Legionella – an open and closed case


14 April 2016
​Buildings that have high levels of insulation can lead to the legionella thriving. Here, Anthony Barnett, Marketing Manager at Armacell, looks at ways to reduce the risk.

​“Legionnaires’ disease is a severe pneumonia caused by the legionella bacterium. People are most commonly infected when they inhale aerosols from a contaminated source. These fine sprays are created by heating and plumbing pipework, air-conditioners, swimming pools and whirlpools.
The bacteria multiplies where the water temperature is between 20-45°C and nutrients are available. As more insulation is incorporated into building designs it can result in cold water pipes warming above 20°C and in these situations legionella is a risk.

​When this is combined with extended periods of stagnation it results in optimal conditions for bacteria growth. Traditionally, flushing systems have been used to beat legionella, although this is water intensive and relies on personnel carrying out the task on a regular basis.

​Summer presents peak conditions for growth of legionella bacteria because during the holiday period water stagnates and super-insulated buildings, combined with improved air tightness, cause internal temperatures to soar, making cold water pipes an ideal breeding ground for this hazardous bacteria.

​Elderly people are usually worst at risk of legionella or those who have pre-existing conditions, such as diabetes, lung or heart disease or have problems with immunosuppression. That makes it a particular concern in multi residential homes as well as hospitals and schools. On average there are around 15,000 legionella cases each year.


  • The temperature of hot drinking water pipes should not fall below 55°C
  • The temperature of cold drinking water pipes should not exceed 20°C
  • Hot and cold water pipes should be laid a sufficient distance apart to prevent temperature transfer
  • When installing pipework, ensure that there is enough space for insulation
  • Only closed-cell insulation materials should be used on cold water pipes to prevent condensation and energy loss
  • The insulation thickness on cold pipes should be roughly equal to the pipe outer diameter

Insulate hot and cold pipes

​One of the most effective preventative measures for reducing the risk of legionella is to insulate both hot and cold water pipes with suitable materials such as closed-cell insulation. That is because poorly chosen insulation can lead to the HVAC system operating in optimum conditions for the bacteria, which usually means the ‘cold’ water temperatures regularly rising to above 20°C, although poorly insulated hot water pipework could also create the same conditions if temperatures fall below 45°C.

There are a number of steps that heating engineers can take to maintain water temperatures at a level that doesn’t support bacteria growth.

In terms of pipework layout too, there are some simple steps that can be taken, such as ensuring that pipes for cold drinking water don’t follow the same routes or run adjacent to space heating or hot water pipes. Cold water pipes should also be protected against heat gain by ensuring sufficient clearance or, preferably, by insulating properly. Similar requirements apply for hot water pipes to protect them against heat loss.
​If hot and cold pipes are laid in one duct or wall cavity, or wherever water is not circulated regularly, we recommend using what is known as 100% insulation. That is where the insulation thickness roughly corresponds to the pipework outer diameter. For example, if the pipe is 22mm diameter, insulation thickness used would also be 22mm. This not only prevents legionella in hot water pipes but also protects the pipes against unnecessary energy losses. For cold water pipes, the insulation provides protection against freezing in cold weather as well as providing protection against unwanted temperature rises.

Suitable insulation materials also prevent condensation. The best material for this is again closed-cell insulation, which has a high resistance to water vapour diffusion, making it ideal for use on cold water pipes where condensation is likely to occur. Plus, by preventing moisture ingress and providing no ready food source, closed cell insulation discourages the growth of microbes. AF/Armaflex Class O closed cell insulation also has an additional anti-microbial protection built in, giving active protection against microbial growth.

Conversely, open-cell insulation materials, whether specified with or without a vapour barrier, do not sufficiently prevent moisture ingress as a result of diffusion. This means that there is a danger of water vapour in the air penetrating open cell insulation, condensing and saturating the insulation material. As the material becomes damp, its thermal conductivity increases and insulation properties deteriorate, causing greater energy losses, and a greater risk of hot water pipes creating conditions for legionella bacteria. Furthermore, corrosion and other expensive consequential damage can occur.

Contaminated air-conditioners, showers, drinking water, swimming pools and whirlpools are well-known sources of Legionnaires’ disease. However, the root cause is commonly errors made in the choice of insulation. One of the most important preventative measures to avoid this potentially fatal disease is to insulate hot and cold water pipes with closed-cell insulation.

To find out more about Armacell high performance closed-cell insulation, visit: www.armacell.co.uk.
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