Are We Heading For a Condensation Crisis?

Gary Bundy, Technical Director of Sto, looks at the issue of condensation in buildings. With increasingly stringent requirements for thermal insulation and airtightness, are we heading for a condensation crisis? And how can this be avoided?

It is estimated that 15% of homes in England and Wales are affected to some degree by condensation. With building regulations demanding increasing levels of airtightness, condensation is an issue that needs consideration.

Buildings can suffer from two types of condensation – interstitial and surface.

Interstitial Condensation

The Problem

Interstitial condensation is perhaps the more insidious, as it is difficult to detect until significant damage has occurred to the building fabric.

Interstitial condensation occurs where warm, moisture-laden air from the interior diffuses into a vapour-permeable material such as fibrous insulation or a porous brick wall. This is most common in cavity walled structures where the cavity is filled with insulation. The air reaches its dew point within the cavity, depositing water in the insulation layer.

The result is the insulation material can become saturated, so it no longer functions as an effective insulation layer. Water droplets can also form within the cavity, leading to damage of structural elements: corrosion of metal wall ties or rotting timber frames. These may not be evident until significant deterioration has occurred.

When the dew point is reached in the outer brick wall, water will be deposited within the porous brick. In very cold weather, the resultant freeze/thaw action can cause spalling or cracking of the brick.

The Solution

One way to prevent the water vapour entering the wall and reaching the cold surface is to apply a vapour proof barrier on the internal wall underneath the plasterboard and decorative finishes.

A more reliable option is to ensure that the wall is sufficiently warm to prevent condensation forming, which is one of the objectives of external wall insulation. Using EWI, the temperature throughout the whole cross-section of the wall structure is above the dew point. Where EWI is applied to a masonry wall, any water vapour passing through the wall structure will reach the dew point on the outer face of the render. From here it can dry out and evaporate harmlessly.

In some structures, particularly light steel framed buildings, there may be parts of the construction where the temperature is always likely to be below the dew point. In these buildings, a vapour barrier applied to the warm side of the wall is the most commonly used solution.

Interstitial condensation is a particular challenge for existing structures where increasing the insulation in the cavity or on the internal wall could easily create a problem where none existed previously. This is why we at Sto believe that the application of external wall insulation is the most effective way to provide increased insulation to existing buildings.

Surface Condensation

The Problem

Surface condensation is more obvious to building occupants as it forms on interior surfaces that are significantly colder than the room temperature. Typically, this will build up first on wall areas that are hidden behind large pieces of furniture, such as wardrobes or mirrors, where air circulation is reduced.

The Solution

Increasing the insulation of the building fabric eliminates this problem, providing the detailing is accurate. Where external wall insulation is used, any problems are most likely to occur at ground and roof level.

It is essential that the insulation layer continues down to the damp proof course to avoid a thermal bridge developing at the bottom of the walls and that this joint is carefully detailed to prevent moisture bridging the DPC. This may mean continuing the EWI below ground level, and Sto provides special insulation, render and adhesive products designed to work in these permanently damp conditions.

At roof level it is equally essential that the insulation forms a continuous layer with the insulation fitted to the ceilings or underside of the roof. There are robust details for these junctions that need to be closely followed, relying on the expertise of the applicator to ensure the detailing is accurate.

The importance of detailing

Always ensure that there is a consistent depth of insulation fitted uniformly to the facade. This can be a challenge around openings, such as doors and windows, and is an issue that needs to be considered carefully at the design stage. In Passivhaus designs that use extremely thick insulation, it is not uncommon to find the window hung out of the structure, sitting within the insulation to avoid a thermal bridge.

Any penetrations of the external wall are potential areas of thermal bridging, but this risk can always be eliminated by using the right components and techniques.

Condensation risk will be increased if there are extreme fluctuations of temperature within a building. The most comfortable and best-performing houses are likely to be those that achieve a relatively stable temperature.

The best type of structure to achieve this stabilisation will be one with a high thermal mass, as this mass will absorb higher temperatures during the day (capitalising on passive solar gain, for example) and release the warmth during the night. A solid wall structure, insulated on the exterior face, provides an effective all-round solution: a wall that is consistently warm enough to remain above the dew point while also allowing the interior environment to capitalise on the benefits of high thermal mass.

New design and building techniques are leading us down a path that will end with zero carbon homes. So far, attention has been focused on how we achieve exceptional thermal insulation and airtightness. Increasingly common Passivhaus designs demonstrate how this can be achieved. The next focus will be on ventilation: to ensure that condensation is not caused simply by failing to extract water vapour from the building.