Just About Everything You Need To Know About Cavity-Wall Insulation
Unlike solid walls, cavity walls are made up of two walls; an outer wall and an inner wall with a hollow ‘cavity’ space in between. The wall material on either side of the cavity is often referred to as “skins” or “leaves” and is usually made of bricks or blocks.
Usually, it’s required for the cavity space to be a minimum of 50mm in width in order to receive cavity wall insulation, but properties might have up to 100mm cavities or more.
When it comes to leaking heat, the walls of your home are responsible for a whopping 30% of heat loss according to the SEAI. However, filling the cavity walls with insulation is one of the most cost-effective of all energy saving measures, (especially with the help of the SEAI grant).
It can also have other benefits such as preventing dampness, improving the structural integrity of the wall, and providing sound insulation so that less noise gets in from outside.
To determine if your home has cavity wall construction, follow these steps:
- Check the Age of Your Home: Most homes built after the 1940’s in Ireland have cavity walls. If your home was built after this period, it’s likely to have cavity walls.
- Examine the External Walls: Look at the pattern of the bricks on the external walls. If the bricks are laid lengthways in a simple alternating pattern, it’s likely that you have cavity walls. (If your bricks are laid lengthwise and widthwise, it might be a solid wall).
- Measure the Wall Depth: If you cannot see any exposed brickwork, then measure how wide your external walls are. Walls more than 260mm thick are most likely to be cavity walls.
- Check all External Walls: Some houses have a cavity wall to the front but none at the sides and back, (they may be cavity block which cannot be filled). For cavity wall insulation to be effective, all external walls must have cavities suitable for filling. Therefore check the brick pattern & depth for each. If still unsure, a site visit by an installer can help determine the construction type.
Other houses have a different type of wall structure altogether. If your house is a steel or timber-framed building, or is made from prefabricated concrete, then you will need to ask a specialist insulation installer to advise you. If an installer suggests injecting wall cavity insulation between the outer and inner leaves of your timber-framed home, don’t do this as it can cause serious damage.
To determine if your home previously received cavity wall insulation, follow these steps:
- Check for Small Holes: Inspect the external walls for small, evenly placed resealed holes. These holes are drilled during installation to pump in the cavity wall insulation.
- Review Your Property Paperwork: Check your Building Energy Rating (BER) certificate or other property documentation. It might mention whether such insulation has been installed.
- If you are still unsure, a site visit by an insulation installer can determine if your walls have already been filled.
The cavity walls to be filled must be structurally sound. The installer will check the type and condition of the wall to determine if it is suitable for filling. As part of this process, the installer must ensure that:
- The cavity to be filled is at least 50mm in width, is unfilled and is clear of rubble
- The outer leaf is free from cracking, defective mortar, damaged rendering and spalled bricks. It should be free from a discharge of water from building features and that gutters, downpipes, overflows, etc, are in good order
- The inner leaf is free of dampness, and any visible gaps in the inner leaf are sealed so that the insulation doesn’t seep into the home
- The tops of cavities in the attic are sealed to prevent the fill being blown into the attic
- There are no areas of steel or timber-framed construction
If you have any damp patches on your internal walls then they should not be insulated until the problem is resolved. Speak to a builder who specialises in damp prevention.
The main types of pumped cavity wall insulation for a retrofit installation are mineral wool fibre, polystyrene beads and polyurethane foam (PUR). Choosing the best type will depend on a number of factors such as your local environment, the thermal performance you wish to achieve, and the overall cost.
- Mineral Wool Fibre – Including rock wool and glass wool types, this is the cheapest of the three main pumped cavity wall materials and is also the most environmentally sustainable. However, it can be the most susceptible to clumping and dampness issues, and can suffer from cold gaps if poorly installed.
- Polyurethane Foam (PUR) – PUR has the highest heat retention rate, but is also the most expensive material, and being made from petrochemicals, isn’t particularly eco-friendly. When injected, this fluid-like material expands and fills all gaps, adhering to both leaves and bonding them together This can help strengthen walls. It is inherently waterproof and resistant to fungal attack. However, it may lead to inadequate cavity ventilation, preventing water build-up from drying out, resulting in mould and mildew growth.
- Polystyrene beads or granules – Similar to mineral fibre, these beads or granules are blown into the cavity using compressed air. They may be supplied loose, or in a light sticky resin to hold them together. They are easily installed, inherently waterproof and resistant to fungal attack. This material can accommodate ventilation for your cavity wall, thus reducing the risk of dampness build-up. However, it costs more than mineral fibre and requires a completely cleaned cavity space for proper installation. While polystyrene is also made from petrochemicals, expanded polystyrene (EPS) beads used in this type of insulation are often produced from recycled polystyrene.
The drilling process can create some vibration so it may be a good idea to remove any items from shelves and take down pictures to protect against accidental breakages caused by vibrations. Ornaments on external walls should also be removed for their safety.
As the insulation is only effective if all walls are done, the installer will need access to any attached garages, conservatories, lean-to sheds, etc. If you have a wall right on the boundary with your neighbour, you will need to ask for your neighbour’s permission to allow the installer onto their property to complete your installation.
- Once the walls are checked and prepared, the installer begins by drilling small holes about 22-25mm wide in the mortar between the brickwork of the outer walls, each about a metre apart. This is to access the inner cavity space from the outside of the property, a non-invasive and mess-free approach to injecting insulation.
- Then, a special applicator hose is used to inject insulation material into the cavity. Periodic checks are made during installation to ensure the wall is filling up evenly.
- The installer seals the holes with mortar or render after all checks and tests are complete, using a mix that closely matches the existing colour and texture. On pebbledash finishes, the installer may apply pebbles to the wall’s surface to match the existing finish. After weathering, the holes are difficult to see.
The whole process usually takes between 2-4 hours, although time will depend on the size of the property and other factors like access.
Air needs to flow in and out of your house so that it stays fresh, dry and healthy. A good installer will be sure not to block or seal any intentional ventilation, including underfloor grilles and airbricks which help keep wooden beams and floors dry, as well as wall vents which let small amounts of fresh air into rooms.
The installer will check that all necessary vents are already sleeved. If they are not, the installer will remove and seal around them to stop them being blocked by the insulation. Other vents which may be redundant, such as cavity vents or vents that used to supply air to open fires in bedrooms, may be closed off.
There has been a lot of misinformation about cavity wall insulation causing damp. Modern cavity wall insulation in itself is very unlikely to cause this. The vast majority of issues tend only to arise from bad installation or poor understanding of your property on the part of installers. If not installed correctly, it can obstruct healthy airflow through the cavity space, leading to moisture and dampness buildup. This is why it’s important to always use a SEAI accredited insulation company.
Properly installed cavity wall insulation should help prevent dampness in most environments. A responsible installer will make sure:
-  Not to block any necessary ventilation (such as vents and airbricks)
-  To address any pre-existing issues with damp before installing any insulation
-  The cavity is free of rubble or debris
-  The exterior walls are in good condition
-  To check if the property receives high levels of wind-driven rain
If your home is semi-detached, the installer will insert special wall cavity barriers to ensure the insulation material stays in your part of the building so your neighbour’s home will not be affected.
If you think your neighbour may be interested in warming up their home, let them know about your plans to install cavity wall insulation. If both homes are to be insulated at the same time, cavity barriers would not be needed.
Cavity wall insulation should last for the lifetime of the building.
Planning permission is not required for cavity wall insulation. However if your home is a protected structure or in a conservation area, you should enquire with your local council before commencing with your insulation work.
This varies based on property size, cavity width, materials used, accessibility and labour. The larger the surface area of your home, or the wider the cavity gap is, the larger the cost of the project will be. A rough cost estimate for the following house types would be (Jan 2025 estimates):
- 2-Bed Semi-Detached House: €1,500 – €1,900
- 3-Bed Semi-Detached House: €1,900 – €2,200
- 4-Bed Semi-Detached House: €2,200 – €2,600
Detached properties will need to be surveyed in order to give an accurate assessment. These are only rough estimates, and proper pricing will vary based on the above factors.
The SEAI (Sustainable Energy Authority of Ireland) offers grants to those who wish to install cavity wall insulation in their home. To qualify for this grant you must:
- Be the owner of a property built and occupied before 2011. (This is defined as the date your electricity meter was installed).
- Use an insulation contractor from the SEAI’s registered list.
- Use newly fitted materials and products.
- Comply with building regulation standards for any insulation works.
- Get a BER assessment done after the works are completed using a BER assessor from the SEAI’s National Register.
Please note: If your home has already received a particular type of insulation grant through any government scheme, it cannot receive additional support for further insulation works of this type. (However, if a grant was previously received for attic insulation for example, the homeowner is still able to claim for a cavity wall insulation grant).
For more detailed information, visit the SEAI insulation grants page here.
The value of the 2025 SEAI cavity wall grant varies according to your type of home.*
- Apartment – €700
- Mid-Terrace – €800
- Semi-Detached or End-of-Terrace – €1,200
- Detached – €1,700
* Where actual expenditure is lower than the grant value, only the lower amount will be paid.