Condensation – A Case Study.

Original Article
December 11th, 2021


In this article, Alan McKeown, Building Surveyor at Plumb Stone Surveyors, shares a case study of a non-invasive damp survey he carried out. The scope of the survey, requested by the landlord of the property, was to determine the cause and extent of any dampness and recommended repairs. 

The property

The property was a mid-Victorian, two-bedroom semi-detached house that was occupied by a family of four: two adults, a young child, and a newborn baby. 

Figure 1: Front elevation

The wall construction was solid brick with a pitched roof covered in the original plain clay tiles. There is an extension to the rear and an attached outbuilding. The property was renovated around five years ago, including damp-proofing measures to the floors and main walls. The main walls were lined internally with insulated plasterboards. The property was in generally good condition, with some maintenance required to clear rainwater gullies and gutters. Biological growth was evident at the base of the main walls.

Figure 2: Biological growth externally 

Black spot mould was present to the door and window reveals on the gable side elevation and condensation was evident on the window surfaces and on the sarking felt within the roof space.

Figure 3: Condensation to windows internally

Figure 4: Black mould to corners of door reveals 

The electronic moisture meter readings taken recorded no significant dampness to the internal wall surfaces or skirting boards. The relative humidity was recorded between 70-73% throughout, and internal air temperature ranged between 19-23 degrees Celsius.

Based on the relative humidity and air temperatures in the property, dew point ranged between 13-17 degrees Celsius (dew point is the temperature at which water vapour in the air condenses). 

Figure 5: Dew point measurement 

Humidity explained
As we all know, water can exist in three different states – as a solid (ice), liquid (water) or gas (steam from a kettle). Air always contains some water in a gaseous form which we refer to as humidity. The warmer the air, the more water it can hold (so air which is at 20°C will be able to hold more moisture than air at 10°C). Relative humidity is the term used to describe how much moisture air contains at a given temperature, or to put it another way, if the relative humidity is 100% then the air is holding as much moisture as it can at that temperature.

A good range of indoor humidity is needed for both comfort and health and is between 30-60% during the winter months. Where relative humidity is high for long periods of time, mould growth can occur. By controlling humidity levels, you can reduce the risk of both mould growth and also dust mites. But houses can be too dry and where humidity levels are consistently below about 30% this can lead to an increased risk of respiratory illnesses.

Dew Point
The ‘dew point’ is the temperature that air needs to be cooled to (at constant pressure) for a relative humidity of 100%.

At this ‘point’, the air cannot hold any more water in the gas form, so water vapour will be released in its liquid form. To put it another way, condensation will occur when the dew point is reached and describes the capacity of the air to hold the water.

Let us assume the ‘weight’ of the water in the air is the same in both instances and is 8.7g/Kg dry air – then when the air is 20°C, it will be 60% relative humidity, but when it is 12°C, it will be 100% RH and condensation will occur.

Condensation is associated with ‘problem areas’ in a property because the temperature of those areas/points causes the air to cool to a lower temperature than the rest of the room, thereby releasing moisture in the form of condensation which would be retained in the air as water vapour in other parts of the room. So, condensation is often noted on or around windows or where there are ‘cold bridges’, perhaps where insulation has been breached.

Thermal imagery was used confirming areas of ‘cold bridging’ to window and door reveals and the rear of the fireplace.

Figure 6: Thermal image showing cooler ‘blue’ areas around window where condensation was forming

Figure 7: Note thermal difference between the uninsulated ‘blue’ backing to the fireplace (where mould was present) and warmer yellow areas

Most wall surfaces recorded in excess of 20 degrees Celsius. The areas of cold bridging identified all recorded below the dew point level – between 13-16 degrees Celsius.

The causes
The issues present are the result of high humidity and resultant condensation. A healthy dry home requires a perfect balance between heating, insulation and ventilation.

During refurbishment, the owners included significant levels of insulation to the walls and roof space, and a modern central heating system.

However, the ventilation measures are inadequate. These include an intermittent extract fan to the bathroom and trickle vents to windows. The extract fan was recorded as moving air at 8 litres per second (regulations for bathrooms are a minimum 15 l/s).

Due to inadequate ventilation, airborne moisture produced by the occupants is not effectively purged from the building and consequently, the levels of humidity increase.

This high humidity condenses against the cooler surfaces. In this case, the increased insulation to wall surfaces highlights the cooler nature of the reveals and rear of the fireplace – areas not covered by insulation. Water vapour condenses in these areas and eventually forms as mould growth.

Figure 8: Water marks visible

Figure 9: Black mould evident to corners of ceilings

Further moisture rises into the roof space and condenses against the cool felt and timbers. This rises through unsealed down lights and loft hatch.

Figure 10: Condensation to rafter felt

Figure 11: Mould growth evident to rafters

The remedy
To remedy the problem the humidity levels need to be reduced. This can likely be best achieved by improving internal ventilation. As a minimum, humidity activated extract fans to the kitchen and bathroom will purge moisture at the source. Other measures, such as Positive Input Ventilation (PIV), can improve air circulation throughout and help in reducing condensation.

Whilst ventilation will significantly improve the situation, it will not prevent areas of cold bridging or balance heating. However, by reducing humidity, the significance or impact of the cold spots will be greatly reduced.

Achieving even surface temperatures can be difficult without significant alterations of the surface coverings. There are other measures, that although not a complete solution, can improve thermal values. These include thermal paints or cork boards (as insulation).

Lifestyle habits in the home inevitably affect levels of moisture and humidity. To an extent, these habits can be better managed. However, this can often be difficult in busy households with newborn babies, particularly when the kitchen has no extraction system.

In summary, improvements in mechanical ventilation will be the most effective remedy.