What Is Interstitial Condensation?

This is a common consideration in modern construction but for those who aren’t clued up on what causes interstitial condensation, here’s a brief guide…

When we breathe we omit moisture into the air, when we have a bath or shower moisture will form and even when we wash the dishes or empty the dishwasher, steam may form. It’s this combination that may be attributed in making the internal air more moisture rich.

As air cools, it is less able to “hold” moisture and when the humidity reaches 100%, the air is saturated, known as its “dew point” temperature. If the air continues to cool, moisture will begin to condense. Where this condensate forms on a surface, it can be described as ‘dew’, hence the term ‘dew point. 

Interstitial Condensation

When the dew point temperature is reached within the fabric of a building’s construction, either on the surfaces of components that make up the fabric, or sometimes within the components themselves, this is known as ‘interstitial condensation’.

Interstitial condensation commonly occurs when moist air permeates through elements of the building fabric, across which there is a temperature difference, typically, but not always, when warm, moist, internal air moves towards the cooler outer parts of external walls or roofs, driven by a pressure difference.
If the temperature drops sufficiently, the dew point may be reached within the fabric and condensation will occur. If this happens, for example, in the outer leaf of a brick cavity wall it may cause no more problems than rain does when it wets external walls simply evaporating over time.

However, if it occurs in the inner components, or in insulation it can cause problems which include:

  • Mould growth, which is a cause of respiratory allergies.
  • Mildew and staining.
  • Corrosion and decay of the building fabric.
  • Frost damage.
  • Poor performance of insulation.
  • Damage to equipment and belongings.
  • Electrical failure or faults.

To overcome the risk of Interstitial condensation, buildings need to be designed so that the temperature profile across the construction remains higher than the dew point temperature. This can be achieved by implementing some or all of the following:

  • Vapour Control Layers (VCL’s) can be positioned on the warm side of insulation, however, these layers must be carefully sealed and penetrations avoided.
  • Materials with low vapour resistance can be positioned on the cooler side of the construction (although this can be problematic where for example the external cladding is impermeable).
  • Ventilated cavities can be provided near the cooler side of the construction.
  • The moisture in the building itself can be reduced by replacing flue-less gas or oil heaters, improving ventilation and so on. Ventilation can be humidity activated.
  • The internal temperature of the building can be increased. Heating can be thermostatically controlled.
  • A HVAC (Heating, ventilation and air-conditioning) system can be installed to help mechanically control the internal temperature of a building.

When we specify Spray Foam Insulation, our process undertakes calculations to determine the risk of Interstitial Condensation and what controls can be put in place to manage, eliminate or counteract the threat.

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