Insulation Calculator (R-Value)
Converts insulation thickness and material into R-value and U-value, tells you the thickness for a target, and counts the rolls or boards for an area. The target itself comes from your code.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
R (m²K/W) = thickness (m) ÷ λ (W/m·K) R (US) = R (SI) × 5.678 U = 1 ÷ R
R-value is thickness divided by conductivity, so doubling the thickness doubles the R-value and halving the conductivity does the same. The US unit is the same quantity in imperial units, which is why the conversion factor is an odd 5.678.
What to watch out for
What the numbers mean:
- λ (lambda) is the material’s conductivity. Mineral wool is around 0.040; PIR board is 0.022, so 100 mm of PIR insulates like 180 mm of wool.
- R-value is resistance — thickness ÷ λ. Higher is better. US figures are 5.68 times the SI ones.
- U-value is the inverse, for the whole element including plaster, cladding and air films. Building codes usually set a maximum U rather than a minimum R.
Compression kills performance. Mineral wool squashed into a cavity half its thickness loses roughly half its R-value, and a gap at the edge of a batt lets heat go round it entirely — fit matters as much as thickness.
Frequently Asked Questions
How do I calculate R-value from thickness?
Divide the thickness in metres by the conductivity. 100 mm of mineral wool at λ 0.040 is 0.1 ÷ 0.040 = R 2.5 in SI, which is about R-14 in US units.
How thick does insulation need to be?
Target R × λ. For R 4.5 in mineral wool that is 180 mm; in PIR board it is 99 mm. The target R depends on your building code and the element — lofts generally need more than walls.
What is the difference between R-value and U-value?
R is resistance to heat flow and applies to a layer; U is the heat flow through a whole element and is 1 ÷ total R. Codes usually specify a maximum U-value for the complete wall or roof, not an R-value for the insulation alone.