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folder Humidity

Staying on top of humidity on site

Humidity is an unwanted problem for many construction sites, yet errors can be made in its removal. Ryan Stanley, moisture control sales & product manager Northern Europe at Aggreko, provides some insight into the issue and best practice for resolving humidity concerns

Our weather is becoming more extreme, with summers getting hotter and periods of heavy rainfall now increasingly common. Such extremes make humidity a more pressing issue for heating professionals and building engineers, which, if left unchecked, could impair both construction site work and finished properties for years to come.


Humidity is sometimes seen as a winter-specific problem, as summer’s less frequent rain leads to lower humidity levels. Yet the opposite is true – as the warmer weather expands the air, it also increases its humidity capacity. Consequently, while relative humidity may decline in percentage, the physical volume of moisture in the air remains the same as it would be in cooler conditions. 

Heating professionals therefore face a consistent challenge throughout the year. Indeed, the relative humidity (RH) of the UK’s largest cities is around 76%, according to World Weather Online. As the comfortably dry range for RH typically falls between 40% to 60%, these heightened levels are clearly an issue.

Moisture types and sources

Whether as free moisture – the surface water or water that has been absorbed into a material – or as water that is chemically bound into the cells of materials in the room, moisture can appear from several sources on a construction site.

Site workers, for example, can cause humidity levels to rise simply by breathing and perspiring, or by leaving windows and doors open, causing external, moisture-filled air to travel into the room.

Yet this is not an issue isolated to just personnel, as machinery and ventilation systems can also contribute to rising humidity. If plant equipment is operated in enclosed, poorly-ventilated spaces, this can affect the moisture levels in the air. Similarly, if ventilation systems installed on-site cannot keep up with high moisture levels in a space, humidity in the room will remain high.

Finally, the building materials themselves can cause humidity to rise. Specifically, water vapour external to the building can permeate into the space through material absorption or through cracks in surfaces, in a process known as water vapour diffusion. Water introduced into a space for cleaning or mixing into building materials can also contribute to room moisture as it evaporates, and materials introduced to hotter sites may release water previously absorbed into its cells, further increasing humidity.

Damaging humidity

Minimising moisture generated through these on-site sources is vital to building professionals, as it can find its way into even the smallest spaces, affecting things like electrical components. This poor humidity can, in turn, leave corrosive deposits that cause damage after drying, while also extending time required for adhesives, paintwork and cement drying and curing processes.

Yet approaching moisture control incorrectly may exacerbate these problems and cause additional, costly issues. This is especially a concern on high-value builds, like constructing luxury developments or restoring historic buildings. In such circumstances, costs can easily snowball.

Typically, building professionals may turn to vigorous heating to dry and reduce moisture in a space, but this is inadvisable. Heating moisture simply turns it into gas, which remains in the space. When the heat is removed, moisture will then return from the air and go back into the building fabric within 48 hours.

Best approach

Moisture equilibrium like this can only be truly disrupted using dehumidification equipment. Specifically, by addressing three distinct factors in harmony – heat, air movement and moisture removal – existing water can be extricated from buildings.

To permanently resolve the issue, a carefully controlled process is required. Firstly, molecules of water present within materials can be excited by adding heat into the space. These particles are then drawn out of the building structure. While this may have already happened to an extent in hotter summer months, additional heaters are needed to hasten this process in winter. After this, air must be circulated using fans to remove moisture and avoid leaving, before a dehumidifier can eliminate energised water from the area, permanently reducing humidity.


Essentially, dehumidification removes water vapours from the air completely by drawing air from the environment over a coil and reducing to a very low temperature via a refrigeration system. By cooling the air below the dew point temperature, it condenses and drains away. With airflow capacities up to 7,000 m3/hr, Aggreko’s industrial dehumidifiers are capable of drying out very large areas. 

Humidity is clearly an issue for UK building engineers over the winter, and will remain so into summer. Because of this, careful temperature and moisture control strategies should always be in place.


27 January 2021


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