Last edited 10 Jun 2018

Interstitial condensation

Air will generally include moisture in the form of water vapour.

When air cools, it is less able to “hold” moisture. When its relative humidity reaches 100%, the air is saturated, and this is described as its ‘dew point’ temperature, or ‘saturation 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’. This is common for example when moisture condenses on cold windows.

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:

Mould growth, which is a cause of respiratory allergies.
Mildew.
Staining.
Corrosion and decay of the building fabric.
Frost damage.
Poor performance of insulation and reduced thermal resistance of other elements of the building fabric. This in turn can reduce the temperature of the building fabric, exacerbating the condensation problem.
Migration of salts.
Liberation of chemicals.
Damage to equipment.
Electrical failure.

Interstitial condensation tends to occur on the warm side of vapour resistant layers.

To prevent this, buildings need to be designed so that the temperature profile across the construction remains higher than the dew point temperature:

Vapour control layers (VCL) 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.
Cold bridges can be eliminated.
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.

Dew point calculators are available that can predict the likelihood of interstitial condensation in different constructions and for different environmental conditions.

Condensation in buildings is regulated by Approved Document C (Site preparation and resistance to contaminates and moisture) and Approved Document F (Ventilation) and further guidance is available in BS 5250 Code of practice for the control of condensation in buildings.