Interstitial condensation

Interstitial condensation is where vapour forms within the building material.

Example include metal RSJ, attached metal stairs and cold chimney voids

Vapour condenses where the temperature of the vapour drops below dew point. This may not happen on the surface, but within the building.

Dew line

A series of dew points within a wall is known as the dew line. Interstitial condensation takes place when the temperature within a wall drops below the dew line.

Tips for interstitial condensation

  1. Identify a heat loss (thermal bridge) with a laser thermometer, when it is cold outside. RSJ (metal joist) are imbedded in walls and ceilings, typically used in rear extensions and loft conversions). RSJ are particularly prone to heat loss causing interstitial condensation when it is cold outside.
  2. Unlike normal condensation, there is often a stain associated with interstitial condensation. Furthermore it only normally happens when it is exceptionally cold outside.
  3. In a similar way to condensation and direct rainwater, interstitial condensation is unlikely to contain nitrates salts.
  4. Improve ventilation. The best solution is reduce humidity through improved ventilation from the source of moisture, bathroom, drying clothes, kitchens and respiration.
  5. Insulate around thermal bridge.
  6. In addition improve distribution of heat.
  7. Sometimes you need to stand back to understand the building’s construction, looking for likely thermal bridges, considering how it may have been extended or otherwise changed over time, consider hidden chimney breasts.

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