After discussing the basics of condensation in the previous section, let's see now what are the main drivers behind it. Because condensation occurs as a result of an interaction between temperature and humidity, condensation can be driven by any of these two factors, namely:

  • Low temperature
  • High humidity

Condensation can be driven by either low temperatures, high humidity or a combination of both

1. Low Temperature Driven Condensation

This is the most obvious and most known form of condensation. It occurs when temperatures drop, typically during winter time, between October and April in the UK. So this is a seasonal problem that typically manifests during the winter months.

There is always a certain amount of humidity in the air. When temperatures are higher (e.g. in comfort range), the warm air "holds" the humidity in vapor form. Once temperatures starts dropping, the volume of the air and with this its moisture holding capacity decreases, discharging some of its vapor in form of condensation.

Where condensation occurs? On the coldest surfaces: typically bottom of walls, corners, cold window panes etc. The reason behind it: temperatures in a room are not uniform. Just because your thermometer shows 22°C for the room, it does not mean that you have the same temperature everywhere. Due to the cooling effect of the environment the external envelope of the building (walls, windows, roofs) can cool down significantly during wintertime. As a result, the temperature of the air also drops nearby cold surfaces, making these areas most liable to condensation. A temperature gradient can be observed nearby cold walls, temperatures dropping mm by mm in the immediate vicinity of a cold wall or a cold surface.

Temperature gradients in a room

Condensation can occur both on the surface or under the surface, so we can have surface or interstitial (in between the pores or under-the-surface) condensation. 

2. High Humidity Driven Condensation

We can have condensation occurring even in well-heated environments if there is high enough humidity present. In old buildings this is rarely due to internal moisture sources (e.g. drying clothes indoors during wintertime), but the most common cause is an external (hidden) dampness source, often from some underlying condition. These can include:

  • Some moisture ingress: often hidden or undetected; resulting in high indoor humidity. It can come from chimneys, gutters, water leaks etc.
  • Rising damp: there is an ongoing evaporation from the soil into the air. If a porous wall is built onto the soil, this evaporation will still take place but now into and through the wall capillaries. Due to their very small size and volume (typically micrometer to nanometer size), pores or capillaries connected to the ground become instantly saturated with humidity, resulting in an ongoing condensation. The evaporation of the wall surface when rising damp present is a very significant moisture source that is often misdiagnosed and completely neglected.
  • Traditional flooring built on soil: brick floors or flagstones simply laid on the soil (without a vapor barrier underneath) act as a large evaporating surfaces.
  • Cellars: cellar walls and floors are large evaporating surfaces that create high indoor humidity. A large percentage of this moisture condenses and recycles back into the walls.

Identifying & Solving the Problem 

In real life, the two sources of condensation often combine as both cold temperatures (cold walls) and excess humidity can be present. However, one of these factors often dominates and for the long-term resolution good diagnosis of the problem is vital.

Cold Walls

The most common sign of condensation created by low temperatures (cold walls) is the presence of black mold. Mold appears on cold surfaces, typically in corners, around window frames or behind large pieces of furniture.

The most common factors that can contribute to the cooling of the building fabric and the appearance of condensation, are: 

  • External rendering: the main role of a render is to protect the building from driving rain. As nowadays most rendering is cement-based - cement being a compact, heavy material with little porosity, thus very poor thermal insulator - it can significantly cool down the building fabric.  
  • Wall construction: solid walls with no cavity can be significantly cooled down by rain and winds, resulting in colder internal surfaces.
  • Modern cement-based plasters: similar to cement renders, modern cement plasters are much colder than their more porous lime counterparts.
  • Wallpapers: wallpapers act like giant "plastic bags" in a room. Any condensed humidity - instead of being freely absorbed by the porous wall fabric and gradually evaporated out - stays on the wallpaper's surface in form of tiny water beads, providing a breeding ground for black mold to thrive on.
  • Modern non-breathable paints: modern paints probably have the least contribution to the problem, yet painting is most often tackled first as it's one of the easiest things to address.

The root-cause of the problem here is the poor thermal insulation of (mostly) external walls.

High Humidity

Condensation can also be caused by the presence of high indoor humidity, which can be the sign of some other underlying problems. Condensation arising from high humidity or excess evaporation can be recognized from (some of) the following signs:

  • High indoor humidity: one just becomes aware of this issue while living in a building.
  • Damp or musty smell: damp or musty smell is a sign of ongoing moisture evaporation, usually from the walls (rising damp), floors or both. This becomes more pronounced behind closed doors in lack of ventilation or airing.
  • Humidity goes up in a room when the door is closed: as discussed above.
  • Dehumidifier extracts large amounts of water: when a dehumidifier is needed in a room and if that extracts a significant amount of water on a daily basis (e.g. about 1 liter or more per day), you almost certainly have an external moisture source that keeps "dumping" humidity invisibly (in vapor form) into that internal space. 

Heating and ventilation often won't solve these high humidity / evaporation related problems satisfactorily (although they can alleviate it) as it doesn't address the source of the problem. High humidity problems must be diagnosed, understood and if necessary, addressed as most of the time they are not "normal".

The diagnosis is not always easy as the source of the problem is often not obvious and/or requires specialist knowledge, measurements and a holistic understanding of buildings, materials and moisture movement. Do your best to debug it on your own using common sense, however don't hesitate to get professional help. A competent holistic dampness survey can save you time, frustration and money by helping you to get to the bottom of the problem quickly, avoiding (time consuming or sometimes costly) trial-and-error experimentation.

Summary

  • Condensation can be driven by either low temperatures (excessive cooling of surfaces) or excess humidity (excessive evaporation from the walls or ground), or both (in combination).
  • Condensation due to low temperatures indicates to a building insulation problem. Various solutions are available to alleviate or "manage" the problem, but to completely eliminate it the thermal insulation of the affected walls need to be addressed eventually.
  • Condensation in the presence of high humidity is often due to the presence of an external (invisible) moisture source. Ideally, this should be diagnosed, understood and if necessary, addressed - with professional help, if needed.