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Interstitial Condensation

Condensation occurs when water vapor in the air condenses into a liquid, and usually occurs when there is a temperature difference between the air itself and the surface it lands on.

A good example of condensation in effect is when you have a cold can of juice on a warm day and water droplets form on the outside. This type of condensation is known as surface condensation.

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Interstitial condensation uses this same principle; however, it occurs when the moist air vapour penetrates through the elements of a building’s fabric, experiencing a temperature difference along the way, and condensing not on the surface but within the building fabric itself.

Therefore, in terms of condensation within a property, there are two main types of condensation: surface condensation and interstitial condensation.

Surface Condensation

Surface condensation occurs on the visible surfaces inside a building, just like when the windows inside your home steam up on a cold day. The warm air inside your home condenses on the cold surfaces, and if left to continue it can damage the interior fabric and lead to mould growth.

Interstitial Condensation

Interstitial condensation happens within the thickness of a building’s elements, for example inside the floors, walls or roof. The warm air vapour passes through the surface and cools down within the structure, leading to condensation which can result in the deterioration of the property fabric and structure.

What is Interstitial Condensation?

When air cools down, it releases moisture as condensation, and this is known as the dew point temperature. If this dew point temperature is reached within the fabric of a property such as roof structure, walls, and floor cavities, it will deposit water within these components and cause what we call interstitial condensation.

It is common in properties with cavity walls with insulation inside them, which allows the air to reach the dew point temperature inside the walls, depositing water in the insulation itself. In this case the insulation becomes soaked and therefor will lose its insulation properties and its thermal efficiency. This then has a knock-on effect as the condensation problem is exacerbated due to the cold walls and cool interior of the building.

Interstitial condensation can be tricky to diagnose, so a specialist surveyor with experience in property damp is needed to accurately assess potential condensation problems. Some of the main signs of interstitial condensation include:

  • Corrosion of building components
  • Damage to the building fabric and structure
  • Salts appearing
  • Reduced thermal efficiency as fabric and insulation becomes saturated

How Do I Stop Interstitial Condensation?

The most effective way of preventing interstitial condensation is by controlling the moisture levels within the building itself. There are a few ways in which this can be done, but one of the most popular ways is to install a ventilation system that can remove the moisture laden air in these humid areas.

Also improving the thermal efficiency of the walls and floors of a property will increase the temperature within the structure taking it above the dew point temperature when condensation would occur.

Interstitial condensation can occur on both old and new properties. Newer properties can suffer from it if they have property defects. Older properties can also be affected due to property defects or lack of insulation causing cold walls or floors.

Positive Input Ventilation (PIV)

Positive input ventilation (PIV) works as a complete property ventilation system, supplying fresh, filtered air into a building at a continuous rate. This constant flow of fresh air drives out stale and moisture laden air that builds up inside houses due to cooking, washing and general human activity.

By controlling the level of moist air in the property to normal levels stops condensation forming within the walls and surfaces of the property. It also has other positive effects such as improving indoor air quality and energy saving benefits.

The average family can release over 15 litres of water a day into the air, just by breathing, cooking, showering and washing. This is enough to raise the air moisture level, known as the relative humidity, inside the home to over 70%, which can result in condensation and mould growth. A PIV system can bring this level back down to an acceptable figure of around 50%, which is considered ideal.

PIV systems are a very common method of ventilating homes. As a ventilation method, it is supported by a BBA certificate and it is recognised as an alternative method to extraction-based ventilation.

Damp Source Detection

Damp in the form of interstitial condensation can be hard to accurately diagnose, so specialist equipment and experience is needed to confirm if a property has this type of condensation. There are three main ways that interstitial condensation can be tested:

Surface Moisture Testing

This is done with an electronic moisture meter which indicated the level of moisture on the wall surface.

Below Surface Moisture Testing

This is done with an electronic moisture meter which indicated the level of moisture in the thickness of the wall by drilling small holes in the wall and inserting deep probe sensors.

Infrared Cameras

Infrared cameras are used to detect the surface temperatures of the building and thermal performance. Since damp or wet walls are cooler, they will show up on the infrared camera. They are a good method of finding moisture quickly, however they may or may not be sensitive enough to find a damp area and should always be used alongside the other methods.

If interstitial condensation is suspected then you may require to open up sections of the walls and floor to inspect the void or material within the wall thickness to visually inspect the condition of the structure.

What to Do Next?

If you are concerned that you may have a damp problem in your property, or you have noticed any of the signs of condensation that we have mentioned, it is important to get the problem looked at by a professional damp expert.

Our qualified damp surveyor will investigate your property and identify the cause of the condensation. At Richardson and Starling, our experienced and PCA certified surveyors are highly trained in the types of damp and rot problems that could affect your property. This means that our surveyor will be able to complete the survey and give the appropriate remedial recommendations no matter what problem is identified.

No charge will be made for the survey if you own the surveyed property and it is located within 25 miles from our nearest branch. An administration fee may apply if a written report is required (a charge may apply if the property is rented out or is situated further than 25 miles). Contact your local branch for advice or a property survey.

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