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R-Value Calculator – Materials & Assemblies

Quick answer

R-value by material stack: fiberglass 3.2, rock wool 3.7, EPS 4.0, XPS 5.0, closed-cell foam 6.5 per inch - sum every layer including air films.

Figures are estimates from this calculator’s standard formula — adjust the inputs in the tool below for your exact number.

How does the R-value calculator work?

R-Total = Σ (Layer Thickness × R/inch) + Air Films (0.68 in + 0.17 out)

This tool applies the formula R-Total = Σ (Layer Thickness × R/inch) + Air Films (0.68 in + 0.17 out) directly to the measurements you enter, so the result is an exact quantity rather than a rough guess. Because R-value is priced and delivered per unit - cubic meters, square meters, bags or tonnes - a small measurement error multiplies across the entire order, which is why the calculation is worth doing properly. Worked example: 140 mm fiberglass (R-17.6) + 50 mm XPS (R-9.8) + films (0.85) = R-28.3 wall assembly. One note from practice: Add a 5-10% waste margin to R-value before ordering. Enter the layer thicknesses and materials, check the totals, then round up to the nearest whole bag, sheet or roll before you place the order.

  1. 1

    Take dimensions

    Note the layer thicknesses and materials for the area you are covering or filling.

  2. 2

    Run the calculation

    The calculator applies R-Total = Σ (Layer Thickness × R/inch) + Air Films (0.68 in + 0.17 out) the moment you type.

  3. 3

    Check the breakdown

    Compare base quantity and waste-adjusted totals side by side.

  4. 4

    Plan the purchase

    Use the final figure to price and schedule the delivery.

Use Cases

Waterproofing wet areas

Membrane and corner counts for R-value showers.

Siding replacements

Square counts for R-value walls with 10% waste.

Energy audits

Document R-value areas before and after upgrades.

Common Mistakes

  • Seams left untaped

  • Measuring the unit not the opening

  • Ignoring fire-rating requirements

FAQs

R-value of 100 mm fiberglass?

R-12.6 at 3.2 per inch - half a code attic target but a solid wall filler.

Best R per cm - foam or mineral?

Closed-cell spray foam at R-6.5/inch (2.55/cm) leads; XPS R-5.0; batts R-3.2 - foam buys R where depth is tight.

Does an air gap add R-value?

Yes - still air films add about 0.17 outside and 0.68 inside (R units); ventilated cavities add none.

R-value drop when insulation gets wet?

Fiberglass loses 30-50% R wet; closed-cell foam keeps 90% - wet climates favor foam and XPS.

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