Use this calculator to work out how thick your insulation needs to be for a target R-value or RSI — and to compare rigid PU/PIR foam against XPS, EPS, mineral wool and fiberglass at the same thermal duty. Or flip it around: enter a material and thickness and read off the R-value, RSI and U-value.
| Insulation material | Typical k (W/m·K) | Required thickness | vs rigid PU |
|---|
Thermal conductivity figures are typical published values for each material class: rigid PU/PIR 0.022–0.026, XPS 0.029–0.033, EPS 0.033–0.038, mineral wool 0.035–0.040, fiberglass batt 0.040–0.044 W/m·K. The calculator uses the mid-range figure shown. Aged lambda of PU foam depends on blowing agent, density and facings — always confirm the design value against the TDS of the specific system you buy.
U = 1 / RSI. RSI × 5.678 = imperial R-value. Figures exclude facings, air films and thermal bridges — a full assembly calculation belongs to the panel designer.
Why rigid PU foam wins on thickness
Thermal resistance is thickness divided by thermal conductivity (k). Rigid polyurethane and PIR foams sit at the low end of the k scale for bulk insulation — typically 0.022–0.026 W/m·K against 0.033–0.038 for EPS and 0.040–0.044 for fiberglass batt. In practice that means a PU panel reaches the same R-value at roughly two thirds of the EPS thickness, which is why PU dominates applications where every millimetre of wall or cabinet space carries cost: refrigerators and freezers, cold-store sandwich panels, refrigerated transport and pipe-in-pipe insulation.
Free-rise density, moulded density and lambda
The conductivity a panel actually achieves depends on the blowing agent, the moulded density and how well the foam fills the cavity. Cyclopentane-blown rigid systems give lower aged lambda than all-water formulations; higher over-pack improves fill and surface but adds weight and cost. The figures in the calculator are class-typical planning values — for a design submission, take the aged lambda from the technical data sheet of the specific system, not from a generic table.
From R-value to a purchase order
Once the thickness is fixed, the material quantity follows from panel volume and target density — our PU foam density & material calculator does that arithmetic and splits the tonnage into A (polyol blend) and B (isocyanate) drum counts. If you would rather hand the whole sizing problem over, send us the assembly drawing and the thermal target through the quote button above — we will come back with a system recommendation, the supporting TDS and a price.
FAQ
What R-value does rigid PU foam give per inch?
At a typical aged k of 0.024 W/m·K, one inch (25.4 mm) of rigid PU foam provides about R-6 in imperial units. Values from R-5.6 to R-6.8 per inch appear on datasheets depending on blowing agent, density and facings.
Is R-value the same as RSI?
They measure the same thing in different units. RSI is the metric figure in m²·K/W; multiply RSI by 5.678 to get the North-American imperial R-value.
Does higher foam density mean better insulation?
Not directly. Within the normal rigid-panel range (30–45 kg/m³) lambda changes little with density; density mainly buys compressive strength and dimensional stability. Going far above the range can even worsen lambda because more polymer conducts more heat than trapped gas. See our PU foam density chart for the working ranges by application.
