How to remove harmful moisture from historic churches

Churches across Europe are generally heated in the winter and left unheated throughout the summer. On top of this, they accommodate varying influxes of people at different times of year. For example, a regular service may only fill a small section of the space, whereas a church may be packed during weddings and christenings.

The changes of seasons and the number of people in a church are among the many factors that result in fluctuating temperatures and therefore, dramatic shifts in relative humidity. When left unregulated, this can make the atmosphere feel uncomfortable and cause buildings to deteriorate at a much faster rate.

Why is relative humidity important?

Relative humidity (RH) is a percentage that tells us the amount of water vapour in the air, relative to the amount of water vapour that can be held at its current temperature. RH is directly linked to temperature because warm air can hold more moisture than cold air. For example, there will be more moisture in the air at 50% RH when the temperature is 25°C than there would be at 10°C.

When air rapidly cools, RH levels can reach 100%, or the ‘dew point’. This means that the air, at its current temperature, has reached its maximum water content and will release excess moisture in the form of condensation. This can spell big trouble for any historic building, but there are some common elements in churches that make the danger even greater, including:

• Architecture
  When heating is turned on, a church’s high ceilings will warm at a faster rate than the parishioners sitting below, this can result in varying relative humidity levels in the space.
  
  When warm, saturated air comes into contact with poorly insulated walls or single glazed windows that are colder than the air temperature, condensation is likely to form. This can quickly lead to rising damp and mould growth throughout a building. Moreover, as water seeps into stone and masonry work, cracks will start to appear in the structure and, in severe cases, entire sections of a building can crumble away.

• Furniture and decorations
  Traditional furniture, wall paintings and artwork will often bear the brunt of unregulated humidity levels inside historic churches.
  
  Depending on which way they are facing, wall paintings will be subject to different changes in temperature and therefore varying levels of relative humidity. Over time, this puts them at risk from mould, salt efflorescence and flaking.
  
  Because it is a hygroscopic material, wood expands as it absorbs moisture in the air. As humidity levels drop, the wood will begin to release this moisture back into the atmosphere and contract. During this process of expanding and contracting, wooden furniture can change shape and become fragile and distorted.

• Pipe organs
  A centrepiece for many churches, pipe organs work in unison with the acoustics of the building to fill the room with sound. However, they are also very complex instruments, and the impressive sound they make relies on the air vibrating at a certain frequency determined by the density of the air.
  
  This, in turn, is affected by relative humidity levels and means that an organ will only play at the correct frequency when the temperature and relative humidity is the same as when the organ was first tuned.

How a portable dehumidifier can help

By circulating humid air throughout a space, dehumidifiers can physically remove moisture from the air. Used in combination with heating, this ensures a comfortable environment for people inside a church while protecting the building's historic significance.

Portable units from AERIAL are some of the most efficient and cost-effective systems available. They are designed with ease of use in mind and, once plugged in, they will run quietly in the background with minimal disruption.

How dehumidifiers work

The process of dehumidification involves moisture-laden air (return air) being drawn into a dehumidifier where the air passes across a refrigerated coil. The air is rapidly cooled below its dew point, condensing the water vapour and recovering its latent heat energy for re-use. The cooled air is then passed across the condenser where it is reheated and (supply air) returned to the served area at the required lower humidity.

The process:

1. Damp air is drawn into the dehumidifier and across a refrigerated coil

2. The air is cooled below its dew point, condensing the water vapour

3. Latent heat energy is recovered and re-used

4. Cooled air passes across the condenser and reheated

5. The warm dry air is then returned to the room at the required humidity
   

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