Interstitial Condensation

Interstitial condensation costs homeowners a great deal of money in repairs and increased heating costs each year. The main problem is that many people are unaware of its existence and how it may affect them.

Interstitial or concealed condensation appears inside walls, floors and under roofs causing insulation to lose its effectiveness and can lead to damp, mould and rot. It is formed as moisture-laden indoor air seep through walls and insulation leaving behind a trail of moisture.

This process is normal and helps the building material to breathe. However, if indoor moisture levels are high interstitial condensation will soon begin to build up and cause damage.

The best way to prevent condensation from appearing is to control indoor humidity levels with the help of a dehumidifier. One high extraction dehumidifier should be sufficient for an average UK home provided the airflow between rooms and spaces remain unblocked. A dehumidifier also provides heating gains as the energy used to extract the moisture is returned back as heat.
Interstitial condensation explained

We know air contains moisture to a lesser or greater extent. It is also common knowledge that this moisture sometimes lead to condensation appearing on windows especially during the autumn and winter months. Sometimes it can also be seen on walls and other surfaces. Condensation is formed as air cools down on a surface where it loses its ability to retain moisture and resulting in water droplets forming.

The same process takes place inside walls and other cavities. Most building materials are permeable, that is, they allow air and moisture to pass through. During the passage through the wall the air will cool down and gradually lose its ability to hold moisture with interstitial condensation appearing as a result.

The only difference between interstitial condensation and the one appearing on windows or cars is that this one is hidden and therefore harder to do anything about in time. During warm and dry periods the sun and wind will help dry exterior walls and insulation. However, if the weather conditions do not change or, condensation appears in areas with poor ventilation or is blocked by sheathing paper or other materials it will not dry up. And, when left alone for a longer period, condensation can quickly start to cause damage such as mould, mildew, rot and smells.
Methods of prevention and cure

The process of transferring the air through the wall is pressure-driven with the air normally moving from the inside to the outside of a structure. This means the airborne moisture coming through the wall will originate indoors and therefore, it can be controlled.

The easiest way of controlling interstitial condensation is to control indoor moisture levels with a dehumidifier. The dehumidifier will remove any excess moisture from the air before it permeates the wall. This will see drier air pass through the inner wall and the insulation and while it cools down as normal it will still be able to contain all its moisture throughout the passage. A dehumidifier will also help heat the house by transforming the energy it uses into heat, resulting in a 1-2 degree temperature gain for the average home.

Many people believe heating will remove the threat of interstitial condensation but this is more likely to have an opposite effect. Warmer air can hold more moisture and is therefore likely to release greater deal of moisture during its journey through the structure.

Ventilation is another false friend in this scenario. During warm or dry days it is possible to get rid of moisture by opening door or windows or running an extractor fan. However, on cold or wet days this is can be a very costly and ineffective alternative for two reasons. Firstly, replacing indoor air with outdoor air on a cold day means you are replacing warm air with cold which leads to increased heating costs. Secondly, on damp days it is likely the outdoor air contains more moisture than the indoor which means venting will draw in moisture rather than getting rid of it.

Visible signs of interstitial condensation include buckling of walls or sidings, moisture blisters or staining of exterior paint (if this occurs indoors it is more likely to be a result of damp or wall condensation).
Probable outcomes of interstitial condensation:

Insulation less effective
Buckling of walls and sidings
Blisters or staining of external walls
Mould and musty smells


Vapour barriers
Dehumidification - heating gains - can lead to a saving of 7-10% on total heating cost
Controlling moisture levels indoors and ensures adequate ventilation, but beware of energy losses and security risks

Interstitial condensation - Sources

University of Strathclyde, Faculty of Engineering
Concordia University, Canada: Conceptualized Reference Database for Building Envelope Research
AIVC, Air Infiltration and Ventilation Centre