Insulation is the single most cost-effective home improvement you can make. A well-insulated roof saves €300–€600 per year on heating in an average house. But "how much" is more complex than "how much paint" — because thickness, material, and where you're insulating all matter. This guide breaks it down.
The two numbers that matter: R-value & U-value
Forget everything else for now. There are only two numbers you need to understand:
EU minimum R-values by surface (2026 standard)
| Surface | Minimum R | Recommended R | Typical thickness (PIR) |
|---|---|---|---|
| Roof / loft | R 6.0 | R 7.0+ | 14–18 cm |
| Cavity wall (new) | R 4.7 | R 5.5+ | 10–14 cm |
| Solid wall (interior) | R 3.5 | R 4.5+ | 8–12 cm |
| Floor (above unheated) | R 3.5 | R 4.0+ | 8–10 cm |
| Floor (above ground) | R 3.5 | R 5.0+ | 8–14 cm |
Going above the minimum is usually worth it. The cost difference between R 6.0 and R 7.0 insulation is small (~10%) but the energy savings over 30 years add up significantly.
How to convert R-value to thickness
Thickness = 6.0 × 0.022 = 0.132m = 13.2 cm
Same R 6.0 with glass wool (λ = 0.035) = 21 cm needed.
Lambda (λ) is the thermal conductivity of the material — printed on every insulation product. Lower lambda = thinner insulation needed for the same R-value.
Common insulation materials
| Material | λ value | Cost (€/m²) | Best for |
|---|---|---|---|
| PIR (rigid foam) | 0.022 | 20–35 | Roof, cavity, where space is tight |
| EPS (Styrofoam) | 0.035 | 10–18 | Floor, exterior walls |
| Glass wool | 0.035 | 8–14 | Loft, between studs |
| Rock wool | 0.038 | 10–16 | Soundproofing + thermal |
| Wood fibre | 0.040 | 20–30 | Eco-build, breathable walls |
| Hemp / sheep wool | 0.038–0.040 | 25–40 | Eco, breathable |
How much do you need? Step-by-step
Step 1 — Measure the area
Width × height for each wall. Add roof slope length × roof width for sloped roofs (not the floor projection). Don't subtract small windows — the cuts are waste material anyway.
Step 2 — Pick your target R-value
Use the table above. If you're renovating, aim for the "recommended" column — building back to minimum is a missed opportunity.
Step 3 — Choose material → calculate thickness
R-value × λ = thickness needed. Round up to the next available board thickness (usually 4, 6, 8, 10, 12, 15, 18, 20 cm).
Step 4 — Calculate boards/rolls
Area ÷ coverage per board. Add 5–10% waste for cuts. PIR boards are typically 60 × 120 cm = 0.72 m² per board.
Worked example: small house roof
You're insulating the loft of a 90m² house. Sloped roof, total roof area = 110m². Target R 7.0. Material: glass wool (λ = 0.035).
- Thickness needed: 7.0 × 0.035 = 0.245m → round up to 25 cm
- Standard rolls of glass wool come in 10cm and 15cm — buy two layers (15 + 10 cm, cross-layered)
- Area to cover: 110 m². With 10% waste = 121 m²
- Roll dimensions: 0.4m × 5m = 2 m² per roll. Need 61 rolls total (across both thicknesses)
At €11/m² total material cost, that's ~€1,330 for a complete loft job. Annual heat savings: typically €350–€500. Payback: under 4 years.
Skip the spreadsheets — let BuildExact do it
Enter your surface area, choose target R-value and material, and BuildExact calculates the exact thickness, board count, total cost and CO₂ savings. Includes Pro tips for finishing layers, vapor barriers and air gaps.
Download free5 things people get wrong
- Forgetting air gaps. Cavity walls need an air gap between insulation and the outer leaf for ventilation. Without it: condensation, mold.
- Skipping the vapor barrier. On the warm side of the insulation (interior). Without one, moisture condenses inside the wall.
- Compressing rolls into joists. Wool insulation only works at its rated R-value when at full thickness. Squashed = useless.
- Bridging — uncovered joists. Wooden joists conduct heat much better than insulation. Cross-layer to cover them.
- Buying just enough. 5–10% waste is normal. Always buy one extra board/roll.
The takeaway
Insulation is one of the highest-ROI things you can do to a house. The math sounds intimidating but boils down to two formulas: R × λ = thickness, and area × 1.10 = material needed. Get the target R right for your country and surface type, pick a material whose lambda value works with your available space, and you're done.
If you'd rather skip the math entirely, calculator apps like BuildExact handle every variable for you — including material costs, CO₂ savings, and payback period.