Cavity wall insulation is one of those home improvements that sounds like a no-brainer, promising a warmer home, fewer draughts and lower energy bills . In most cases, that’s exactly what you get, provided your home is actually suitable for it and the job is done properly.
Unfortunately, many homeowners jump in too quickly, tempted by government grants or persuasive sales pitches, and end up with problems. Damp patches, crumbling plaster and not achieving the promised energy savings are all possibilities.
This article sets out the most common cavity wall insulation mistakes and, more importantly, explains how to avoid botching when adding this type of home insulation.
1. Not checking if your home is suitable
The first and biggest mistake is assuming that cavity wall insulation is right for every home. It isn’t. Some properties just aren’t designed for it, and installing cavity wall insulation in the wrong type of wall can cause serious problems.
Older houses might have smaller cavities within the walls which can’t be properly filled with insulation. If the brickwork is particularly porous, or if the walls are regularly exposed to driving rain, then this could make cavity wall insulation an unsuitable choice. Timber frame properties are another big red flag. Their walls rely on ventilated cavities to keep the timber dry. Filling this space with insulation can trap moisture, leading to rot, structural problems and difficulties with securing insurance cover and mortgages.
Before carrying out cavity wall insulation, always have a professional survey. A proper inspection will identify the cavity width, check for any existing damp, and assess the general condition of the external walls. A reputable installer will insist on this as they won’t want long terms defects just as much as you.
2. Choosing the wrong insulation material
Even if your home is suitable, choosing the wrong insulation material can result in unexpected problems. The right type of insulation will depend on how your home is construction and also how exposed it is to severe weather conditions.
Mineral wool insulation is one of the most common options. It’s breathable and a good insulator, but it can absorb water if moisture gets into the cavity. Polystyrene beads are another popular choice and are water-resistant and able to fill awkward spaces easily. Polyurethane foam, expands to fill the cavity completely but has greater risk of defects particularly if the cavity isn’t constructed correctly.
Homeowners often go with whatever product the installer recommends without questioning its suitability. If the installer has limited product options or isn’t technically competent the wrong insulation product could be proposed. For example, recommending mineral wool or eco insulation in a wall with cracks or weak pointing that allows moisture to enter the cavity. If your home faces frequent wind-driven rain or you live in a coastal area, the installer should take that into account and recommend that repairs are undertaken.
It’s always best to ask why a particular material is being proposed and how it’s expected to perform. A good installer will be happy to explain all the options and provide a comparison of the features and benefits they offer. A bit of research before signing off will save a load of problems and hassle later.
Shop insulation types
Knauf Insulation
Knauf Insulation Dritherm
The Knauf Insulation DriTherm® 37 Standard Cavity Slab is a water-repellent glass mineral wool slab, designed for use in external full-fill masonry cavity walls, that offers thermal performance. It is non-combustible with the best possible Euroclass A1 reaction to fire classification
Knauf
Earthwool Dritherm Cavity Insulation Slab
Knauf Earthwool DriTherm Cavity Sheet 37-50mm (6.55m²/M Per Pack) which is 50mm Thick Hollow Wall Insulation. Easy and quick installation with excellent thermal performance.
Generic
Pir Insulation Board 25mm
This 25mm PIR insulation board provides excellent thermal insulation properties to help reduce energy costs. Suitable for use in floors, walls, and roofs in new-build and renovation projects. Lightweight yet sturdy boards measure 400mm x 1200mm for convenient handling and installation.
3. Poor installation techniques
Even the best insulation can fail miserably if it’s not installed properly. A common mistake is incomplete filling of the cavity. If the insulation doesn’t reach every part, voids will form, creating cold patches that allow heat to escape. These cold spots attract condensation and eventually mould will form.
Blocked vents are another common issue. Air bricks and ventilation openings should never be blocked as part of an installation. Blocked vents reduces airflow, increases condensation risk, leading to mould and damp problems.
Incorrect drilling patterns are also surprisingly common. Installers should follow a precise layout of drill holes across the external walls to ensure the insulation is evenly distributed. Drilling insufficient injection holes results in uneven insulation filling and if the holes aren’t properly sealed afterwards, rainwater can seep in leading to damp.
Good installers will use thermal imaging cameras and borescopes to checks and confirm that the cavity has been completely filled. If your installer doesn’t offer this, then ask questions quickly.
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TOPDON
Topdon Thermal Camera
This thermal imager allows you to measure temperature differences from a safe distance of up to 16.4ft (5m), making it an ideal tool for home inspectors, HVAC technicians, electricians, beekeepers, automotive technicians, and even farmers looking to protect crops and livestock.
The H128 infrared camera elevates your thermal imaging clarity from 128×128 to 240×240 resolution. With a thermal sensitivity of 40mk, it captures the slightest temperature differences.
BSIDE
Thermal Imaging Camera
The thermal image camera is equipped with a high-speed, high-precision sensor that can detect temperatures from -4°F to 1022°F (-20-550°C) within ±2 %. Max, min, and center temperature auto-recognition and auto-tracking functions shine in tasks such as heat leak detection, electrical issue identification etc.
4. Ignoring existing damp issues
It sounds obvious, but if you already have damp problems don’t install cavity wall insulation until the problem has been fixed. Despite what promotional literature might say, insulation isn’t a magic cure and poor installation can make things worse. Once insulation is in place, moisture that would have evaporated in an adequately ventilated cavity can become trapped. If there’s rising damp, penetrating damp, or even leaks from damaged guttering, these must be fixed well ahead of cavity wall installation taking place.
Installers sometimes downplay the importance of preparation works as this makes jobs more expensive and longer to complete. A good building survey should identify any damp issues and make recommendations for carrying out repairs. It’s far better to wait a few weeks for these to be done, rather than rushing and having to deal with saturated insulation and mould growth.
5. Lack of proper ventilation
Cavity wall insulation will make a home warmer and more airtight, which can be a double-edged sword. Older homes tend to rely on natural ventilation through air leaky walls to keep them well-ventilated and dry.
If this ventilation isn’t maintained, the result can be condensation on cold surfaces, particularly in areas with high humidity such as bathrooms and kitchens. Maintaining good ventilation is essential so check that air bricks that bridge the cavity aren’t blocked by the insulation. Also make sure that extractor fans are working correctly, trickle vents on windows are open, and loft spaces with insulation have adequate eaves ventilation.
6. Not using a certified or competent installer
There’s no shortage of stories about poor workmanship in the insulation industry. Some homeowners are lured in by cheap deals for insulation costs or slick sales pitches promising hassle free installations and significant savings on energy bills.
Using an unqualified or uncertified installer can lead to disastrous results. Without the necessary technical know-how, installations might not meet industry standards and resolving problems later, can become a complete nightmare.
Look for installers who are registered with recognised bodies such as CIGA (Cavity Insulation Guarantee Association), TrustMark or work with reputable organisations such as RetrofitWorks. Member installers are often required to be signed up to recognised industry schemes, have to follow specific standards and provide guarantees for their work.
Whilst cavity wall insulation might seem straightforward, DIY installations are best avoided. Doing the job correctly requires specialist equipment and training and the risk of causing damage far outweighs any savings from trying to do it yourself.
7. Failing to maintain and inspect
Once cavity wall insulation is installed, it’s completely out of sight and out of mind. Most people forget about it completely, but like any part of a house, it needs a little maintenance to keep it performing well and as intended.
Over time, insulation can settle, become damp, or degrade if it’s exposed to water. Cracked render, leaking downpipes, or deteriorating mastic pointing or mortar joints can allow rainwater to enter the cavity, soaking the insulation and reducing its effectiveness.
It’s a good idea to inspect exterior walls at least once a year. Look for cracks, damaged pointing, or anything that might allow moisture to get through to the cavity. If you notice cold spots or damp patches, have a professional carryout an inspection to find out why.
Remember, early intervention will stop minor issues turning into a major headaches. Simple maintenance tasks, such as cleaning gutters or repairing leaking downpipes will keep water under control and avoid damage to the cavity wall insulation.
8. Over-insulating or under-insulating
It’s possible to get insulation wrong even when it’s installed correctly. Too little, and you’ll not realise the planned energy savings. Too much could create other problems.
The more common mistake is under-insulating, usually because the installation wasn’t properly carried out. That leads to patchy performance, with some areas of the wall staying cold whilst others are warm.
Over-insulating isn’t usually a problem, but adding large amounts of insulation without a proper design can trap moisture between the insulation and wall layers, which again risks damp and further problems
The key is design and compliance with manufacturers installation recommendations. Firstly, the correct insulation product should be used to suit the cavity size and wall type. Assessments should support the design proposal such as U value calculations and condensation risk analysis. A competent installer will complete these in advance of an installation to ensure optimal thermal performance is achieved without compromising the building’s ability to breathe and dissipate moisture.
Cavity wall insulation can make a huge difference to a home’s comfort and energy efficiency, but only if it’s done right. The biggest issues tend to come from skipping surveys, using the wrong materials, or cutting corners on when installation is carried out. Mistake might at first seem small but the impact of getting things wrong can be expensive.
If you’re considering cavity wall insulation, my advice is to take your time, do your research and undertake it properly to make sure it lasts. You can read more on how to insulate a house as well as how long does insulation last in my in-depth pieces on the topics.
Have a professional survey carried out, check installer’s credentials, verify that the materials will be properly handling and installed. Once the installation is complete, keep an eye on how it performs and carry out simple maintenance to prevent problems developing.
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