Cavity wall insulation problems – what can go wrong

Discover the most common complications of cavity wall insulation and how to solve them

Cavity wall insulation (CWI) is a great option to make your home more efficient and save on energy bills. Most installations go without a hitch, leaving homeowners to enjoy the benefits of a warmer, more comfortable living environment. Cavity wall insulation problems do crop up so we're here to tell you why.

'We’ve issued 6.2 million guarantees for cavity wall insulation, and have a claims rate of just 0.43%,' reassures Nigel Donohue, CEO at the Insulation Assurance Authority.

Occasionally, however, things can go wrong. Most cavity wall insulation problems are the result of installation and maintenance issues, which can be easily avoided through reliable pre-installation assessments and good installation practice.

What are the most common cavity wall insulation problems?

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(Image credit: Future plc/Simon Whitmore)

Here are some cavity wall insulation problems and what to do if you face them.

1. Damp and mould cavity wall insulation problems

Properly installed, insulation can reduce issues with damp and mould and make your home a more comfortable place. You might have cavity wall insulation problems occurring if the work is poorly done or the house is unsuitable.

Damp and mould are usually easy to spot. Often the issue stems from poor walls not being repaired prior to fitting cavity wall insulation. Cracks in the brickwork or render, crumbling mortar and leaky guttering can all allow water into the cavity. This will soak the insulation, meaning it won’t perform as it should, ultimately leading to issues.

A good company will complete a pre-installation survey and advise you on what needs to be done before CWI is fitted or inform you if cavity wall insulation isn’t suitable. Post-installation, good general property maintenance, such as repointing mortar joints and regularly checking guttering, will help to avoid future problems. Ventilation is also important for preventing damp and mould. Look into the best dehumidifiers to help, too

2. Inadequate ventilation

Good airflow is vital inside your home, to ensure it stays fresh and free from issues such as mould. When you insulate a house, you will be making it more airtight, so installers must ensure existing airflow is maintained and additional ventilation fitted where necessary.

'We install ventilation as standard as part of a CWI job,' says Ed Andrews, Commercial Director at InstaGroup. 'But homeowners need to be aware of what they can do to reduce the risk of damp issues, such as opening windows and using extractor fans.'

The cavity wall insulation problems related to ventilation problems can be exacerbated if it is poorly installed or slumps over time. As well as causing cold spots, this could also block vents, so proper specification and installation are key.

3. Poorly-filled cavity wall insulation

When retrofitting insulation, obstructions within the cavity, such as pieces of mortar, can snag materials such as mineral wool. If a cavity is too thin, the insulation may also not spread as expected.

Using the right type of insulation for your house is the first step to avoiding this. Good installation is essential, too. A cowboy installer might not pay due care and attention to ensuring the whole wall is properly treated.

As the cavity is hidden, there’s no easy way to spot these kinds of issue. Sometimes you’ll detect cold spots, but you might not get wind of a problem until you’ve had several months’ of utility bills. The thing to look for is whether your energy usage (rather than the price, which can fluctuate) hasn’t come down as much as expected compared to the previous year.

4. Unsightly retrofit installations

CWI is installed by drilling holes in the exterior wall and blowing or injecting the insulation into the cavity. Good contractors will do a decent job of patching these up, and the holes do tend to fade with age; but poor cover-ups can be unattractive. 

As well as speaking to your installer about how they’ll hide their work, do your research and ask former clients if they’re happy with the insulation performance, the finished appearance and aftersales care.

Who should I contact about cavity wall insulation problems?

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(Image credit: Future plc/Simon Whitmore)

Speak to the original company who provided your cavity wall insulation cost and installation and ask them to assess the issues and carry out any remedial work. If they are unwilling to provide support, or have gone out of business, then your next option is to contact your guarantee or warranty provider. If all else fails, appoint your own surveyor to make an independent assessment.

Which homes are inappropriate for cavity wall insulation?

Properties in exposed locations, subject to wind-driven rain or without a damp-proof course might not be suitable for cavity wall insulation. 'Some properties may also use particularly porous bricks in the external construction or feature a very narrow cavity, which is unsuitable for CWI,' says Matthew Evans, Head of Technical GB at Kingspan Insulation.

In the worst cases, unsuitable insulation may need to be removed – a process which can cost £1,000s. That's why  it’s important to use a registered installer whose work will be covered by a 25-year CIGA guarantee.

If there are obstructions inside the cavity, water can bridge across insulation. Snots (lumps) of mortar are the most common cause. This may mean it’s not possible to install CWI, so another solution (such as external wall insulation) may be your best bet for upgrading energy efficiency.

Whether a house is appropriate for cavity wall insulation or not should be determined by a pre-installation assessment from a reputable company. This should include inspecting the cavity using a probe camera.

Rhoda Parry

Rhoda Parry was the Editorial Director of Ideal Home and its sister titles, 25 Beautiful Homes and Style at Home from 2021-2022. She wass also Editorial Director for Gardeningetc, Amateur Gardening and Easy Gardens. Rhoda is a highly experienced editor and journalist and has worked on many women's lifestyle media brands throughout her career. For the last 20 years, she has specialised in homes, interiors and gardens. A storyteller at heart, Rhoda is passionate about championing, crafting and creating exciting written and visual content for digital, print and experiential audiences.