Thermal resistance and thickness
The calculation is d = R × lambda. R is the added thermal resistance in m²K/W, lambda is the declared conductivity in W/(m·K), and d is thickness in metres. For R-3.5 with lambda 0.040, d is 0.140 m or 140 mm. A 100 mm layer at that lambda provides R-2.5; a 150 mm layer provides R-3.75 before installation losses.
Use the declared lambda for the exact product
Lambda is not a universal number for ‘foam’, ‘wool’, or ‘fibre’. Board, loose fill, and faced products can differ. Copy the declared conductivity from the technical sheet, not a U-value, density, or an imperial R-per-inch rating. Lower lambda means more resistance per millimetre. Entering 0.40 instead of 0.040 makes the thickness ten times too large.
A wall has more than one heat path
The d/lambda formula describes a continuous ideal layer. Timber studs, metal fasteners, crushed batts, gaps, electrical boxes, and wind washing create paths with different resistance. Vapour control, ventilation, fire protection, and condensation risk are assembly questions. A nominal thickness does not prove a wall is compliant or moisture-safe.
Where the number helps
Use it to compare products before quoting, check whether a cavity can hold a target resistance, or add resistances of genuinely continuous layers. Measure the tightest available depth after allowing for battens, service voids, linings, and required ventilation. If the supplier is not fixed, calculate a range using the candidate lambda values rather than inventing a precise material.
Not the insulation-pack calculator
The existing insulation calculator starts with surface area and pack coverage to count packs. This page starts with thermal performance and conductivity to calculate material thickness. It does not count packs, estimate a room heat loss, price materials, or calculate a full wall U-value. Those are separate questions even when a renovation needs answers to both.
Before construction
Check the product instructions for compression, stacking, moisture conditions, and required clearances. A thermal upgrade can alter surface temperatures and condensation behaviour, especially in roofs and external walls. Follow local building rules and obtain an appropriate design or inspection where required. The browser keeps dimensions local but cannot inspect the construction.