Interstitial condensation is the term for water condensing within a material (for our purposes – within a wall or ceiling). Interstitial condensation a complex concept in the context of a damp survey.

There are 3 distinct types of interstitial condensation

1) Interstitial condensation around external metal

condensation around a cold spot
Condensation on cold metal – inside / outside.

Easiest to see is condensation around metal objects touching a wall creating a localised cold spot in and on the wall. Metal is a poor thermal insulator cooling more rapidly than the wall. Water condenses out of humid air around the coldest spot first.

Typical places to find this type of interstitial condensation is;

  1. Wrought iron downpipes and fixings.
  2. Electrical wires and around electrical boxes.
  3. Embedded cold water pipes.
  4. In older buildings, pipes left by gas lighting.

The easiest way to check for this is type of interstitial condensation is at the other side of a damp wall for the metal attachment, use a thermal imaging camera, trace pipes and electrics and use a metal detector.

2) Interstitial condensation around metal joists

interstitial condensation around metal
Interstitial condensation around metal joist.

When metal is extremely cold, such as during the “Beast from the East” in March 2018 and humidity high, then condensation can form within a wall or ceiling, also known as interstitial condensation. Notable characteristics of interstitial condensation are;

  • No single entry point – here the RSJ is consistently cold and causing condensation throughout its length.
  • Light staining, often rust, but not tea-bag like of penetrating damp.
  • Dribble marks (bottom right corner of image),

Interstitial condensation is common with rear extensions or support for cut away chimneys.
Example of Damp Survey and report for interstitial condensation London March 2018

3) Interstitial condensation forming within a stone or brick wall

Interstital condensation
Interstital condensation – what is happening?

Harder to recognise and understand is interstitial condensation within a homogenous substrate (such as a wall – where the thermal properties of the wall are consistent – no cold spots).

Looking at the right-hand image in Cambridge. There is a band of calcium sulphate salts, but the line clearly is not horizontal. Why?

The reason is the plant marginally changes the heat and evaporation (or airflow) characteristics across the wall. Interstitial condensation causes salts within the wall to move to the evaporation surface at the dew point line. The dew point line is modified by the the plant.

Interstitial condensation Corpus Christi College - Cambridge
Interstitial condensation Corpus Christi Cam.

Look around you and you’ll see examples of salts bands that are not horizontal, like this picture taken at Corpus Christi College Cambridge.

So what does it say about salts bands that are in fact horizontal? Could these be signs of Rising Damp?

NO, salt bands are generally good examples of interstitial condensation. The reason that they are horizontal and at the bottom of a wall is that;

It will be horizontal if wall and air temperatures rise horizontally and consistently, which they do normally. The base of a wall has less external air movement (evaporation) and therefore higher humidity.

Interstitial condensation

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