Klimabonus Wunnen 2026: Roof Insulation R-Value 6 or 7? What the Law Really Requires

Klimabonus Wunnen 2026: What Changes for Insulation

Since January 2026, the Klimabonus Wunnen has applied new rates, set by the law of 24 July 2026. The Klimabonus name itself dates from January 2022, when it replaced PRIMe House. For roof insulation, the aid is an amount per m² insulated, from EUR 35 to 115/m² depending on the insulant and the performance standard reached (up to EUR 140/m² with the transitional increase), capped at 60% of costs excluding VAT. You often read that you need R ≥ 6 m²K/W for sarking or R ≥ 7 m²K/W for lost attics: these thresholds are not in the law. What it requires is a minimum insulation thickness and, for the higher standards, a maximum U-value for the roof. Here is what that means in practice.

Thermal resistance R measures a material's ability to resist heat flow. The higher the R, the better the insulation. The law, however, does not reason in R: it sets a minimum insulation thickness, required whatever the standard, and, in addition for standards II and I, a maximum U-value for the insulated roof. R remains useful for comparing insulants, but only as an indicative equivalent.

R 6 or 7, or 20 cm: What the Law Really Requires

R (thermal resistance) is expressed in square meters kelvin per watt (m²K/W). It depends on the thickness and thermal conductivity (lambda) of the insulation. Annex II of the law of 24 July 2026 sets, for a pitched or flat roof as for the floor of a lost attic, a minimum thickness of 20 cm of insulant at lambda = 0.035 W/mK. For another insulant, the thickness is converted pro rata to its conductivity: about 22.9 cm at lambda = 0.040 and 17.1 cm at lambda = 0.030. As an indication, 20 cm at 0.035 is a thermal resistance of about 5.7 m²K/W for the insulant alone: this figure is for comparison, it is not a legal threshold.

Source: law of 24 July 2026, Annex II.

So there is no higher threshold for lost attics: the minimum thickness is the same as for sarking (20 cm at lambda = 0.035); only the standard I U-value differs (0.10 instead of 0.11 for a roof). Above all, this thickness applies whatever the standard: even if your roof reaches U ≤ 0.11, new insulation thinner than the standard III minimum makes it ineligible. Standard I is the most demanding and the best funded. If the loft is heated, the roof renovation must also include replacing roof windows older than 10 years with a U-value above 1.4 W/(m²K) by windows with U ≤ 1.4 W/(m²K).

Concretely, a well-insulated roof significantly reduces heat loss. A poorly insulated roof can account for up to 30% of a house's heat loss. By aiming for standard II or I, you improve comfort in both winter and summer and reduce your heating bill significantly.

Klimabonus Wunnen 2026 Grants for Insulation

Klimabonus Wunnen 2026 offers a generous subsidy for roof insulation, provided the legal minimum requirements are met. Here are the key points:

• Amount: a rate per m² insulated. Pitched or flat roof: EUR 35 / 45 / 60 per m² (fossil-based insulant more than 50% recycled), 60 / 70 / 85 (mineral) or 90 / 100 / 115 (ecological) for standards III / II / I. Floor of a lost attic: EUR 20 / 30 / 45, 25 / 35 / 50 or 40 / 50 / 65 per m². These amounts are increased if the agreement in principle is applied for between 01/01/2026 and 30/06/2027 and the invoice is issued by 31/12/2030 at the latest.
• Cap: 60% of the costs of the renovation work excluding VAT.
• Conditions: at least 20 cm of insulant at lambda = 0.035 (or the equivalent thickness) in every case; in addition, U ≤ 0.13 for standard II, U ≤ 0.11 (roof) or 0.10 (lost attic) for standard I.
• Combination: possible with the super-reduced VAT of 3% for homes meeting the age requirement.

To benefit from the aid, if only the roof is renovated, the work must be carried out by a company approved under the law of 21 April 1993 or followed by an energy adviser; as soon as several elements are renovated, energy advice is mandatory. You must also provide performance proof (technical data sheets, insulant thickness and conductivity, U-value calculation). The application is made online via Guichet.lu or Klima-Agence. Payment occurs after validation of the work.

Note: PRIMe House is a closed scheme, replaced by the Klimabonus as early as January 2022. Since January 2026, the rates of the law of 24 July 2026 apply to new applications for an agreement in principle: submit yours under this scheme, before work begins.

Which Thickness for Which Insulant? Choice of Materials

To reach the legal minimum thickness, several solutions are available. The choice depends on your roof configuration and budget. Here is the standard III minimum thickness converted for the conductivity of common insulants:

As the table shows, the lower the conductivity, the smaller the required thickness. For sarking as for lost attics, 20 cm of mineral wool at lambda = 0.035 is enough for standard III. For standards II and I, what counts is the U-value of the whole roof: your craftsman or energy adviser must calculate it. Beware of polyurethane: although thinner, it does not qualify for the aid unless more than 50% recycled.

It is essential to have your craftsman check the thickness and the U-value precisely. A thickness error could compromise eligibility for the subsidy. Moreover, insufficient insulation will lead to thermal bridges and excessive condensation, harming the durability of your roof.

Typical Example: How Much Does Eligible Insulation Cost?

Let's take an illustrative case: a single-family house in Luxembourg City with a 100 m² pitched roof to insulate with sarking. The goal is to reach at least standard III. The craftsman proposes 22 cm of rock wool at lambda = 0.035, above the legal minimum of 20 cm, with new covering. The quote amounts to EUR 18,000 including VAT (materials and labor).

With a mineral insulant at standard III, Klimabonus Wunnen 2026 pays EUR 60/m², i.e. EUR 6,000 for 100 m², and EUR 7,000 with the transitional increase (EUR 70/m²). This amount stays below the cap of 60% of costs excluding VAT. Additionally, the super-reduced VAT of 3% applies (home meeting the age requirement), reducing the initial bill. In the end, your out-of-pocket cost is about EUR 11,000 to 12,000, with estimated energy savings of EUR 1,200 per year on heating. The return on investment is about 9 to 10 years.

Another illustrative case: an 80 m² lost attic. 30 cm of cellulose wadding (lambda = 0.040) is blown in, above the legal minimum of 22.9 cm for this conductivity. The cost is EUR 6,000. Note that the floor of a lost attic falls under the rate for an upper slab against an unheated zone, lower than the roof rate. At standard III, if the cellulose is recognised as an ecological insulant (check with your energy adviser), the aid is EUR 40/m², i.e. EUR 3,200; with the transitional increase (EUR 50/m², i.e. EUR 4,000), it is limited by the cap of 60% of costs excluding VAT, about EUR 3,500 here. With 3% VAT, the out-of-pocket cost is about EUR 2,500 to 2,800. Annual savings are about EUR 600.

These examples show that Klimabonus Wunnen 2026 makes high-performance insulation very accessible, especially when combined with reduced VAT.

Other Klimabonus Wunnen 2026 Grants

Klimabonus Wunnen 2026 is not limited to roof insulation. It also includes subsidies for other energy renovation work:

• Windows: EUR 70 per m² of window, measured by its external dimensions (EUR 80 with the transitional increase), for glazing with Ug ≤ 0.6 W/(m²K), in practice high-performance triple glazing.
• Photovoltaic panels: EUR 8,250 for a 10 kWp installation, then a flat EUR 10,000 from 15 kWp. In 2026, the legal basis for this aid is still the amended law of 23 December 2016: the PV provisions of the law of 24 July 2026 only apply from 1 January 2027. No agreement in principle is needed: you apply after the work, or an installer listed in the Ministry of the Economy register deducts the aid from the final invoice under the optional pre-financing.
• Home battery: for a battery linked to a PV installation, EUR 1,805.56 for 5 kWh of usable capacity and a flat EUR 2,250 from 9 kWh.

Each of these aids has its own conditions and caps. For example, you can insulate your roof, replace your windows, and install solar panels, all subsidized; as soon as several elements of the envelope are renovated, energy advice is mandatory, and window replacement must in principle go together with external wall insulation or the installation of mechanical ventilation. The 3% VAT applies to the insulation and window work in homes meeting its conditions (main residence, age of the building), and to the supply and installation of photovoltaic panels on a private home, applied directly by the installer; for a battery, ask your installer which rate applies.

To benefit from these grants, you must meet the technical criteria specific to each equipment. For example, for windows, it is the thermal transmittance of the glass (Ug) that must be less than or equal to 0.6 W/m²K; the law sets no Uw for façade windows. For photovoltaics, the installation must have at least 2 kWp and run in self-consumption, and you must give up the regulated feed-in tariff.

How to Apply?

The application for Klimabonus Wunnen 2026 is done in several steps:

• Obtain a detailed quote from a certified craftsman, stating the insulant thickness and conductivity and the target U-value.
• Submit the file before starting work on the Guichet.lu portal or via Klima-Agence.
• Provide paid invoices and performance proof after work.
• Receive the subsidy payment within a few weeks.

It is recommended to be accompanied by an energy advisor to prepare the file. Klima-Agence offers free consultations. Also, ensure your craftsman is properly registered and complies with Luxembourg standards.

Warning: applications must be submitted before work begins. No subsidy will be granted for work already done without prior authorization.

Conclusion: A Profitable and Ecological Investment

Meeting the minimum thickness of 20 cm of insulant (at lambda = 0.035) is not just an administrative requirement to obtain Klimabonus Wunnen 2026. It is a guarantee of durable energy performance. By properly insulating your roof, you reduce your heating consumption, improve your comfort, and add value to your property. With a per-m² aid capped at 60% of costs excluding VAT and 3% VAT, the return on investment is quick. Don't wait to consult a professional and take advantage of these generous grants.