Insulation is designed to make heating our homes more efficient, but if it isn't installed properly it can sometimes lead to condensation, damp and mould. Recently UK government ministers revealed the scale of the issue and reported that thousands of homes may have had botched insulation fitted since 2022 under a government scheme called the Energy Company Obligation .
Author
- Sarah Price
Research Fellow at the Leeds Sustainability Institute, Leeds Beckett University
Many people have also told journalists they were hoping the insulation would make their homes warmer in cold weather, and were confused and alarmed to find sub-standard installation made mould and damp worse.
To understand why, we first need to think about where moisture in our homes comes from.
There's rain and ground water, of course, but we often forget about moisture in the air. Air is great at holding moisture . Every time we breathe, we give off carbon dioxide and water vapour. Bathing, cooking, washing-up and drying clothes indoors add even more moisture to the air.
Normally, ventilation - the flow of fresh air in and stale air out - helps carry some of this moisture away. Some people open their windows regularly and some homes are naturally "leaky" , with gaps in walls, roofs and floors that let air in and out.
Now imagine a home that was once well ventilated because it's leaky, which then has insulation fitted to the walls. It's going to get more sealed up, and this can upset the balance of moisture in the air inside the home, making it more humid.
And if the occupant isn't good at opening windows enough, particularly in the colder months, (which most of us aren't!), this can lead to condensation, damp and mould, especially in colder corners of rooms where insulation is often missing . It takes more skill and can cost more to install insulation in corners.
Moisture is more likely to condense when there are cold surfaces inside - like a pint of beer in a pub, or a pair of glasses steaming up when you come in from outside. These cold spots, where insulation is missing, are called thermal bridges , and they stay cold while the rest of the wall warms up.
This is unfortunately a common theme in retrofitting insulation to homes. Insulation is installed, gaps in the material are left, and ventilation is forgotten about. This is why housing experts now say " no insulation without ventilation ".
Walls and moisture
There are other ways in which insulation can lead to damp and mould. These usually relate to moisture being trapped inside the building fabric. For example, if external wall insulation is not properly sealed at the top, rainwater can be absorbed into the bricks behind the external wall insulation.
The wall has a hard time releasing moisture because the external wall insulation slows down drying to the outside. The wall tries to release moisture to the inside instead, which means that the ventilation system needs to work extra hard and may get overloaded - if there even is one installed.
This is what led to the mushrooms growing in people's homes in the 2013 Preston external wall insulation debacle. It involved a botched external wall insulation retrofit of properties with many vulnerable residents.
Moisture can also get trapped behind internal wall insulation on solid masonry walls. It makes the existing solid brick wall colder because it's being insulated from the inside warmth of your home. Tiny gaps in the insulation system can allow warm moist air from inside the house to get to the back of the internal wall insulation and to reach the colder external brick wall. If that happens, condensation, and therefore mould, could form inside the wall.
The solution is to make sure there are no gaps in the insulation, and to use the right kind of insulation. Some types of insulation such as wood fibre or calcium silicate board are better at managing moisture . It's also important to make sure the wall is in good condition to start with, with no cracks and to install an internal air barrier to stop any of the warm moist air getting in behind the internal insulation in the first place.
This all might sound a bit alarming, but the good news is that we've learned a lot about moisture in retrofit.
If you're thinking about retrofitting your home with insulation, ask the contractor if they've considered the things listed in the infographic below:
A ventilation system can be easily retrofitted to any home. It can be done before, during or after installation of the insulation. It should include extraction fans in kitchens and bathrooms to extract moisture at source, combined with supply vents or trickle vents in all the other rooms.
Research from the Leeds Sustainability Institute has shown that taking a whole-house approach to installing insulation can reduce the risk of condensation. The whole-house approach is something experts have been talking about for a while and just means thinking of the house as a system including ventilation and cold spots, and not just focusing on the bit being insulated.
Sarah Price works for Leeds Beckett University
