a specific build-up must be checked by the project's facade or envelope consultant.

Standards referenced

  • ISO 13788 — Hygrothermal performance of building components — internal surface temperature and interstitial condensation, calculation methods (ISO)
  • EN 15026 — Hygrothermal performance — assessment of moisture transfer by numerical simulation (CEN-CENELEC)
  • ECBC 2017 — Energy Conservation Building Code — building envelope (Bureau of Energy Efficiency)

Frequently asked

The temperature at which air holding a given amount of water vapour becomes saturated and starts to release liquid water. The more humid the air, the higher its dew point. Inside a wall or roof, condensation happens wherever a layer is colder than the dew point of the vapour reaching it.

From the warmer, more humid side to the cooler, drier side. In a cold climate that is from the heated inside outward. In hot and humid parts of India, with air-conditioned interiors, it is mostly the reverse for much of the year: vapour drives from the humid outside inward towards the cool interior. Guidance written for cold climates can therefore put the vapour retarder on the wrong side.

Generally on the warm, humid side of the insulation, so vapour is stopped before it reaches a cold layer. For an air-conditioned building in a hot humid climate that usually means towards the outside. Climates and uses vary, so the position for a specific build-up should be checked by a condensation calculation rather than copied from another project.

ISO 13788 gives a steady-state calculation, often called the Glaser method, which compares the temperature and vapour pressure through each layer over monthly conditions. Where moisture storage and drying matter, EN 15026 describes dynamic numerical simulation, which is more realistic but needs specialist software and input data.

In metal-roofed industrial buildings, where warm humid air reaches the underside of a cold sheet; in cold stores and server rooms surrounded by warmer space; in insulated facade spandrels; and around cold chilled-water pipes and ducts where the insulation's vapour seal is broken.

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