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What Causes Mould & Biological Growth On Walls?

Mould and biological growth will always crop up as an issue when temperatures rise. The issue of condensation and water ingress are consistent problems in the EWI community. We will look to address how mould and biological growth occurs and affects systems, and how you can treat and prevent them.

What is mould & how does it occur?

Mould, in biological terms, is a formation certain fungi take under certain atmospheric conditions. The conditions best suited to the propagation of fungi include high moisture and warm environments. It will tend to present itself in the form of patches on walls, and can potentially form in as little as 24-48 hours.

Walls tend to be porous by nature, which means they absorb moisture. This is a particularly pertinent issue in the UK as the housing stock is the worst insulated in Europe. The weather also does us no favours, given that it tends to rain an awful lot. As a result, walls can become saturated directly, or through the phenomenon of rising or penetrating damp.

graphic showing the comparison of home temperature loss after 5 hours
Comparison of home temperature loss, Mission Zero, Independent Review of Net Zero, Rt Hon Chris Skidmore MP

Damp and mould are ultimately caused by excess moisture in the area. Moisture can be produced through simple things like boiling a kettle or drying clothes indoors. Walls are porous from both sides, therefore can absorb moisture from both sides. Any introduction of heat to the environment will cause the water present to evaporate. If the ventilation strategy is not sufficient, mould is more likely to grow. Ventilation may not be sufficient due to a number of issues; some obvious facets to note are the absence of vents and ventilation bricks. However, you also need to consider issues like blocked-off chimneys. The chimney columns act as stores of cold air, which, in a warm home, present an area for more condensation to gather.

What is biological growth & how does it occur?

Biological growth refers to any form of growth that requires some combination of light and nutrients. This can include moss, fungi, algae, and various plants or weeds. Some of these can be highly destructive to the structural integrity of your home. For example, Japanese Knotweed is notorious for being essentially impossible to remove and disrupting brickwork. However, types of biological growth vary in what they require to propagate. Moss stays dormant in dry weather and then revives in rainy conditions. It proceeds to thrive in shady and moist conditions. Therefore, if a part of your wall is shaded by a tree and has no insulation, it can be susceptible to the growth of moss. The walls stay cold and moist, unable to stay warm whilst allowing the water vapour to evaporate. Other forms of biological growth are different;

‘Algae need moisture to grow, as well as light and nutrients. Walls which have surfaces which remain wet for long periods are likely to be more hospitable for algal growth than those which are totally dry or experience major swings in surface moisture levels.’ (SoGE, University of Oxford)

Mould spreads by airborne spores that settle on moist surfaces. Biological growth is similar in that sense, as it also uses airborne transmission.

  • Japanese Knotweed wall damage
  • Moss on a wall
  • Algae on wall

There are two ways to avoid running into these issues. You can take a remedial approach if your walls are already subject to mould and biological growth. You can also take a preventative approach; this takes several forms, ranging from insulating your walls to using a fungicidal wash or special silicone renders.

The remedial approach

Cleaning mould from walls is a fairly simple task. Mixing 1 part bleach with 4 parts water, and then scrubbing the areas affected gently with a cloth will permanently remove mould. However, it is crucial to dry the area completely afterwards so as to not provide a suitable environment for any more growth. Biological growth is similar; some form of fungicidal wash and scrub will do the trick. A pressure washer on a lower fan setting also removes any growth on external walls. Again, the walls need to dry fully afterwards.

EWI Pro’s Fungicidal Wash is the ideal solution in that it is very simple to apply, starts working within 24 hours, and crucially, prevents further regrowth for a period of 6 months.

The problem with the remedial approach to treating mould and biological growth is that it does not address the root issue. Walls will remain cold and can provide a haven for condensation and mould growth. The solution is to take a preventative approach to the issue; despite this, you will also require a remedial approach to prepare the house for the installation of preventative measures.

The preventative approach

The best way to approach the prevention of further growth is twin-pronged. Firstly, try to improve the insulation and ventilation strategy of your home. Secondly, render your home in products that actively resist the growth of mould and microorganisms. Both these solutions can be paired together to provide a unified resolution.

Insulation – External wall insulation installed on your property will lower the overall U-value. It will contribute to the capability of your walls to act as heat stores. The walls will retain warmth and proceed to release it back into the home. Insulation will also address the issue of thermal bridges, which are essentially heat highways; heat inevitably seeks the path of least resistance when trying to escape the thermal envelope. As a result of having warmer walls, condensation cannot settle on them, therefore you eliminate the environment for mould and biological microorganisms to propagate. Moreover, most external wall insulation systems will be breathable or vapour permeable, which contributes to the overall ventilation strategy of your household. All of this feeds into ensuring that mould and microorganisms cannot grow on your walls. A home that stays warmer for longer also requires less heating, therefore you save on bills.

Ventilation – There are various forms of ventilation, from passive to active. Passive ventilation includes features like air bricks or passive stack ventilation; ‘Passive stack ventilation (PSV) is the most effective natural ventilation strategy as it uses a combination of cross ventilation, buoyancy (warm air rising) and the venturi (wind passing over the terminals causing suction) effect.’ (Passivent)

Rendering as part of the preventative approach

Rendering is currently a highly technical process and has a rich history. With an abundance of products at various price ranges, renders deliver fantastic benefits and can mirror heritage features. However, when you are looking to protect your property from issues like damp and water ingress, silicone renders are the best option. Silicone renders, as you can tell by their name, contain silicone fibre additives that enhance certain properties. Silicone is a naturally rubberised substance that is more resistant to higher temperatures. This is not the major benefit in this instance, as most silicone renders are hydrophobic. The surface of the render acts as a barrier upon which water gathers into droplets and proceeds to wick off.

Certain renders have other additives that act in a fungicidal fashion. The render will resist biological growth, and releases additives to actively break down any biological growth. As a result, renders like EWI Pro Premium Bio Silicone Render and EWI Pro Nano Drex Silicone Render are extremely popular in areas of high vegetation and harsh weather. This includes areas like the seaside, which is subject to more adverse weather and can have higher water salinity levels.

Overall, it’s crucial to maintain your walls and prevent any biological growth, and the EWI Store Silicone ranges are the premier choice for this. If you have any more questions, leave us a comment!

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