Interstitial condensation is a form condensation that occurs in the wall itself and can’t be seen until water damage is caused. Waterproof render can be used to combat it.

Combatting the ‘invisible’ damp threat when walls can no longer breathe

It’s said that condensation issues affect around 15% of properties, and with the more recent buildings regulations demanding new builds should be more air tight, it’s an issue that’s likely to increase. Two types of condensation can affect a building – interstitial and surface.

Interstitial condensation – the ‘stealth dampness’ enemy

Unlike surface condensation that can easily be seen, interstitial condensation is less apparent because it forms inside the wall itself.

Many houses and buildings are designed to ‘breathe’ so that air from the inside should naturally pass through the wall to the outside. If it can’t do so – perhaps due to cavity wall insulation or when the outside wall has been covered in a non-breathable material such as cladding – the air becomes trapped within the wall structure.

Once the air reaches its dew point and moisture forms, then problems can begin; timbers can eventually rot, metal ties will soon corrode, and insulation will become saturated with moisture so unable to do its job effectively. Brick work can also eventually crack as the ‘freeze thaw’ effect strikes when temperatures change.

Modern living exacerbates interstitial condensation

Everyday living activities such as cooking, drying clothes on radiators, showering and some heating generates moisture. If the building has no way to breathe then moisture entering the wall can’t escape, and so increasing the amount of moisture and adding to the problem.

How can you tell if there’s a problem?

Eventually mould will likely appear on a surface. Unlike mould from surface condensation that tends to form at the corners of a room, mould caused by interstitial condensation usually forms across a large surface area of a wall – maybe from top to bottom in severe cases.

How can interstitial condensation be prevented?

Waterproof rendering can be applied to walls, both internal and external, to stop moisture from penetrating through and inside a wall. It is made up of a sand, cement and water mix, the quantities and the even mixing of which is vital to its performance.

There are different renders available and it is a silicone render which is water repellent and breathable meaning it allows moisture to escape from the wall rather than becoming trapped. Lime mortar can be used instead of cement and is commonly used when rendering older buildings. Additives can also be added to the mix which can affect bonding and drying times.  A waterproof render is also an essential component when treating a damp problem such as rising damp.

Adding ventilation

In the warmer summer months, you may have windows open so natural ventilation will help moisture to escape. In winter, extra ventilation may be required in the form of Positive Input Ventilation (PIV) units or Heat recovery units. These help to control indoor moisture when it’s at its most severe. It is also essential to evacuate moisture laden air at source. Therefore it is essential a suitable extraction fan is installed in areas which produce high volumes of moisture laden air such as bathrooms and kitchens

Diagnose carefully

If you think a property is suffering from condensation issues, the ultimate advice is to diagnose carefully. A damp and condensation survey is always advisable as the approach to treat interstitial condensation and the resulting problems can be different to treating surface condensation. A wrong diagnosis can end up being a costly exercise.