Avoiding the Risk of Mould and Underheating

The homes we live in should obviously play a positive role in our health and financial wellbeing. In 21st century Britain you would assume this is a given for everyone, whether you own or rent your home. Well sadly that is not the case, as was recently highlighted by the tragic death of two-year-old Awaab Ishak, resulting from prolonged exposure to mould in the housing association home he shared with his family. 

How common are unhealthy homes in modern Britain? 

According to the 2019-2020 English Housing Survey, 3.5 million occupied homes did not meet the Decent Homes Standard with serious damp problems affecting almost a third of these. Unfortunately it is often the most vulnerable in society that pay the price for poor living conditions.  

Why are the poorest in society at highest risk? 

Damp and mould occur disproportionally in the homes of the poorest in our society. Whilst this is caused by a number of different factors, the inability to heat homes to a safe level underpins most occurrences. An underheated home, which includes any home heated to less than 18°C as highlighted by the Public Health England 2014 Report, also has a direct effect on your wellbeing, Health problems and diseases related to a cold home include increased blood pressure, more susceptibility to the common cold, pneumonia and even an increased risk of heart attacks. 

Why are mould and damp bad for your health? 

They significantly increase the risk of respiratory problems, respiratory infections, allergies and asthma. Damp and mould can also affect the immune system. This is because mould produces allergens and irritants, and can even create toxic substances. The Housing Ombudsman Service’s Spotlight on Damp and Mould 2021 report highlights that health and wellbeing is in fact one of the areas most affected by damp and mould in almost every case that they assessed. 

The cost of poor housing in England 2021 by the BRE highlights that the combined effect of these health-related issues currently costs the NHS in England £1.4bn a year directly. When societal costs, such as those relating to care, loss of economic potential, poorer educational achievement, loss of productivity, career prospects and mental health are included they estimate that £18.5bn is lost as a result of sub-standard housing. 

What causes mould in homes? 

Mould is caused by excess moisture and condensation in the home. These are best explained in the following way: 

  • Rising damp is when moisture travels from the ground up through a masonry wall, by a mechanism known as capillarity. It normally rises about one metre above the ground, however the size of the pores in the structure dictates the extent of the moisture being able to rise. The smaller the pores, the greater the height 
  • Penetrating damp / Falling Damp occurs when water from outside penetrates into the fabric of a building causing damp internally. There are many possible causes such as cracks in render, mortar pointing erosion, blocked gutters, a leaky downpipe or a slipped roofing tile. 
  • Condensation generally happens when a property can’t cope with normal levels of water vapor that occur from ordinary occupation of the building. This can happen when there is insufficient insulation, ventilation, heating, or excessive moisture creation by occupant behaviour such as drying clothes on radiators, shutting trickle vents or a combination of these issues. 

So in summary, rising damp and penetrating/falling damp are caused by a failure in the fabric of the building, whilst condensation is a failure in its design and/or use. 

Mould – Understanding the enemy

It’s not unusual to find mould growth where there has been no condensation at all, certain moulds can start to form at 65% Relative Humidity (RH). This is because moulds don’t actually require physical water for germination, they only require the presence of moisture which can also be supplied in the form of water vapour. Xerophilic moulds such as, Aspergillus Flavus can grow and reproduce in conditions with a low water activity as little as 80% AW/RH, whilst hydrophilic moulds such as, Stachybotrys Chartaram require a much damper environment with conditions above 90% AW/RH. 

When mould does occur it often germinates in corners, particularly by external walls as they are often comparatively colder than the rest of the room. As heated air currents move around it sweeps the corners leaving the air within them still and slightly cooler. If the air has a high enough RH and is cooled sufficiently this can cause the water vapour in the air to condense into liquid form, i.e., creating condensation. This means the air within the corners is a little damper than the air in the rest of the room. 

Surely the government should be doing something to help people? 

A combination of these UK wide unhealthy home statistics, and recent media headlines, has indeed prompted the government to take action. In February 2023 it was announced that an amendment to the Social Housing (Regulation) Bill will mandate that landlords must investigate and fix serious problems within strict time limits. It will also give new powers to the Housing Ombudsman to help landlords improve performance. 

Whilst these proposals are a step in the right direction, there is no one silver bullet to solving the UK’s unhealthy homes. We need to ensure all involved in managing, maintaining and retrofitting homes understand the following key things: 

  • The serious health risks mould and damp create in homes 
  • The root causes in a home that create such conditions 
  • The role fuel poverty plays in increasing these risks 

What more needs to be done beyond a tougher Social Housing Bill? 

One of the greatest travesties in this situation is we already have many of the elements to solve the problem of unhealthy homes. But sporadic political action and woefully poor industry practice mean families still have to live in such appalling conditions. The route to a better future needs three elements: 

1 – Policy makers linking decarbonisation to health and funding as a package 

Important lessons were learned during the Each Home Counts report in 2016. The resulting creation of PAS 2030/2035, whilst not universally agreed upon, has given industry a framework to try and reduce the risks of unintended consequences. It aims to ensure that retrofit projects actually deliver their intended outcomes whilst being cost optimal. It also helps to ensure that planned energy savings measures actually happen in practice, and that retrofit measures are considered as part of a whole property system. 

It is always essential to ensure that the chosen retrofit measures themselves do not cause increased risks to the occupants or the property. For example, thermal improvements are often the first retrofit measure needed; this will increase the airtightness of the property, making it essential that improvements to the ventilation system are conducted at the same time to ensure no risk of mould and compliance with building regulations. And it is here where we see a disconnect in major funding streams: 

  • The latest Energy Company Obligation 4, funds a range of retrofit measures including insulation, heating systems and PV, but it doesn’t cover ventilation systems. Given the proven role these play in a health home this really needs to change. 

To truly tackle the problem of damp and mould, control of the heating and ventilation in homes needs to be such that it automatically prevents the conditions arising, as well as ensuring the physical fabric and building services are working as a system. 

We find a similar disconnect when approaching this problem from the human perspective. Numerous trials have been run over the last 5 years to understand how NHS Health Professionals might prescribe energy efficiency retrofits for the most vulnerable people: 

  • The Warm Home Prescription trial saw the NHS in Gloucestershire and Severn Wye energy charity prescribe retrofits to around 150 people. Energy Systems Catapult are testing a scaled up version with around 1,000 people in Tees Valler and Aberdeenshire. We clearly need this health targeted funding as part of a national strategy to eliminate unhealthy homes.

2 – Landlords caring for their tenants by providing well planned maintenance 

We are yet to see the detailed results of new legislation for landlords to investigate hazards and repairs linked to Awaab’s Law. But it is safe to assume Landlords should expect some form of continuous monitoring of their properties, to provide indications of a property and resident at risk.  

The task of solving this problem may seem overwhelming but knowledge and expertise is available to help target real world actions. As an industry we know that older properties are certainly more at risk of both underheating, due to inadequate insulation, and mould caused by penetrating or rising damp (see below)

  • The English Housing Survey 2019 to 2020 shows that houses built before 1990 are most likely to fail the Decent Homes Standard for thermal comfort, and that older properties were “more likely to have defects to the damp proof course, roof covering, gutters, or down pipes, which could lead to problems with rising or penetrating damp affecting at least one room in the property.” Finding approximately 690,000 homes suffered from damp, with around 490,000 also suffering from mould.
  • The Welsh Housing Conditions Survey 2017-18 highlights that “Wales has the oldest stock and the lowest proportion of dwellings with an EPC rating of band C or above” with around 84,000 properties suffering from damp.
  • The Scottish house condition survey: 2019 found that about 234,000 homes suffered from damp.
  • The 2016 Northern Ireland House Condition Survey found only around 8,200 homes were suffering from damp.]

Landlords should not underestimate the financial and reputational risk of not acting now to improve people’s living conditions. The Homes (Fitness for Human Habitation) Act 2018  came into force in 2019 requiring all landlords, both private and social, to ensure that their properties, including common areas, are fit for human habitation at the beginning and throughout the course of a tenancy. Failure to comply with this statutory obligation will lead to enforcement action, possibly legal proceedings and financial compensation for the tenant.

The Housing Ombudsman Service from April 2019 to March 2021 enforced over £123K in compensation across 222 cases, with sums over £1,000 being ordered in 21 cases. However, this was prior to the previously mentioned high profile court case for Awaab Ishak’s death. We can rightfully expect to see significantly harsher penalties for damp and mould related cases in the future.

3 – Construction professionals owning the risks and learning what actually works

The construction industry is rapidly becoming aware of the role digitisation, IoT devices and data analytics will play in its future. Forward thinking funding, such as the Welsh Government’s Innovative Housing and Optimised Retrofit Programmes, have since 2019 required high quality performance monitoring as part of grant offers. When added to developments like the BEIS S-Meter and the recently launched BS 40101 mean we have never been in a better position to capture data in a consistent manner so lessons can be learned across the UK.

Now is surely the time to start making building performance evaluation ‘business as usual’ across the sector. Especially targeting those data streams, such as internal temperatures, relative humidity and CO2, that we know will help us truly understand how best to make unhealthy homes a thing of the past for everyone living in the UK.

If we have sparked your interest and you would like to know more, please visit our Ventilation Systems & Moisture section.

Author: Colin King is Sero’s Technical Lead for both new build and retrofit schemes. A true industry expert, he is our go to resource for anything hygrothermal, whilst ensuring the highest standards are met on all Sero projects.

Contact – PR@Sero.Life


  1. Approved Document L for England [link: https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l]
  2. Approved Document L for Wales [link: https://www.gov.wales/building-regulations-guidance-part-l-conservation-fuel-and-power]
  3. Building Standards Technical Handbook for Scotland [link: https://www.gov.scot/publications/building-standards-technical-handbook-2020-domestic/documents/]
  4. Technical Booklet F1 for Northern Ireland [link: https://www.finance-ni.gov.uk/publications/technical-booklet-f1]
  5. Approved Document F for England [link: https://www.gov.uk/government/publications/ventilation-approved-document-f]
  6. Approved Document F for Wales [link: https://www.gov.wales/building-regulations-guidance-part-f-ventilation]
  7. Building Standards Technical Handbook for Scotland [link: https://www.gov.scot/publications/building-standards-technical-handbook-2020-domestic/documents/]
  8. Technical Booklet K for Northern Ireland [link: https://www.finance-ni.gov.uk/publications/technical-booklet-k]