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Why does insulation make mould worse?

We've all heard of new insulation being installed, only for huge mould problems to erupt. But why is that happening and what can be done to prevent it?

Alex Brodholt
Alex Brodholt

Oct 20, 2020

It is a long-held tradition in social housing that when government funds are made available for improving the quality of housing stock, the main installations conducted are related to insulation. The current Green Homes Grant is no different and ECO3 before it. The uncomfortable truth, though, is that insulation is often causing more problems than it is solving. There is no denying that insulation helps reduce the cost of heating a home - which in turn reduces the rates of fuel poverty. This is a great thing - especially for those who are making the difficult choice between eating and heating. Unfortunately, as most housing providers have learned, it also can have dire knock-on effects on the health of a property and its residents. So why exactly does the installation of insulation cause mould?

The Basic Answer

The most simple answer is that by installing insulation, a housing provider is improving how airtight the property is. Take, for example, a property that has just had cavity wall insulation installed. Where before, the walls and windows were semi-permeable - allowing small amounts of air to flow between the inside and outside of the house, the new insulation has sealed them tight. These holes (usually around windows and doors) allowed for the natural ventilation of the property. To understand the full relationship between ventilation and mould you can view our piece on the ‘mould triangle’ here. The gist is that ventilation, no matter how small, allows lower humidity air from outside the home to replace the higher humidity air from inside the home. Every inhabitant in a property is producing moisture that is absorbed by the air in the home, and when that gets to a certain level, it produces condensation on surfaces in the house - the perfect conditions for mould to form. Despite the thermal inefficiency caused by a lack of insulation, in very basic terms, the ventilation was keeping the humidity levels from getting too high. Unfortunately, the solution to this isn’t just ‘don’t install insulation’. Residents deserve to be able to heat their homes appropriately and as efficiently as possible - which means that we have to dig a little deeper and look at a more complicated answer to the problem.


The More Complicated Answer

The truth is that if you as a housing provider want to install solutions that don’t cause additional problems, you can’t use a one-size-fits-all solution. By attempting to install the same solution across your portfolio, you are installing expensive measures in properties that don’t need them, as well as installing the measure in places where it will be actively detrimental to the property’s health. The complexity for retrofit installs is driven by factors that are outside of your control. Taking two identical properties, installing the same measures and then monitoring the health of the property over a year would see two very different results depending on resident behaviour, the health of the installed measures and the level of maintenance being done to the property. To truly improve a property without negatively impacting the quality of life of your residents, you need to look at properties on a one-to-one basis and be proactive in fixing unexpected problems. This ultimately brings us to the ultimate solution - taking a holistic view of a household.


The Ultimate Solution

Understanding that no two properties are alike now opens up a tonne of different opportunities to improve properties without installing unnecessary measures and without causing additional problems. Instead of using all of your retrofit budget to install the same (often expensive) measure in your properties, you can now deploy it only where it is truly necessary. In the case of insulation, this will mean focusing retrofit budget on the worst-performing properties in terms of heat retention. This will likely mean the installation of insulation in a smaller number of properties, but with additional ventilation systems (for example) to offset the reduced natural ventilation. The same budget will have permanently solved the problem in a proportion of properties instead of spending it on a larger number with fewer positive results. This does mean that you need accurate data on a properties condition (luckily Switchee can help with that), but once you can look at individual property problems the number of retrofit projects causing additional problems will drop.

Find out more about combating mould with our ultimate guide.mould-whitepaper-promo-min

Alex Brodholt

Alex is Switchee's marketing lead. He has a BA (Honours) History International degree from the University of Leeds. Prior to Switchee, Alex worked for property tech startup Home Made as well as for Farmers Weekly Magazine and leading AgriTech business Proagrica.

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