Having cavity wall insulation installed: is your wall suitable, which material, the risks and who does it

A cavity wall consists of two walls with a layer of air between them: an inner leaf that carries the building and an outer leaf of brickwork that keeps out the rain. That layer of air, the cavity, can be filled with insulation material afterwards. This is called retrofit insulation or cavity wall insulation. The work is relatively simple and is done from the outside, without much needing to happen indoors. Yet it does not work out well for every home: a lot depends on the cavity itself and on the condition of the brickwork. This article explains how to assess that, which materials are available and where the risks lie. Where cavity wall insulation fits among the other insulation measures is explained in Improving your energy label: which measures really count.

Does your home have a cavity wall?

From around 1920, cavity walls were used increasingly often in the Netherlands. Homes from before then usually have solid walls without a cavity; those can only be insulated on the inside or outside. According to Milieu Centraal (the Dutch independent information centre on energy and the environment), homes from the period 1920 to 1975 usually have a cavity without insulation. In homes built between 1975 and 1991, the external walls were almost always insulated, usually in the cavity, but that insulation can often be improved. According to Milieu Centraal, homes built after 1991 already have good wall insulation. The year of construction is therefore a first indication, but not a certainty: within the same period it differs per contractor and per project, and later owners may already have had the cavity filled.

If you see only the long sides of the bricks in the facade, it may be the outer leaf of a cavity wall, but it may also be a single half-brick wall. A solid wall usually also shows the short ends of bricks. Also measure the thickness of the wall next to a window or door: if it is clearly more than 20 centimetres, then according to Milieu Centraal you probably have a cavity wall. You only know for certain after an inspection.

Is the cavity suitable? What the insulation company investigates

A reliable insulation company always starts with a preliminary survey. It drills a small hole in a mortar joint and looks into the cavity with a camera or endoscope. This establishes the following:

• The width of the cavity. A cavity is usually a few centimetres wide; Milieu Centraal cites 4 to 6 centimetres as typical. Not every material is equally suitable for a narrow cavity; the company chooses the material partly on the basis of the measured width.
• A clean or contaminated cavity. During bricklaying, mortar droppings and rubble often fell into the cavity. Where rubble or mortar bridges the cavity, moisture can travel from the outer leaf to the inner leaf, and the insulation material cannot reach those spots properly. Sometimes it is better to have the cavity cleaned first.
• Existing insulation. If there is already insulation in it, whether topping up makes sense depends on the remaining air gap. Milieu Centraal cites a remaining air gap of at least 4 centimetres for EPS boards and at least 5 centimetres for glass or stone wool. Insulation material installed in the 1970s and 1980s may have slumped over the years, leaving the upper part of the wall unprotected.
• Wall ties. The metal ties that connect the inner and outer leaves can rust in older cavity walls. A facade with corroded ties must be repaired first; how that is done is explained in Repairing pointing and cleaning the facade.
• Protected animals. Bats can roost in cavity walls, and swifts and house sparrows can also nest there. These species are protected, and under the nature rules of the Omgevingswet (the Dutch Environment and Planning Act), a cavity in which they live may not simply be closed off. Ask the insulation company how it investigates this; Milieu Centraal mentions an eDNA test, carried out by a certified company, as a way of establishing the presence of bats. If traces are found, then according to Milieu Centraal an omgevingsvergunning (environmental permit) and an ecological survey are normally required.

Also tell the company beforehand whether you have damp patches in your home, on walls, floors or ceilings. An existing damp problem must be solved first; insulating will not cure it and may make it worse. How to distinguish damp problems by symptom is explained in Recognising damp and mould in your home.

First get the facade in order: pointing, bricks and frames

The most important condition is not in the cavity, but in the facade around it. In its suitability check, Milieu Centraal asks: "Are the bricks, pointing, frames, roof and gutters in order?" There is a reason for that. In an empty cavity, water that comes through the outer leaf runs down the inside of the outer leaf and is drained away at the bottom; the air gap keeps it away from the inner leaf. According to Milieu Centraal, with a properly executed fill no moisture from outside comes through the filled cavity to the inside, but that only applies if the brickwork is in good condition and not frost-sensitive. If the outer leaf lets a lot of water through, that water can get closer to the inner leaf via the insulation material. Sandy or eroded joints, cracks, broken bricks, leaking gutters and poor seals around frames must therefore be repaired before insulating.

Have a bricklayer assess the facade if you are in doubt. If the wall is going to be repointed anyway, plan the cavity wall insulation after the pointing, so that the drill holes are filled with mortar that matches the new joints. Paint or a coating on the outer leaf also deserves attention: a facade that lets vapour through poorly may dry out more slowly after insulation.

Which material: wool, beads or foam

According to Milieu Centraal, glass wool, stone wool and EPS beads are the most commonly used materials and they insulate about equally well. The differences lie in how they are applied, how they behave with moisture and fire, and their suitability for your cavity.

• Glass wool and stone wool flakes. Loose fibres that are blown into the cavity with air. Mineral wool is non-combustible. The flakes are treated to be water-repellent, so that in principle water runs past them instead of soaking in. The material must be blown in densely enough; at too low a density it can settle.
• EPS beads. Small polystyrene granules that are blown into the cavity, often with a binder that makes the beads stick together. Bonded beads stay in place and do not pour out if a hole is drilled in the wall later. EPS absorbs hardly any water. It is a plastic and therefore combustible, although it is enclosed in the cavity behind the brickwork. With sloppy work, loose beads can end up in the garden or the crawl space; Milieu Centraal therefore advises using a recognised company.
• PUR or PIR foam. Foam that is injected in liquid form and expands and cures in the cavity. Milieu Centraal describes foam as a less commonly used material. The foam bonds to both leaves, which can be a disadvantage in the event of penetrating damp or later repairs to wall ties, and as a plastic it is combustible.

One material to be wary of is urea-formaldehyde foam, UF foam for short. Milieu Centraal reports that the GGDs (the Dutch municipal public health services) advise against it for the time being, following reports of serious problems after its use. Since 1 April 2026, UF foam has also no longer been eligible for an ISDE grant. So always ask exactly which material will be used and have it stated on the quote.

What does it achieve? As an indication, Milieu Centraal states that an uninsulated cavity wall has an insulation value of about 0.35 and that 5 centimetres of insulation adds an Rd value of about 1.2 on average, bringing the total Rc value to about 1.55. That is a substantial improvement, but a cavity wall does not reach the level of a new-build wall with it. According to Milieu Centraal, how high the insulation value becomes depends mainly on the width of the cavity; insulating thicker than the cavity is wide is impossible, and the differences between the materials are small.

How the work is done

The insulating itself is usually done quickly. The process is roughly:

1. Preparation. The company determines where the drill holes will go, screens off ventilation openings and openings to the crawl space, and checks whether there are openings in the cavity through which material could escape, for example at the roof edge or into an adjoining home.
2. Drilling. Small holes are drilled in the mortar joints on the outside, according to Milieu Centraal about a metre apart.
3. Filling. The material is blown or injected into the cavity through the holes, until the cavity between the holes is completely filled.
4. Sealing. The holes are filled with mortar in the same colour as the existing joints. When done well, they are barely visible afterwards.
5. Inspection. A good company checks the fill, for example with a thermal imaging camera during the heating season, and supplies a guarantee certificate with the details of the work.

Indoors you notice little of the work. In a terrace of houses with a continuous cavity, the cavity must be closed off at the party wall, otherwise you will also fill part of the neighbours' cavity; in that case, consult the neighbours or have the work carried out together.

Risks: damp, thermal bridges and ventilation

Cavity wall insulation goes well in most homes, and according to Milieu Centraal a well-insulated cavity wall in fact helps prevent damp problems. Even so, there are points where things can go wrong:

• Penetrating damp with poor pointing. As described above: a facade that lets water through can cause damp patches indoors after insulation. Repairing the facade beforehand is the most important measure.
• A colder and wetter outer wall. After insulating, less heat from the home reaches the outer leaf. As a result, the outer leaf may stay colder and dry out more slowly after rain. On the north-facing wall and in places that get little sun, this can lead to more green algae or moss. With soft, porous bricks there is also a greater risk of frost damage. Have the company assess whether the brick is suitable.
• Thermal bridges. Where the cavity is interrupted, such as at lintels above windows, around frames and where the wall meets the foundation or the floor, the insulation remains interrupted. The inside stays colder there than the rest of the wall, and with insufficient ventilation, condensation or mould can form there.
• Ventilation openings becoming blocked. Crawl space vents and ventilation openings in the facade must remain open. A crawl space without ventilation becomes damp, with consequences for the floor and the indoor climate. Check after the work that all vents are clear. Open perpend joints (weep vents) at the bottom of the wall and above frames, which are intended to drain water from the cavity, must not become blocked either.
• Ventilation in the home. An insulated home is often also less draughty. Milieu Centraal stresses that enough fresh air must enter the house both before and after insulating. Keep trickle vents open and use the mechanical ventilation, if you have it.

ISDE grant, Warmtefonds and VAT

Cavity wall insulation is covered by the Investeringssubsidie duurzame energie en energiebesparing (ISDE, the Dutch investment grant for sustainable energy and energy saving) from RVO (the Netherlands Enterprise Agency). The conditions change regularly; check them on rvo.nl before you commission the work. At the time of writing, the conditions include the following:

• you have at least 10 square metres of cavity wall insulated;
• the newly installed material has an Rd value of at least 1.1 and is on RVO's notification code list (meldcodelijst);
• the home was built before 1 January 2019, or the omgevingsvergunning (environmental permit) was applied for before 1 July 2018;
• a company carries out the work; doing the insulation yourself is not eligible;
• the scheme is intended for homeowners who live in the home themselves; other rules and application channels apply to landlords and VvEs (owners' associations);
• you apply for the grant after the work has been carried out, within 24 months, with, among other things, the invoice, proof of payment and a photo of the work.

You only receive a grant for cavity wall insulation once, and you receive no grant for insulating a part with which you enlarge the home, such as an extension, a new storey or a garage that you incorporate into the home. Adjusted conditions apply to listed buildings. Ask the insulation company for an invoice stating the date, the area, the thickness, the Rd value, the type of material and the notification code (meldcode). How the ISDE relates to municipal schemes is explained in Sustainable renovation: grants and options.

For the rest of the investment, you can turn to the Nationaal Warmtefonds (the Dutch national heat fund), which offers a loan for energy-saving measures. The conditions and the interest rate can be found on the fund's website.

As for VAT: the labour costs for installing insulation material in the walls of a home that is more than two years old fall under the reduced rate of 9%; the insulation materials themselves are subject to 21%. The invoice must therefore split the two. More on this in 9 or 21 percent VAT on renovations.

VvE and rented homes

If you live in an apartment, the facade is normally a shared part and the vereniging van eigenaren (VvE, owners' association) decides on cavity wall insulation. You cannot then have only your part of the facade insulated yourself. Moreover, the cavity often runs continuously past several apartments, so that in practice the work is done for the whole building at once. Raise the proposal at the meeting and see whether it fits into the long-term maintenance plan, for example together with repointing the facade. More about decision-making can be found in Renovating a VvE apartment.

If you rent, insulation is normally a matter for the landlord. If you want to have the cavity filled yourself, first ask for written permission; it is normally required for changes that you cannot easily undo at the end of the tenancy, and certainly on the outside of the home (article 7:215 of the Dutch Civil Code). If the landlord has the cavity insulated as an improvement, they may propose a reasonable rent increase (article 7:255 of the Dutch Civil Code); this does not apply to remedying a defect. Make sure you are properly informed about this beforehand.

Listed buildings and protected townscapes or village views

For an ordinary home, cavity wall insulation usually needs no permit, because the appearance of the facade does not change. If protected animals live in the cavity, however, an omgevingsvergunning (environmental permit) may be required under the nature rules. Things are different for a rijksmonument (national listed building). There, only necessary regular maintenance aimed at preserving the monumental values is permit-free, as long as detailing, profiling, design, type of material and colour do not change (article 13.11 of the Besluit activiteiten leefomgeving, the Dutch Environmental Activities Decree). Filling a historic cavity does not normally count as such maintenance and therefore usually requires an omgevingsvergunning. The drill holes in historic pointing and the effect on how old bricks handle moisture also play a role in the assessment. For municipal listed buildings, the rules are in the municipality's omgevingsplan (environment plan) or heritage by-law, and additional rules may also apply in a protected townscape or village view. So always consult the municipality beforehand; it can seek advice from the Rijksdienst voor het Cultureel Erfgoed (the Cultural Heritage Agency of the Netherlands). What is generally permit-free is explained in When do you need a permit.

Who does it and what should you look for in the company?

Cavity wall insulation is usually carried out by a specialist insulation company. Repairing the facade beforehand, such as repointing, replacing bricks or repairing wall ties, is work for a bricklayer or a facade specialist. If you want to combine insulation with other work, such as new frames, floor insulation or a renovation, a contractor can coordinate the work and subcontract the insulation to a specialist.

When choosing the company, look at certification. Milieu Centraal recommends preferably a company with a certificate from SKG-IKOB or Insula. SKG-IKOB certifies companies with a KOMO process certificate based on assessment guideline BRL 2110 for retrofitting cavity wall insulation; Insula Certificatie certifies companies for cavity wall insulation with bonded EPS beads based on BRL 2121. According to Milieu Centraal, either of the two certificates is sufficient. A certified company works according to fixed rules for the preliminary survey, the execution and the inspection. Also ask about the guarantee, references from similar homes and how the company deals with protected animals.

What belongs in the quote?

Ask for two or three quotes based on an on-site inspection, and check that they contain the same components:

• the outcome of the preliminary survey: cavity width, contamination, existing insulation, condition of the facade and any repairs required beforehand;
• the area in square metres, per facade, and which parts will not be insulated;
• the material with brand and type, the thickness and the Rd value;
• how ventilation openings, crawl space vents and open perpend joints are kept clear, and how the cavity is closed off at the neighbours';
• sealing the drill holes in a matching colour;
• the survey for protected animals;
• the certification, the guarantee and the guarantee certificate;
• an invoice with the details you need for the ISDE, and the split between labour and materials for the VAT.

How to compare quotes is explained in Comparing quotes: how to read a building quote.

In summary: have the cavity inspected first, make sure the facade is in order, choose a certified company and check after the work that all ventilation openings are clear. Keep the invoice and the guarantee certificate. On VerbouwPro you post your job free of charge and bricklayers and contractors from your area respond.