Condensation and Mould in Old Buildings

What is Condensation?

Condensation is the liquification of invisible water vapours present in the air in contact with cold surfaces.

Condensation is often associated with the growth of mould as the tiny liquid droplets (condensation droplets) on the surface provide bacteria the necessary "fuel" to thrive, resulting in mould growth.

Condensation and mould growth is a common seasonal problem in Britain, primarily affecting old buildings during the colder winter months between October to April. Condensation is generally not a problem during the warmer summer months.

What Causes Condensation?

Condensation occurs when warm humid air comes in contact with cold surfaces, mostly external walls. Water vapours in the air are slowed down near cold surfaces then attracted to them, making vapours to accumulate and liquefy on cold surfaces.

The 2 key variables determining the extent of condensation are:

  1. The surface temperature of the wall fabric: colder walls trap more of the available moisture, causing more condensation.
  2. The indoor humidity: higher internal humidity means more available moisture to liquefy or condense.

Of these two variables, low surface temperatures or cold wall surfaces - usually attributed to insufficient heating - are considered the main cause of condensation. However, this is only half of the story - there is more to it.

In our experience, especially in old buildings with solid walls, the other variable, high indoor humidity, tends to be a more important factor in the formation of condensation - something that is frequently overlooked or misdiagnosed.

To better understand the relation of these two variables and how they impact old buildings, let's look at them in a bit more detail.

1. Cold Wall Surfaces

External walls can be cooled down by the external environment, leading to condensation problems driven by low temperatures

But which part of the walls is the coldest and thus most susceptible to condensation? Long-term measurements done with embedded micro-sensors, however, revealed an interesting fact: the core of old walls was warmer in wintertime than the surface. Although this might sound counter-intuitive at first sight, it does make sense as old walls also accumulate and store some heat (thermal mass) keeping the core of the walls warmer than the environment. What cools down wall surfaces is the ongoing evaporation, making evaporative wall surfaces susceptible to condensation.

This condensation is common in newer buildings, typically built from the 1930s onward, often with external cement render, internally decorated with modern cement plaster, modern emulsion paint or wallpaper.   

mould modern

Condensation and mould in a 1940s building

And this leads us to the second variable... high indoor humidity.

2. High Indoor Humidity

What can cause high indoor humidity? In addition to man-made moisture, evaporating damp walls and floors are one of the largest sources of indoor humidity in old buildings. How much water can old walls absorb from the ground, store and evaporate out? Lots! These calculations from a British scientific research paper will give you an idea.

Those solid walls standing on the damp soil connected to the water table are constantly absorbing moisture from the ground (rising damp), then evaporating it out into the internal space. This results in constant high indoor humidity in the building, which condenses on colder wall surfaces. Hence condensation can also occur as a consequence and accompanying phenomenon of rising damp. The lack of a damp proof course makes old buildings susceptible to rising damp, resulting in (secondary) condensation as a result of high indoor humidity.

This condensation can often present in several hundred year-old buildings, which despite being built with lime (breathable) and being "leaky" (not airtight by design), can have very high indoor humidity as a result of rising damp. This effect of rising damp - high vapour content indoors leading to condensation - are often overlooked or misdiagnosed.  

old condensation

Condensation in an old listed building due to excess indoor humidity

The two leading causes of condensation - cold walls and high indoor humidity - can overlap and be present at the same time and both contribute to condensation problems - so good diagnosis is essential for the full resolution of the problem.

Diagnosing Condensation Problems

When diagnosing condensation and mould problems it is important to do both temperature and humidity measurements on the property to establish whether the condensation is primarily caused by cold walls, high indoor humidity or both. High indoor humidity can indicate the presence of some underlying hidden moisture source in the building, this can be rising damp, complete lack of ventilation or something else - there is always a cause behind it that needs to be found.

Just installing robotically some extra ventilation in the building to lower the high humidity won't necessarily solve the problem and without understanding hat's going on it's poor practice.  

Here is a simple test one can do to diagnose the problem. Run a dehumidifier in the building or in the affected room(s). If the dehumidifier consistently extracts large amounts of water from the air - typically half litre or more per day - that's a strong indication that there is a hidden moisture source present somewhere in the building that keeps generating the excess humidity the dehumidifier collects.


Dehumidifier in an old farm building extracting a lot of moisture on a daily basis

Additional tests can be done by taking absolute humidity readings:, this usually require specialist instrumentation. One often finds that when a hidden moisture source is present in the building, the absolute moisture inside the building is higher than outside. This, in plain English means that there is higher humidity inside the building than outside; or the air inside the building is more saturated than outside. This is not a good thing.

The differences between the absolute and relative humidity is explained in detail here

absolute humidity

Professional humidity measurement showing very high indoor absolute humidity - 11.21 g/m3 

Solving Condensation Problems

Although the theory behind why condensation occurs is simple, solving it or eradicating it can sometimes be more complicated.

One is generally recommended to start with simpler actions and gradually progresses towards more complex interventions until the problem is eliminated. Some of these simpler actions one can take can be:

  • Improving the ventilation of local areas by pulling the furniture awa from the walls.
  • Improving the general ventilation of the building by opening the trickle vents
  • Improving the heating in the building

If these do not resolve the problem, more complex actions are required that need professional help. Some of these can include:

If you have condensation problems and need professional help or advice, feel free to get in touch - we will do our best to assist you; we have the expertise and an extensive range of tools to diagnose virtually any type of dampness problem in any building.

Related Pages

You can read more about condensation on the following pages:

Val Juhasz is an Electronic Engineer with a Master’s Degree from the Manchester Business School (MBS), currently undertaking a PhD in Architectural Conservation. He is researching various aspects of moisture movement in porous materials and the effect of the electromagnetic environment onto the movement and retention of moisture in old buildings. He is the co-founder of Core Conservation Ltd, a building conservation company advocating traditional, building-friendly materials and practices. He regularly attends conservation workshops and seminars in Italy and Venice, learning about the traditional Roman architecture, materials and technologies. He held numerous talks and CPDs to private architectural and surveying practices throughout the UK.