Condensation could be one of the simples and also one of the most complicated forms of damp to resolve. In principle condensation occurs when warm moisture loaded air comes into contact with a cold surface. The air temperature immediately to the cold surface suddenly drops and water condenses.  There are two basic kind of condensation, interior and interstitial.


Humidity is usually measured as relative humidity, which is the actual amount of water vapour in air expressed as a percentage of the maxim amount of water vapour that the air could hold. In terms of individual perception the relative humidity between 90-100% can be described as steamy; 70-90% as humid; 50-70% as comfortable; 30-50% as dry as below 30% as dehydrated.

Interior condensation

Interior condensation occurs as the name suggest on the internal surfaces in buildings. It is highly dependent on the level of humidity, temperature, ventilation, permeability of materials,  their texture and insulation value.

In recent years there has been an increase in condensation in domestic buildings mainly due to the rising standard of accommodation. Insulation properties of buildings have been increased and ventilation has been reduced which has subsequently led to condensation and very often mould contamination.

Apart from the obvious water droplet on the window surfaces the presence of condensation can be often diagnosed by the presence of specked mould. Condensation is typically associated with the coldest, most impervious surface and areas of the stillest air. In single pane windows the effects of poor insulation and increased ventilation counter act and mould contamination or condensation does not occur. However when old windows are replaced, the ventilation rates are drastically reduced especially if such windows are fitted with blinds and curtains.

Once double glazing is installed, the next coldest surface will most likely be the wall, ceiling or floor next closes to the exterior. Particularly at risk are wall corners, alcoves, voids, cupboards and wall behind large pieces of furniture.

Preventing interior condensation

There are several measures that can be employed to tackle interior condensation:

Interstitial Condensation

This is a hidden kind of condensation which happens in between layers of construction or decorative components. If it happens on moisture resistant surfaces its effect are negligible but if vulnerable materials are involved the effects can be disastrous.

As water penetrates the indoor surface finishes and travels deeper into the building structure it encounter progressively colder surfaces. If a dew point temperature is reached on any of these surfaces interstitial condensation will occur.  Timber framed houses with impervious external rendering are most vulnerable to this kind of condensation.

Condensation very frequently appears in wrongly constructed or refurbished lofts where bituminous sarking has been applied just below the tiles and mineral wool insulation was put between the floor rafters. Water condenses on the bitumen sarking and drips onto the insulation causing damp and rot.

Solution to this condensation problem is installation of permeable sarking , impervious vapour check between ceiling and roof rafters and increase in ventilation of the loft.

Similarly the same problems affect the wall structures and in particular if impervious external cladding has been added to the already existing timber structure building.