Roof Insulation Guide Luxembourg 2026: Interior vs Sarking

Why Insulate Your Roof in Luxembourg?

The roof is the primary source of heat loss in a poorly insulated house: up to 30% of heat escapes through the roof. In a country with harsh winters and high energy costs, effective insulation is a worthwhile investment. It improves comfort in both summer and winter, reduces energy bills, and increases property value.

In Luxembourg, thermal regulations impose minimum requirements for new constructions, but for renovations, homeowners should aim for higher performance to benefit from the Klimabonus Wunnen 2026 subsidies. This program, which replaced the former PRIMe House in January 2026, offers attractive grants for roof insulation.

The Two Techniques: Interior Insulation and Sarking

Two main approaches exist for insulating a pitched roof: interior insulation and sarking (external insulation). The choice depends on several factors: roof condition, roof structure, available space, budget, and architectural constraints.

Interior Insulation

This method involves placing insulation between and under the rafters, from inside the attic. It is often used for attics that are finished or can be finished. Common materials include glass wool, rock wool, wood fiber, or polyurethane.

• Advantages: lower cost, quick installation, does not alter the exterior appearance of the roof.
• Disadvantages: reduces living space (insulation thickness can reach 20-30 cm), risk of thermal bridges at rafters, requires careful vapor barrier to avoid condensation.

Sarking (External Insulation)

Sarking involves placing rigid insulation above the rafters, under the roofing. This technique is increasingly popular for full renovations. Insulation panels are often made of wood fiber, fiberboard, or polyurethane.

• Advantages: almost complete elimination of thermal bridges, continuous and efficient insulation, protection of the roof structure against humidity and temperature variations, does not reduce interior space.
• Disadvantages: higher cost (materials and labor), requires removing the existing roofing, longer and more complex work.

Selection Criteria Based on Your Situation

To choose between the two techniques, evaluate:

• Roof condition: if the roofing needs replacement, sarking is often more relevant as you can install external insulation during the work.
• Living space: if the attic is finished or cramped, interior insulation can reduce ceiling height. Sarking preserves space.
• Architectural constraints: for listed buildings or those in protected areas, the exterior appearance must be preserved, often requiring interior insulation.
• Budget: sarking typically costs €80-150/m², compared to €40-80/m² for interior. Subsidies can reduce the gap.
• Thermal performance: sarking offers better air tightness and more homogeneous insulation, crucial for achieving high R-values.

Technical Requirements and Recommended Materials

To qualify for Klimabonus subsidies, insulation must achieve a minimum thermal resistance (R):

These values correspond to varying thicknesses depending on the material: for example, glass wool requires about 24 cm for R=6, while polyurethane needs only 12 cm. Bio-based materials like wood fiber offer a good balance between performance and summer comfort.

It is essential to use a qualified professional to ensure proper installation, especially regarding air tightness and ventilation. A defect can lead to condensation and mold, compromising durability.

Subsidies and Financial Aid in 2026

Since January 2026, the Klimabonus Wunnen program (managed by Klima-Agence) replaces the old PRIMe House system. It offers aid for many energy renovation works, including roof insulation.

• Roof insulation: subsidy up to 50% of eligible costs, capped at €15,000.
• Energy audit: subsidy up to 50% of cost, capped at €5,000 (required for some aids).
• Photovoltaic panels: up to €10,000 (approx. €8,300 for 10 kWp, max from 15 kWp).
• Home battery: up to €2,250.
• Window replacement: €70 per m² of replaced glazing.

Additionally, the reduced VAT of 3% applies to renovation work in dwellings over 2 years old, significantly reducing total cost.

To prepare your application, consult guichet.public.lu and have an energy audit done by an approved professional. Subsidy applications are submitted online via the Klima-Agence portal.

Illustrative Example: Sarking vs Interior Insulation for a Typical House

Consider a single-family house from the 1970s with a pitched roof of 150 m² in Luxembourg. The owner wants to improve insulation.

Option 1: Interior insulation (wood fiber, R=7): average cost of €60/m², totaling €9,000. After 50% subsidy (capped at €15,000), out-of-pocket cost is €4,500.

Option 2: Sarking (wood fiber, R=6): average cost of €120/m², totaling €18,000. After 50% subsidy, out-of-pocket cost is €9,000.

The extra cost of sarking is €4,500, but it offers better performance and does not reduce living space. Moreover, if the roofing needs replacement, the additional cost is lower as you avoid removing and reinstalling the roofing twice.

This example is illustrative; actual costs vary based on materials, project complexity, and contractors.

Key Steps in an Insulation Project

1. Energy audit: have a professional audit to assess current state and prioritize work.
2. Choose technique: based on criteria above, decide between sarking and interior insulation.
3. Request quotes: obtain at least 3 detailed quotes from qualified craftsmen (RGE certification or equivalent).
4. Submit subsidy application: before starting work, apply to Klima-Agence.
5. Carry out work: entrust the project to a professional for quality.
6. Claim payment: after work, submit invoices and documents to receive the subsidy.

Mistakes to Avoid

• Neglecting air tightness: a poorly installed vapor barrier can cause moisture damage.
• Choosing too thin insulation to save money: performance may be insufficient.
• Ignoring ventilation: an insulated roof must be properly ventilated to expel moisture.
• Not checking subsidies before starting: some must be applied for before work begins.
• Using an unqualified contractor: guarantees and quality may be compromised.

Conclusion

Whether you choose interior insulation or sarking, the key is to carry out quality work adapted to your home and budget. The Klimabonus Wunnen 2026 program offers a significant financial opportunity to offset these costs. Don't hesitate to consult an energy advisor to guide your choices and procedures.