The 8 key criteria for choosing thermal insulation

Choosing insulation is about more than comparing two columns of figures in a catalogue. In practice in Luxembourg, eight criteria determine whether a material suits your project and whether you can get the maximum financial support from it. Understanding how they fit together will help you make an informed choice and maximise the return on your insulation investment.

Klimabonus 2026: which insulation maximises your grant in Luxembourg?

The Klimabonus Wunnen scheme, overhauled with effect from 1 January 2026, distinguishes between insulation materials based on their origin, as the 2022 scheme already did, and, new in 2026, on how they are fixed (mechanically fixed non-fossil cladding). Understanding this classification before you choose your material is essential if you want to maximise your grants. A crucial reminder: the Klimabonus approval in principle must be obtained before any work starts — no exceptions are possible.

Source: Guichet.lu — Klimabonus Wunnen, scheme in force since 1 January 2026.

Complete comparison table of thermal insulation materials 2026

This table brings together the 8 insulation materials most widely used in residential renovation in Luxembourg. The λ values are typical declared values — check your product’s technical documentation (ACERMI, DTA or FDES). The prices shown are ranges for the whole job (material + installation + finishes) on a standard residential project in Luxembourg in 2026, including 3% VAT.

* Mineral wool: €60–85/m² depending on the standard achieved (+€15/m² if more than 50% recycled material). EPS: €0/m² if standard; €35–60/m² if it contains more than 50% recycled material. Higher amounts if the approval in principle is applied for before 30 June 2027. For a wall insulated exclusively from the inside (IWI), apply a 25% reduction. Sources: Guichet.lu Klimabonus Wunnen — scheme in force since 1 January 2026; price data collected from Renov.lu’s verified partner contractors — Luxembourg 2026.

Glass wool — the versatile budget option

Glass wool is the most widely used insulation in Europe. Made from recycled glass (60 to 80% of its composition), it comes as rolls, semi-rigid batts or blown loose-fill. Its thermal conductivity ranges from λ = 0.032 to 0.044 W/m·K depending on density (from 10 kg/m³ for lightweight rolls to 32 kg/m³ for semi-rigid batts). It is stocked by every builders’ merchant in the Grand Duchy, and virtually every insulation contractor knows how to install it.

Strengths: lowest purchase price on the market, good fire resistance (class A1 or A2 depending on the product), wide range of products and thicknesses, easy for most contractors to install, compatible with every structural configuration. Weaknesses: low thermal phase shift (2 to 5 hours for lightweight rolls), vapour barrier required for IWI, irritating to the skin and mucous membranes if handled without suitable protection. Not suitable for damp areas without additional protection.

Recommended uses in Luxembourg: insulating unconverted lofts (as blown loose-fill or rolls), acoustic insulation of internal partitions, and IWI on metal stud framing in more recent homes (concrete or aerated concrete) with low moisture constraints. For older stone or brick houses prone to rising damp, choose hygroscopic insulation instead. A typical application: soundproofing a bedroom.

Rock wool — the fire-resistant option

Made from volcanic rock (mainly basalt) melted at very high temperatures (around 1,500 °C), rock wool outperforms glass wool on two key points: mechanical strength and fire behaviour. Its higher density — up to 160–200 kg/m³ for rigid EWI boards — gives it better long-term stability and a rigidity that is useful for cladding systems. Its typical thermal conductivity is λ = 0.034 to 0.045 W/m·K.

Its thermal phase shift (3 to 6 hours depending on density) remains lower than that of bio-based insulation, but dense rock wool (80 kg/m³ and above) offers a noticeably better phase shift than lightweight wool. It is widely used in EWI systems with ventilated cladding and in areas that require particular mechanical strength or non-combustibility (boiler rooms, attached garages).

Recommended uses in Luxembourg: EWI with ventilated cladding on detached houses and apartment buildings, insulation of flat roofs (high-density boards), technical areas requiring fire resistance (boiler rooms, garages attached to the house), and IWI in apartment buildings with demanding fire safety regulations.

Wood fibre — the champion of summer comfort

Wood fibre (also known as wood wool) is made by defibrating wood offcuts or timber that could not otherwise be used — often sawmill residues. It is the bio-based insulation that offers the best balance between thermal performance in winter, exceptional summer comfort and hygric properties. Its thermal conductivity ranges from λ = 0.036 to 0.050 W/m·K depending on density (from 55 kg/m³ for flexible batts to 200+ kg/m³ for high-density rigid EWI boards).

Its main differentiator is its thermal phase shift: with 20 cm of dense wood fibre board, the phase shift can reach 12 to 14 hours. In practical terms, if the south-facing façade reaches its peak temperature at 2 pm, the inside of your home will only feel that heat in the middle of the night — by which time night-time ventilation will already have cooled the indoor air naturally. For homes without air conditioning, this property is decisive in Luxembourg’s increasingly hot summers. Wood fibre is also hygroscopic, which allows it to regulate humidity naturally and makes it ideally suited to traditional Luxembourg masonry houses.

Available forms: high-density rigid boards for EWI (rendered or clad), semi-rigid batts for IWI in framing or for sloping roofs, and blown or injected loose-fill for unconverted lofts and timber-frame walls. For roof and loft insulation work, semi-rigid wood fibre between the rafters is often the best all-round choice.

Cellulose — the affordable bio-based option

Cellulose insulation is made from newspaper and recycled paper (80 to 85% of its composition), treated with fire-retardant and anti-pest agents. Its thermal conductivity is λ = 0.038 to 0.042 W/m·K. It clearly stands out on three complementary criteria: thermal phase shift (8 to 12 hours), moisture regulation and an exceptional environmental footprint — it requires around 30 times less embodied energy to manufacture than polystyrene.

It is available as blown loose-fill for unconverted lofts (its preferred use), as semi-rigid batts for IWI in framing, or injected into timber-frame walls. Its purchase price is one of the most competitive among bio-based materials. Blowing loose-fill into an unconverted loft is particularly quick — often a single day for an entire house — and leaves no area uncovered, as it fills every gap and irregularity in the roof timbers.

Hemp — the natural moisture regulator

Industrial hemp is grown without pesticides or herbicides and processed into insulation in the form of rolls or semi-rigid batts. Its thermal conductivity is λ = 0.039 to 0.045 W/m·K. Its most remarkable feature is the way it manages moisture: hemp is one of the most hygroscopic insulation materials on the market, able to absorb up to 30% of its own weight in water vapour and release it gradually with no loss of thermal performance. This makes it the insulation of choice for older stone, rubble-stone or brick houses — typical of Luxembourg’s pre-1950 building heritage — where natural moisture regulation is essential to prevent interstitial condensation, mould and the gradual deterioration of the walls.

Its thermal phase shift (8 to 12 hours) is comparable to that of cellulose. Hemp is often installed without a vapour barrier thanks to its ability to manage water vapour naturally — which simplifies installation and reduces the risk of defects caused by gaps in the vapour barrier. For internal insulation work in an older Luxembourg house, hemp is often the technically safest solution.

EPS (expanded polystyrene) — the budget EWI option, but excluded from the standard Klimabonus

Expanded polystyrene (EPS, also sold under the name Styropor®) has historically been the dominant material for external wall insulation in Europe. Its thermal conductivity is λ = 0.030 to 0.038 W/m·K (grey graphite EPS reaches λ ≈ 0.031 W/m·K). Its light weight, low cost and the maturity of thin-coat render systems still make it the benchmark material for rendered EWI in France and Belgium.

Technical advantages: one of the lowest costs among EWI systems (as a guide, €150–220/m² excl. VAT installed with thin-coat render in Luxembourg), good moisture resistance, a wide network of qualified contractors, easy to cut. Major limitations: very low thermal phase shift (2 to 4 hours), combustible (class E to F) and gives off toxic smoke in a fire, high carbon footprint (petroleum-based material), and — crucially in Luxembourg — excluded from the 2026 Klimabonus in its standard form. Only EPS containing more than 50% recycled material qualifies for a reduced grant of €35–60/m² for EWI, depending on the standard achieved.

Polyurethane (PUR) — top performance at minimal thickness

Polyurethane (PUR) is the synthetic insulation with the best thermal performance on the consumer market: its thermal conductivity reaches λ = 0.022 to 0.028 W/m·K for rigid foams. To reach R = 4, you need only 9 to 11 cm, compared with 13 to 18 cm for mineral wools — a decisive advantage for IWI in small rooms where every centimetre counts.

PUR mainly comes as rigid foam boards (under-screed insulation, IWI over small areas) or as foam sprayed in situ (floor undersides, flat roofs). Significant limitations: a fossil-based material, reaction-to-fire class E or F with extremely toxic gases released in a fire (notably hydrogen cyanide), minimal thermal phase shift (1 to 3 hours) and very low water vapour permeability. Above all, standard PUR is excluded from the Luxembourg Klimabonus, except for walls and floor slabs in contact with the ground and for green roofs.

When should you use it despite its exclusion from the Klimabonus? PUR remains relevant where space is genuinely critical and grants are not the main priority: insulation under an existing floor screed (cellar, basement) where compressive strength is decisive, or insulation under an inaccessible flat roof. For floor and cellar insulation work, see our dedicated guide.

Expanded cork — the premium solution

Expanded cork is made from the bark of the cork oak (Quercus suber), a 100% natural and renewable resource harvested without felling the tree — a cork oak can be stripped every 9 to 12 years for 150 to 200 years. It stands out from all other bio-based insulation materials thanks to one unique feature: natural moisture resistance without any chemical treatment. Cork is rot-proof, unaffected by mould, rodents and insects, and keeps its performance in damp environments or in direct contact with the ground. Its thermal conductivity is λ = 0.036 to 0.042 W/m·K.

Its exceptional thermal phase shift (10 to 14 hours) puts it on a par with dense wood fibre. It also offers excellent acoustic performance — a dual thermal and sound insulation function — that is particularly valued for floors, wall bases and rooms with party walls. Its main drawback remains its higher cost (€100–145/m² installed for IWI), which limits it to projects where long-term quality and durability take precedence over the initial budget.

Which insulation for which part of the house? A practical guide for Luxembourg

The right insulation depends as much on the area being treated as on the material itself. Here are our recommendations for Luxembourg’s specific context: a largely older housing stock (60–70% of homes built before 1980), a temperate continental climate and the Klimabonus rules in 2026. For an overview of strategies to improve your energy rating, see our dedicated guide.

External walls — EWI (external wall insulation)

This technique preserves all of your living space and maximises the Klimabonus (no 25% reduction). The first choice is wood fibre in high-density rigid boards (exceptional phase shift, €115/m² of Klimabonus under render, €130/m² with non-fossil cladding). The second choice is rock wool behind ventilated cladding (up to €85/m², excellent fire behaviour). EPS is only viable if it contains 50%+ recycled material (check the FDES); in its standard form, it qualifies for no Klimabonus grant at all. PUR should be avoided for EWI (excluded from the Klimabonus, fire risk).

Walls — IWI (internal wall insulation)

For older stone or brick houses — typical of the historic districts of Luxembourg City, Esch-sur-Alzette and Dudelange, or of rural villages in the north and east of the country — it is essential to choose breathable, hygroscopic insulation: hemp (best moisture regulation) or wood fibre (best phase shift). For more recent houses built in concrete or aerated concrete (1970s–1990s), rock wool or glass wool on metal stud framing is a cost-effective, high-performing solution. For small rooms where space is at a premium, PUR remains viable even without the Klimabonus. For more details, see our guide to internal wall insulation (IWI).

Unconverted lofts

This is the simplest, quickest and often the most cost-effective application. Blown cellulose is the optimal choice: excellent value for money, an 8–12 hour phase shift, an exceptional carbon footprint and up to €65/m² of Klimabonus on the loft floor (€75/m² if the approval in principle is applied for before 30 June 2027). Glass wool or rock wool rolls remain a budget alternative (mineral category: up to €50/m² on a loft floor). See our guide to roof and loft insulation in Luxembourg.

Ground floors and floor slabs

The main constraint is mechanical resistance to crushing under load. High-density EPS or XPS remain the most technically suitable solutions; they are only supported by the Klimabonus for a slab in contact with the ground (not for a slab above a cellar), unless they contain more than 50% recycled material. High-density expanded cork (120 kg/m³+) is the only Klimabonus-eligible bio-based alternative. For details, see our guide to floor and cellar insulation.

Pitched roofs (sloping roof sections and joists)

Semi-rigid wood fibre between the rafters offers the best overall summer comfort (vital for rooms directly under the roof of a converted attic). Blown cellulose between the rafters is a high-performing, lower-cost alternative. Mineral wools remain the standard budget choice.

Frequently asked questions about insulation materials in Luxembourg