Question 1

How is cavity wall insulation installed?

Accepted Answer

Small holes are drilled in the mortar joints of the exterior wall. Bonded EPS beads are then injected into the cavity in a controlled manner. Once the cavity has been filled, the openings are carefully closed again.

Question 2

What are the disadvantages and risks of cavity wall insulation?

Accepted Answer

The main risks generally result from an unsuitable wall, poor preparation or incorrect installation rather than cavity wall insulation itself. A cavity containing large amounts of mortar debris, a cracked or frost-damaged external wall, a highly vapour-resistant façade finish or a wall exposed to severe driving rain may require additional measures first. Existing insulation should also be inspected. A technical assessment before installation is therefore an important part of a correct cavity wall insulation project.

Question 3

What materials are used for cavity wall insulation?

Accepted Answer

Common materials for retrofitting existing cavity walls include EPS beads, mineral wool and insulation foams such as PUR. EPS beads are blown into the cavity with a bonding agent, while mineral wool is installed as loose fibres. PUR is injected as a foam that expands inside the cavity. Each system has different thermal properties and installation requirements. For suitable existing cavity walls, Nu-Isoleren mainly works with bonded EPS beads after first inspecting the wall and cavity.

Question 4

What is the best insulation for a cavity wall?

Accepted Answer

For many technically suitable existing cavity walls, bonded EPS beads are a very effective choice. Nu-Isoleren therefore mainly uses bonded EPS beads when retrofitting existing cavity walls. The beads are blown into the cavity together with a bonding agent and can distribute around wall ties and other obstructions. Other cavity insulation materials include mineral wool and insulation foams. There is no single material that is automatically best for every property, which is why the cavity width, contamination, existing insulation and condition of the external wall must first be checked.

Question 5

How much does cavity wall insulation cost per m²?

Accepted Answer

At Nu-Isoleren, cavity wall insulation generally costs around €21 to €25 per m² excluding VAT. The total price mainly depends on the wall area, accessibility, the condition of the cavity and any preparatory work that may be required. For 100 m² of cavity wall, this corresponds to approximately €2,100 to €2,500 excluding VAT. Before preparing the final quotation, we inspect the cavity to check whether it is technically suitable and whether contamination or existing insulation is present.

Question 6

Which insulation do you use behind exterior render?

Accepted Answer

For external wall insulation systems, we use EPS insulation boards. The required thickness depends on the existing wall and the desired thermal performance.

Question 7

How is an exterior wall insulated from the outside?

Accepted Answer

Insulation boards are installed against the existing exterior wall. Reinforcement and base layers are then applied before the façade is finished with render.

Question 8

What is the difference between render with and without insulation?

Accepted Answer

Render without insulation mainly renews the existing façade. Render with insulation also adds a new insulation layer to the outside of the wall.

Question 9

Can render be combined with external wall insulation?

Accepted Answer

Yes. Insulation boards are installed against the existing exterior wall before the façade system and final render finish are applied.

Question 10

Can exterior render be installed without insulation?

Accepted Answer

Yes. If the existing façade is technically suitable and additional external wall insulation is not required or desired, render can be installed without a new insulation layer.

Question 11

How wide does a cavity need to be for insulation?

Accepted Answer

For an existing cavity wall to be retrofitted with insulation, the cavity should be at least 5 cm wide according to Belgian technical guidance. Width alone is not enough: the cavity must also be sufficiently clear of mortar deposits, rubble and other obstructions so that the insulation can spread evenly. The external wall, mortar joints, wall ties, existing insulation and possible moisture problems should also be inspected before the cavity is filled.

Question 12

Can an existing cavity wall be retrofitted with insulation?

Accepted Answer

Yes. Many existing cavity walls that are empty or insufficiently insulated can be retrofitted. Before installation, we always check the cavity width and condition, possible contamination, any existing insulation and the condition of the exterior wall to determine whether the cavity is technically suitable.

Question 13

How much does roof insulation cost?

Accepted Answer

The price depends on factors such as roof area, insulation material, required thickness, roof structure, accessibility and finishing work. For an exact price, we assess the property and prepare a tailored quotation.

Question 14

How thick should roof insulation be?

Accepted Answer

The required thickness depends on the insulation material, available space and desired thermal performance. PIR and glass wool have different thermal properties and therefore do not automatically require the same thickness. We determine the appropriate thickness based on the complete roof build-up.

Question 15

Should I choose glass wool or PIR for my roof insulation?

Accepted Answer

This depends on the available space and existing roof construction. Nu-Isoleren works with materials including URSA glass wool and IKO PIR insulation. PIR can be useful where high insulation performance is needed within limited thickness, while glass wool can be a practical solution for other roof structures.

Question 16

How can I recognise rising damp in my home?

Accepted Answer

Common signs include damp patches along the bottom of walls, peeling paint, damaged plaster or moisture marks rising from floor level. Nu-Isoleren first investigates the source so that the actual moisture problem can be treated rather than only the visible damage.

Question 17

What does a water-repellent treatment do to my facade?

Accepted Answer

A penetrating water-repellent treatment makes a suitable porous facade less absorbent to rainwater. The impregnation product penetrates the mineral surface and reduces the amount of wind-driven rain absorbed by the masonry.Unlike a thick waterproof coating or paint, facade impregnation does not cover the wall with a visible sealed layer.Nu-Isoleren.be uses facade waterproofing to protect suitable exterior walls against moisture coming from outside. It does not repair cracks, damaged mortar joints, leaks or rising damp.

Question 18

How do I know which cleaning method my façade needs?

Accepted Answer

Not every façade should be cleaned in the same way. Nu-Isoleren examines the façade material, type of contamination and condition of the surface before choosing an appropriate cleaning method.

Question 19

Why combine render with external wall insulation?

Accepted Answer

Combining exterior render with external wall insulation allows you to renew the appearance of your property while improving the thermal performance of its exterior walls. Insulation is installed on the outside, so no interior living space is lost. Nu-Isoleren also provides rendering without additional insulation when the existing façade is suitable.

Question 20

When should old cavity wall insulation be removed?

Accepted Answer

Older cavity insulation can settle, deteriorate or lose effectiveness over time. Poorly installed insulation from an earlier renovation may also need to be removed. Nu-Isoleren first inspects the cavity to determine whether removal is actually necessary.

Question 21

What is the difference between insulating a pitched roof and a flat roof?

Accepted Answer

Pitched and flat roofs have different build-ups and therefore require different insulation methods. A pitched roof can be insulated between and below the rafters or above the roof structure when insulated externally. A flat roof is commonly built as a warm roof, where the insulation sits above the structural deck and below the waterproofing layer. An inverted roof is another option, with the insulation positioned above the waterproofing membrane.

Question 22

Can rendered and natural stone façades also be cleaned?

Accepted Answer

This depends on the type of finish and the condition of the surface. Render and certain natural stones may be sensitive to particular cleaning methods. We therefore assess the façade before deciding which treatment is appropriate.

Question 23

How much does façade cleaning cost per m²?

Accepted Answer

There is no universal price per square metre. The total area, contamination level, accessibility and required cleaning method all affect the final cost. Nu-Isoleren therefore prepares a quotation tailored to your property.

Question 24

Can a brick facade be waterproofed or impregnated?

Accepted Answer

Yes. Many porous brick facades can be treated with a penetrating water repellent, provided that the bricks and mortar joints are technically suitable and sufficiently absorbent.The exterior wall should also be clean, stable and sufficiently dry. Cracks and damaged mortar joints should be repaired beforehand.Because different bricks and finishes absorb water differently, Nu-Isoleren.be assesses the surface before carrying out brick facade impregnation.

Question 25

What are the disadvantages of facade waterproofing?

Accepted Answer

Problems mainly arise when a water-repellent treatment is applied to an unsuitable wall or without proper preparation.Facade waterproofing does not repair cracks, damaged mortar joints, leaks or rising damp. Certain wall materials and existing coatings may also be unsuitable for penetrating impregnation.Nu-Isoleren.be therefore assesses the exterior wall first. If cleaning, repointing or other repairs are required, these should be completed before the water-repellent treatment is applied.

Question 26

How much does facade waterproofing cost?

Accepted Answer

The cost of facade waterproofing depends on the surface area, accessibility, wall material and condition of the facade. There is therefore no single fixed price per square metre that applies to every property.The price can be influenced by:total facade area;accessibility;surface type and absorbency;level of contamination;condition of mortar joints;facade cleaning requirements;repairs required before treatment;scaffolding where necessary.After inspecting the exterior wall, Nu-Isoleren.be can provide a quotation based on the work actually required.

Question 27

Does facade waterproofing help with damp walls?

Accepted Answer

Facade waterproofing can help when dampness is caused by wind-driven rain being absorbed through a suitable porous exterior wall.It does not treat rising damp, where ground moisture moves upward through the masonry by capillary action.Condensation and leaks also require different solutions. Nu-Isoleren.be therefore identifies the likely source of the moisture before recommending facade impregnation or another treatment.

Question 28

How often does a facade need to be re-impregnated?

Accepted Answer

There is no single fixed interval for every facade. The service life of a water-repellent treatment depends on the product, the porosity of the masonry, exposure to wind and rain and the general condition of the exterior wall.A facade therefore does not automatically need another treatment after a specific number of years.If the wall begins absorbing noticeably more rainwater again, Nu-Isoleren.be can assess the facade and determine whether a new treatment is appropriate.

Question 29

How is a facade waterproofing treatment applied?

Accepted Answer

Nu-Isoleren.be first inspects the facade material, mortar joints, cracks, contamination, moisture problems and surface absorbency.Where necessary, the exterior wall is cleaned and repaired first. The surface must then be sufficiently dry.The penetrating facade waterproofing treatment is applied evenly so that the product can enter the porous mineral surface. Windows, doors and other areas that should not be treated are protected during application.

Question 30

When should an exterior wall be waterproofed or impregnated?

Accepted Answer

Facade impregnation can be useful when a porous exterior wall absorbs significant amounts of wind-driven rain while the masonry itself remains technically sound.A wall that stays wet for a long time after rainfall or visibly absorbs water may benefit from a penetrating water-repellent treatment.Nu-Isoleren.be first checks whether the moisture is actually caused by rainwater rather than damaged joints, cracks, leaks or rising damp. The treatment should always match the actual cause of the moisture problem.

Question 31

What are the guide prices on this website based on?

Accepted Answer

Our guide prices are based on the usual price ranges for the work we carry out. They provide a realistic initial estimate, but the final price always depends on the specific property and chosen solution.

Question 32

Can a roof without an underlay be insulated?

Accepted Answer

A roof without an underlay cannot simply be insulated in the same way as a roof with a correctly installed underlay. The underlay provides a secondary layer of protection against rain, snow and moisture entering beneath the roof covering. If it is missing, the entire roof build-up and the condition of the covering must first be assessed. Simply packing insulation directly against the underside of the tiles without an appropriate moisture-management solution is not a standard approach.

Question 33

Can an old roof still be insulated?

Accepted Answer

Yes. An old roof does not automatically need to be completely replaced before it can be insulated. If the roof structure is sound, there are no active leaks and the covering is still in good condition, a pitched roof can often be upgraded with insulation from the inside. The timber structure, signs of moisture or mould, the roof underlay and the available space between rafters should first be inspected.

Question 34

Do I need a vapour barrier with roof insulation?

Accepted Answer

When a pitched roof is insulated internally, a properly installed air and vapour control layer on the warm side of the insulation is generally essential. It helps prevent warm, humid indoor air from entering the insulation and condensing inside the roof construction. Joints, wall connections and service penetrations must also be made airtight. The exact type of membrane required depends on the complete roof build-up and the vapour permeability of the roof underlay.

Question 35

What R-value should roof insulation have?

Accepted Answer

There is no single minimum Rd value that applies to every roof renovation in Belgium. In Flanders, new roof or attic-floor insulation must achieve an Rd value of at least 4.5 m²K/W to qualify for Mijn VerbouwPremie in 2026, subject to the other eligibility conditions.In Wallonia, the temporary Primes Habitation 2025–2026 scheme ended on 30 September 2026. New projects are subject to different financing conditions through Rénopack and Rénoprêt.The insulation thickness needed to achieve a particular Rd value depends on the material's thermal conductivity, known as its lambda value.

Question 36

Is it better to insulate a roof from the inside or the outside?

Accepted Answer

For an existing pitched roof whose covering is still in good condition, insulating from the inside is usually the less invasive option because the roof covering generally does not need to be removed. If the roof is already going to be completely renovated, external insulation can be attractive because a more continuous insulation layer can be installed above the roof structure and thermal bridges can be reduced more effectively.

Question 37

What is the best roof insulation?

Accepted Answer

There is no single roof insulation material that is best for every roof. Glass wool is well suited to installation between timber rafters, while PIR is particularly useful where space is limited but high thermal resistance is required. Because PIR generally has a lower thermal conductivity, it can achieve a similar thermal resistance with less thickness than mineral wool. The best choice therefore depends on the roof type, available space, required R-value and whether the roof is insulated internally or externally.

Question 38

Will the drill holes remain visible after cavity wall insulation?

Accepted Answer

After the insulation has been installed, the drill holes are carefully filled again, so they are normally only minimally visible. The openings are mainly drilled through the mortar joints rather than randomly through the bricks. Once the cavity has been filled, the holes are repointed. We do everything possible to match the original colour and appearance of the existing mortar joints as closely as possible.

Question 39

How long does cavity wall insulation last?

Accepted Answer

Correctly installed cavity wall insulation is intended to be a long-term, permanent insulation layer rather than something that needs replacing every few years. Its actual service life depends on the material, installation quality, condition of the external wall and whether moisture or structural problems develop later. If an older property already contains cavity insulation, an endoscopic inspection can be used to check whether the material is still present and evenly distributed.

Question 40

Can existing cavity wall insulation be topped up?

Accepted Answer

Yes, existing cavity wall insulation can sometimes be topped up, but the existing material and the remaining free space in the cavity must first be inspected. If the old insulation is still in good condition but leaves gaps or insufficiently filled areas, adding insulation may be possible. If it has collapsed, become damp, contaminated, unevenly distributed or is incompatible with the new system, some or all of the old insulation may need to be removed first.

Question 41

What type of insulation does my home need?

Accepted Answer

The most suitable insulation depends on the age, construction and current condition of your property. In many older homes, the roof and external walls are important areas to check. If the property has a suitable uninsulated cavity wall, cavity wall insulation may be an effective option. If there is no suitable cavity, external wall insulation with render may be more appropriate. The free Nu-Isoleren home check provides an initial indication of which solutions may suit your property.

Question 42

How long does it take to insulate a cavity wall?

Accepted Answer

In most cases, the cavity walls of an average home are fully insulated within a single working day, without any demolition work or inconvenience.

Question 43

What is a good EPC or PEB rating?

Accepted Answer

A lower energy score indicates better energy performance. In Flanders, an EPC score between 1 and 100 kWh/(m² year) corresponds to label A, while a score of 0 or below corresponds to A+. Brussels and Wallonia use their own PEB scales and thresholds, so scores should always be interpreted according to the region where the property is located.

Question 44

How much energy can I save by insulating my home?

Accepted Answer

The amount of energy you can save depends on your home's existing insulation, the size of the roof or external walls, and your heating system.In a poorly insulated property, roof insulation, cavity wall insulation or external wall insulation can significantly reduce heat loss. However, this does not mean every home will achieve the same percentage reduction in energy bills.Nu-Isoleren's free energy savings calculator provides an indicative estimate based on your property and the insulation improvement you are considering.

Question 45

What insulation grants are available in Belgium?

Accepted Answer

Belgium offers various grants, subsidies and financial support schemes for insulation and energy renovation work. The support available depends on the region where the property is located. Flanders, Brussels and Wallonia each have their own rules, eligibility requirements, grant amounts and application procedures. Depending on the property and applicant, support may be available for roof insulation, cavity wall insulation, external wall insulation and other energy renovation measures. Nu-Isoleren provides an overview by region to help homeowners identify which schemes may be relevant.

Question 46

How do I know if my cavity walls can be insulated?

Accepted Answer

Not every cavity wall can be insulated without an inspection. The width and condition of the cavity, the exterior wall and any existing insulation should first be checked. An inspection can determine whether the cavity is suitable for insulation with EPS beads.

Question 47

Can my façade be protected after cleaning?

Accepted Answer

Yes. Where the façade is suitable, cleaning can be followed by a water-repellent treatment. The surface is first cleaned and prepared before the protective treatment is applied.

Question 48

What factors affect the cost of exterior render per m²?

Accepted Answer

The cost depends on the façade area, substrate condition, accessibility, selected finish and any preparation required. When external wall insulation is included, insulation thickness and technical junctions also influence the final price. We therefore provide a quotation tailored to each property and the chosen installation method.

Question 49

Does my facade need to be cleaned before waterproofing?

Accepted Answer

Yes, when a facade is contaminated it should first be properly cleaned and allowed to dry sufficiently before a water-repellent treatment is applied.Moss, algae, dust and atmospheric deposits can prevent the impregnation product from penetrating evenly. Cracks and damaged mortar joints should also be inspected and repaired where necessary.Nu-Isoleren.be therefore checks whether professional facade cleaning is required before the exterior wall is waterproofed or impregnated.

Question 50

Are the prices shown final prices?

Accepted Answer

No. The amounts shown are indicative guide prices. For a final price, we assess your individual project and provide a tailored quotation.

Question 51

How does Nu-Isoleren treat rising damp?

Accepted Answer

Where the wall is suitable, openings are drilled at a controlled level and a damp-proof treatment is introduced. This creates a barrier designed to stop moisture from continuing to rise through the masonry, after which the existing wall can gradually dry.

Question 52

How much can I save with roof insulation?

Accepted Answer

Roof insulation is often one of the most effective measures when a roof has little or no insulation. Around 30% of a home's heat can be lost through an inadequately insulated roof. Actual savings depend on the existing insulation, roof area, heating system and current energy consumption, which is why Nu-Isoleren calculates an estimate based on your individual situation rather than promising one fixed percentage.

Question 53

How does Nu-Isoleren remove old cavity wall insulation?

Accepted Answer

The removal method depends on the type and condition of the existing insulation. The old material is removed from the cavity so the space can be cleared as thoroughly as possible. Afterwards, we can assess whether the wall is suitable for new insulation.

Question 54

Should I insulate my roof or the attic floor?

Accepted Answer

If the attic is heated or will become living space, insulating the roof itself is generally the logical choice. If the attic remains unheated and is rarely used, insulating the attic floor can be an attractive alternative. Nu-Isoleren assesses which solution best matches how the property is used.

Question 55

Where should I start when insulating my home?

Accepted Answer

For a poorly insulated property, the roof is often a logical place to start because roof insulation can significantly reduce heat loss. The external walls can then be assessed for cavity wall insulation or external wall insulation. The ideal order depends on the property itself. Nu-Isoleren uses the home check to assess the roof, walls, façade and possible moisture issues before suggesting the most relevant solutions.

Question 56

Are insulation grants different in Flanders, Brussels and Wallonia?

Accepted Answer

Yes. Insulation grants differ between the three Belgian regions. Flanders has its own renovation support system, the Brussels-Capital Region has separate financial support and financing schemes, and Wallonia operates its own grant system. Grant amounts are not the only difference: income requirements, technical standards, eligible work and application procedures can also vary. Nu-Isoleren therefore recommends checking the rules for the region in which the property is located before insulation work begins.

Question 57

How is an EPC or PEB score calculated?

Accepted Answer

An energy performance score is calculated using the energy-related characteristics of the property. These include the size of the home, roof, wall and floor insulation, windows and doors, heating, domestic hot water, cooling, ventilation and solar energy systems. Standardised assumptions are used so that the energy performance of different homes can be compared.

Question 58

Can I get a grant for roof insulation?

Accepted Answer

Financial support may be available for roof insulation, depending on the region and the applicant's circumstances. Eligibility can depend on factors such as income, the property, the insulation performance and the date of application. In Flanders, roof insulation falls within the roof category of Mijn VerbouwPremie, although eligibility rules changed in 2026. Nu-Isoleren installs roof insulation using solutions including glass wool and PIR insulation boards.

Question 59

How much energy can cavity wall insulation save?

Accepted Answer

Cavity wall insulation can significantly reduce heat loss through external walls when the cavity is empty or insufficiently insulated. The actual saving depends on factors such as wall area, existing insulation, cavity width and heating system. The Flemish government describes cavity wall insulation as a cost-effective way to make an existing home more energy efficient.

Question 60

Can my cavity walls be insulated again after the old insulation is removed?

Accepted Answer

In many cases, yes. Once the old insulation has been removed, Nu-Isoleren reassesses the condition of the cavity. If suitable, the wall can then be re-insulated using a modern EPS bead system.

Question 61

How do I know whether my home has cavity walls that can be insulated?

Accepted Answer

In Flanders, cavity walls became increasingly common from the 1950s onwards and continued to be widely used throughout the following decades, well into the 1990s and 2000s. However, the year of construction alone does not provide certainty. Wall thickness, brickwork and certain openings can provide useful indications. To determine whether the cavity is suitable for insulation, Nu-Isoleren carries out an on-site inspection, checking factors such as cavity width, debris, moisture and the condition of the exterior wall.

Question 62

What does kWh/(m² year) mean on an EPC or PEB certificate?

Accepted Answer

kWh/(m² year) expresses the calculated annual primary energy performance per square metre of floor area. A lower figure generally indicates a more energy-efficient property. It is a theoretical energy performance figure and is not the same as the actual consumption shown on your energy bills.

Question 63

Can cavity wall insulation cause moisture problems?

Accepted Answer

The EPS system used by Nu-Isoleren absorbs virtually no water and retains its insulating properties within the cavity. Before installation, we still check that the cavity and exterior wall are suitable and that there are no existing defects that need to be resolved first. This combines a moisture-resistant insulation material with correct installation.

Question 64

Can Nu-Isoleren insulate an existing roof from the inside?

Accepted Answer

Yes. Many existing pitched roofs can be insulated from the inside without replacing the entire roof covering. Nu-Isoleren first examines the existing roof structure and then selects a suitable insulation build-up and material.

Frequently asked questions about insulation and façade works

General & Grants

Cavity wall insulation

Render

Roof insulation

Rising damp

Façade waterproofing

Façade cleaning

Cavity wall insulation removal

Can't find your answer?

Calculate

About us