Required Insulation Thickness Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/7/2026
The required insulation thickness is calculated with target R × λ (the material's thermal conductivity). For a thermal resistance of 7 m²·K/W in fiberglass wool (λ = 0.035), you need about 24.5 cm of thickness.
Explanation
This calculator answers the reverse question to our insulation thermal resistance calculator, which calculates the R-value achieved from a given thickness: here, you start from a target thermal resistance (often recommended by a professional or a regulation, for example R ≥ 7 m²·K/W for a well-insulated roof in a renovation) to work out the insulation thickness needed, depending on the material chosen. Thermal conductivity λ, expressed in W/(m·K), measures a material's ability to let heat pass through: the lower λ is, the better an insulator the material is, and the less thickness is needed to reach a given thermal resistance. That's why, for the same target thermal resistance, a higher-performance insulator like polyurethane (λ around 0.025) needs markedly less thickness than a more common insulator like fiberglass wool (λ around 0.035) — an important factor when available thickness is limited (a renovation with space constraints, interior insulation that reduces living space). Material choice isn't limited to thermal performance alone, though: cost, ease of installation, fire resistance, environmental impact, and other criteria also come into play in a real insulation choice. The thermal conductivity values used here are median values from commonly cited ranges, which can vary slightly by specific product and manufacturer: the chosen product's technical data sheet remains the most reliable reference for a final calculation.
Example: target R of 7 m²·K/W in fiberglass wool
Inputs
Target thermal resistance: 7 m²·K/W. Material: fiberglass wool (λ = 0.035).
Calculation
Thickness = 7 × 0.035 = 0.245 m, i.e. 24.5 cm.
Result
You need about 24.5 cm of fiberglass wool to reach this thermal resistance.
Frequently asked questions
Why does polyurethane need less thickness than fiberglass wool for the same resistance?
Because polyurethane has a lower thermal conductivity (around 0.025 W/(m·K), versus about 0.035 for fiberglass wool): it lets less heat through per unit of thickness, so it reaches the same thermal resistance with less thickness. This is an important factor when the space available for insulation is limited, even though polyurethane is generally more expensive than fiberglass wool.
What target thermal resistance should I aim for on a roof or attic?
Recommendations vary by context (new construction or renovation, climate zone, financial incentives targeted), but a thermal resistance of 7 m²·K/W or more for a roof is often cited as a good performance level in renovation. A certified energy-efficiency professional or an energy performance diagnosis can pinpoint the target value best suited to your home and your goals.
Is this calculation enough on its own to choose an insulator?
No, thermal performance is only one criterion among several for choosing an insulator: cost, ease of installation, fire resistance, moisture management, manufacturing environmental impact, and long-term durability also come into play. This calculator only gives the theoretical thickness needed from a thermal standpoint, a starting point to complete with these other criteria before a final choice.