Site Humidity Levels: Riding Out the Storm


Managing humidity is an ongoing concern for building professionals working on construction sites that is growing in prominence. With the UK recently struck by storms, many projects have been disrupted by damaging floods and this topic is now rising back in the spotlight. Matt Watson of Aggreko explores how to effectively dry out sites without long-term issues.

The impact of these increasingly common events can be amplified without an effective dehumidification strategy in place, as residual moisture can pose a threat to structures, building materials and electrical equipment.

Heavy rain and increasingly hot summers are staple parts of British life, as our perception of what constitutes ‘normal’ weather shifts due to climate change. Nowhere is this more apparent than with storms since the turn of the year, which buffeted the UK with torrential rain and disrupted travel and services for many.

Site and project managers in the construction industry were one such group impacted by this disruption. Indeed, the humidity brought on by excessive rain did not only make onsite working conditions uncomfortable – in particularly affected areas, flooding was not just a possibility, but an inevitability.

Such events will only rise as British weather becomes more extreme, and in a country already affected by high humidity levels, will require round-the-clock maintenance schedules to be effectively managed.  Yet while humidity risks are most commonly associated with extended periods of wet weather, it is important to recognise that this challenge is not exclusive to rainy winter months. Moisture management is not an issue that peaks and troughs with the seasons, but instead presents persistent and varying challenges in different weather.

Different seasons, same concerns

Indeed, as a nation renowned for its heavy rain, many building professionals may believe winter months present only periods of high humidity levels, whereas summer is drier and so the problem is reduced. Yet this is a key misconception – during the summer months, the warmer weather expands the air, allowing it to have a larger humidity capacity. This thermal expansion means the relative humidity may show a lower percentage, but the physical volume of moisture is still the same as under cooler conditions. It is therefore vital to understand that humidity is a year-round challenge for building professionals.

When it comes to relative humidity – the percentage of water within an air particle – it averages at around 76% in our nation’s largest cities. When considering that the comfortably dry range for relative humidity falls between 40% and 60%, it’s clear that moisture in the air is a significant issue. Such a concern is further exacerbated by extreme weather events such as Storm Arwen and Storm Barra, which can lead to additional spikes in humidity.

Different moisture types

Whether its free moisture – the surface water or water that has been absorbed into a material – or the water that is chemically bound into the cells of materials in the room, there are several sources that are found on construction sites. The building materials themselves, for instance, can impact overall site humidity.

Water introduced to these materials for cleaning or mixing purposes can raise the moisture levels within a room as it evaporates, and these materials may also release water previously absorbed into its cells in hotter site conditions. Furthermore, water vapour from outside the building can permeate into the space via cracks or absorption into these materials, increasing moisture levels in the air. This process is known as water vapour diffusion.

Machinery and ventilation systems may also contribute to higher humidity, as if plant equipment is operated in poorly ventilated or enclosed spaces, moisture present within this space will rise. Similarly, ventilation systems installed on-site may not be equipped to reduce increased moisture levels, meaning humidity in the room will remain high.

Outside of materials and machinery, even site workers can contribute to humidity levels. Whether by leaving windows and doors open that allow moisture-filled air from outside to permeate a room through unintended ventilation, or by simply breathing and perspiring, on-site personnel should also be regarded as a factor when considering humidity.

Damage limitation

Premature deterioration of building materials is something any building engineer wants to avoid. In particular, poor humidity can affect things such as electrical components - the moisture can find its way into the smallest areas and spaces. This in turn can leave corrosive deposits behind that continue to cause damage after drying. Not to mention prolonged time for paintwork, adhesives and cement to dry or cure.

However, improper drying and moisture control can also exacerbate such problems while creating new ones, which could lead to further damage and potentially higher costs. In particular, building engineers working on high value projects such as luxury apartments or historic buildings should be aware of the potential costs that could mount up.

The extreme cost of heating

When it comes to building works, there are a number of common misconceptions that can lead to more time-consuming work, or even further damage. The main one relates to the differentiation between heating and dehumidification. Many believe that vigorously heating a space onsite is the best way to dry moisture, this method doesn’t actually remove the problem.

 By heating the space to ‘blast out’ flood damage, for instance, the atmosphere is expanded, drawing the moisture from affected areas into the air. Once the heat is removed, it only takes around 48 hours for moisture equilibrium to be restored, as it returns from the air back into the building’s fabric. This ‘invisible water’ can damage materials such as wood, plaster and paint, leading to long-term damage that could threaten ever-tightening project deadlines.

Effective removal

The drying process of any space relies on three key factors that must work in harmony to ensure that moisture is drawn out of the structure and is removed effectively – heat, air movement and removal of moisture. By following this process and controlling it carefully, building engineers can ensure the problem has been removed permanently.

Firstly, adding heat into space to excite the molecules of water within materials. It is at this point that they are drawn into the expanded atmosphere and out of the building structure. In hot summer months heat can already be present in the atmosphere, so additional heaters may not even be required as they are in winter. Then, once the water molecules are excited and moving throughout the space, the air must be circulated using fans to aid the removal of moisture and avoid leaving.

Finally, this is the point where a dehumidifier is used to remove the energised water molecules and therefore any moisture from the environment. Two methods can be used to do this, through either desiccant or refrigerant systems. The former draws air over a coil while reducing its temperature below its dew point so it condenses and drains away, with the latter expelling moisture as water vapour once it has been drawn and dried over a desiccant wheel. With airflow capacities up to 7,000 m3/hr, Aggreko’s industrial dehumidifiers are capable of drying out very large areas.

In conclusion, the recent stormy weather has once again placed a microscope on site humidity levels, and the damaging effect it can have on construction projects if left unchecked. However, building professionals must be aware that this issue is not seasonal – it is year-round, and requires action if potential disruption is to be controlled in light of increasingly extreme weather. Temperature and moisture control should therefore be implemented in building works as an ongoing concern, and subject to assessment and optimisation through each stage of the process.

Matt Watson is Moisture Control Expert for Aggreko Northern Europe

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