Required thickness depends on the product’s thermal conductivity and the desired resistance. The example table explains the difference; a roof assessment determines the suitable build-up.
Rd, Rc and lambda in plain language
Lambda (λ), in W/(m·K), describes thermal conductivity. At equal thickness, lower conductivity gives higher resistance. Rd is the insulation layer’s resistance in m²·K/W; Rc refers to the construction and is not automatically the same as a board’s Rd. Framing and junctions matter.
How to read the thickness calculation
For a homogeneous layer: thickness in metres = target Rd × lambda. Rd 4.5 with lambda 0.035 gives 0.1575 m, or 15.75 cm. The table rounds up to whole centimetres. These selected inputs are calculation examples, not product advice, legal minimums or subsidy approval. Order using the exact product’s declared Rd and thickness.
How many square metres are needed?
Measure the actual insulated surface, not just the room’s floor area. A sloping roof has more surface than its horizontal projection. Openings, edge strips and the layout determine cutting waste; one standard waste allowance does not suit every roof. State both net insulated area and supply assumptions.
Moisture and junctions determine performance
Assess existing layers and vapour behaviour before adding insulation. Internal insulation of a flat roof needs particular care and can cause moisture problems in an unsuitable build-up. Coordinate airtightness, vapour control, roof height, outlets and ventilation. A thicker board cannot repair a leak or gap.
Examples: insulation thickness in centimetres
| Selected λ (W/m·K) | Rd 3.5 | Rd 4.5 | Rd 6.0 |
|---|---|---|---|
| 0.022 | 8 cm | 10 cm | 14 cm |
| 0.035 | 13 cm | 16 cm | 21 cm |
| 0.040 | 14 cm | 18 cm | 24 cm |
Insulation materials and checks
| Material | Application | What to check |
|---|---|---|
| PIR / PUR boards | Rigid boards in a compatible roof build-up | Declared Rd, facings, joints and fire performance |
| Glass wool / stone wool | Rolls or boards; product depends on application | Avoid gaps and compression; roof boards need suitable load performance |
| EPS | Lightweight foam boards | Compressive strength, temperature and compatibility |
| XPS | Including suitable inverted roofs | Water exposure, drainage and system design |
| Wood fibre | Flexible or rigid products | Moisture behaviour, weight and suitability |
| Cellulose | Blown into suitable enclosed cavities | Fill density, settlement and airtight enclosure |
| Flax, hemp, cork and sheep wool | Product-specific bio-based options | Lambda, additives, moisture and fire behaviour |
| Cellular glass | Specific load-bearing insulation systems | Complete system and installation |
| Vacuum panels and aerogel | Specialist solutions for restricted space | Detail design, protection and product limits |