Insulation Thickness & R-Value Calculator

Understanding the insulation thickness calculator — concept and principle

This calculator works out how thick an insulation layer must be for a wall, roof or floor to reach a target U-value, and converts the result into an R-value (metric and US units) and a total volume for ordering. It is used when comparing insulation products, checking whether a build-up can meet an energy standard, or estimating how much material a project needs.

How it is calculated

• Required total resistance R_total = 1 ÷ U (m²K/W).
• Insulation R = R_total − R_other (never below zero).
• Thickness d (m) = insulation R × λ, shown in mm.
• US R-value = metric R × 5.678 (ft²·°F·h/BTU).
• Volume (m³) = area × d.

U is the target thermal transmittance in W/m²K, λ (lambda) is the declared thermal conductivity of the insulation in W/m·K, and R_other is the sum of all other layers in the build-up, including the internal and external surface resistances.

The principle

Heat passing through a layered element meets each layer in series, like resistors in a circuit, so their thermal resistances add up: R_total = R1 + R2 + … The U-value is the reciprocal of that total. A single layer's resistance is its thickness divided by its conductivity (R = d ÷ λ), so solving for thickness gives d = R × λ. A material with lower λ, such as PIR board (about 0.022 W/m·K), reaches the same R with a thinner layer than mineral wool (about 0.035–0.040).

Worked example

Default inputs: U = 0.18 W/m²K, λ = 0.035 W/m·K, R_other = 0.5 m²K/W, area 50 m².

1. R_total = 1 ÷ 0.18 = 5.556 m²K/W.
2. Insulation R = 5.556 − 0.5 = 5.056 m²K/W.
3. Thickness = 5.056 × 0.035 = 0.1769 m = 176.9 mm.
4. US R-value = 5.056 × 5.678 = R-28.7.
5. Volume = 50 × 0.1769 = 8.85 m³.

How to read the result

Lower U-values mean less heat loss. For reference, 0.18 W/m²K is the external wall value used for the notional dwelling in England's Approved Document L (2021). In practice you round up to the next available product thickness, often 180 or 200 mm here, or split it into two layers to stagger joints. Surface resistances alone add about 0.17 m²K/W for a wall (0.13 inside plus 0.04 outside under ISO 6946).

Common mistakes

• Confusing metric R (m²K/W) with US R-values, which are 5.678 times larger.
• Leaving out timber studs or metal fixings, which bridge the insulation and raise the real U-value.
• Using a generic λ instead of the declared value on the product label.

Frequently asked questions

What is the difference between U-value and R-value?

R measures resistance to heat flow; U is the overall rate of heat flow, equal to 1 ÷ total R of the whole element.

How do I convert R-value from US to metric?

Divide the US value by 5.678. For example, R-19 is about 3.35 m²K/W.

Does doubling insulation thickness halve heat loss?

Only for the insulation layer itself. Because other layers also resist heat, the total U-value falls by less than half.

Sources: ISO 6946 — Building components and building elements: thermal resistance and thermal transmittance, calculation methods · HM Government, Approved Document L (2021) — Conservation of fuel and power