What can I do about a serious condensation problem in my home?

What is the best way to deal with a serious condensation problem on the inside of the side wall of an end-of-terrace house built in the 1970s? The condensation is very bad upstairs, where heating is limited. The two bedrooms have a vent and fly windows are left open. The wall is very cold to touch. Would external insulation on the side wall of the house be the answer?

I note that the house was built in the 1970s and for the purposes of this answer I am assuming that it is a typical 1970s house which has not been modernised or upgraded in the interim. It’s clear the problem is manifesting itself on the walls within the bedrooms at first-floor level and this is largely because these are typically the colder parts within the house. Moist air tends to rise and condense on colder surfaces.

To begin, we need to look at why condensation occurs. In this respect, there are a number of contributing factors. Firstly, and most significantly, the levels of relative humidity (moisture in the air) will affect the degree to which condensation occurs. Everyday activities such as cooking and showering give rise to high relative humidity levels. Drying clothes within the house is a significant contributing factor to a high relative humidity as all the moisture within the clothes evaporates into the air as part of the drying process. The air can hold only so much moisture, which will be dependent on the temperature, and basically the cooler the air, the less moisture it can hold. When the air becomes saturated, this gives rise to condensation occurring on the colder surfaces.

The second most significant contributing factor is the degree of ventilation. The greater the level of ventilation within the property, the less risk of condensation occurring, as good ventilation, such as extractor fans within a bathroom, ensure the warm moist air is extracted to discharge to the exterior.

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The third factor is the level of heating. As stated above, the warmer the air, the more moisture it can hold and, in effect, a poorly heated house or even the poorer areas such as upstairs bedrooms which are not heated full-time, will be prone to a greater risk of condensation.

The fourth factor is the level of insulation to the property; the warmer the house, the less risk there is of condensation occurring on a surface. In this respect, condensation will occur on the colder surfaces such as the windows or metal frames but also on poorly insulated walls.

In order to address the problem, all of the above factors need to be considered. Where possible, cut down on the levels of moisture vapour being created, so do not dry clothes in the house, for example. Look at improving ventilation levels. This could be as simple as opening windows regularly. However, with modern living, you need to ensure there are good levels of permanent background ventilation and mechanical extract fans in high-humidity areas such as the kitchen and bathroom. Also ensure reasonably good levels of heat are maintained within the environment.

The above issues do not involve significant spending but rather a change of user habit. However, the other issue to consider is the level of insulation and this is where your specific query comes in. Increasing the insulation in one element in isolation could result in shifting the location of the condensation to the next colder surface. So it is important the insulation standard is upgraded to the entirety of the external envelope. Basically, the envelope comprises the roof, the external walls, the windows/external doors and the ground floor.

In practice, the majority of heat loss is through the roof and windows so these are the areas that should be concentrated on first, when trying to manage a condensation problem. The walls are the next most vulnerable area, and there are a number of options to consider here, including applying external insulation, as suggested, or applying an internal insulation in the form of a dry lining on the inside face of the wall, or in the case of a cavity wall by pumping insulation into the cavity. The solution should take account of the specific situation and the lifestyle of the occupants.

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Applying external insulation is very useful, particularly when a house is occupied for long periods and the walls tend to act as a thermal store, in that these have to be heated up first in order for the system to be effective. If the occupants are gone for large periods of the day and just occupy the house in the evenings and early mornings, an internal dry lining system may be a more effective solution as this facilitates a quicker warm-up time, but it does not tend to hold the heat as well.

I note you are suggesting insulating the side wall only. This would be poor practice as it could give rise to cold bridging and, in reality, the net effect would be to change the location of the condensation from the side wall to either the front or the rear wall. For the application of external insulation to be effective, it would have to be a complete wrap around insulation covering the entirety of the external walls.

These are complex issues, and it is important to adopt a holistic approach when trying to resolve the matter. My advice would be to have the situation assessed by a chartered building surveyor or suitably qualified energy consultant. They will be able to advise you on the most appropriate course of action to suit the specific circumstances. There are also grants available for energy upgrades, and your energy consultant will be able to advise you on what specific grants are available and on how best to follow these up.

Val O’Brien is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland

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