R-Value Calculator
Find the thermal resistance (R-value) of any insulation we stock, or work backwards from the R-value you need to the thickness to buy. Every material uses the manufacturer’s declared thermal conductivity (λ), and prices are live from our shop.
Use the Ex VAT / Inc VAT switch at the top of the page to change how prices show. Delivery to England is free on orders of £400 ex VAT or more. Celotex also delivers to mainland Wales and mainland Scotland (£400 ex VAT minimum plus £40 + VAT delivery). Other products on this calculator are England only.
R-value
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R-value for every thickness of this material we stock
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Pick up to three materials to see the R-value each gives at the same thickness, and how thick each needs to be to reach the same R-value.
Price per m² is the live price of the stocked build-up divided by its coverage, before waste and delivery.
Lambda, R-value and U-value: what’s the difference?
Lambda (λ), or thermal conductivity, in W/mK, is a property of the material itself: how easily heat flows through it. Lower is better. Foil-faced PIR is around 0.022, XPS around 0.033, and mineral wool between 0.032 and 0.044.
R-value, or thermal resistance, in m²K/W, is for a particular thickness: R = thickness in metres ÷ λ. Higher is better, and layers add up. 100mm of PIR at 0.022 gives 0.100 ÷ 0.022 = 4.55 m²K/W.
U-value in W/m²K is for the whole floor, wall or roof, including the bricks, timbers, air gaps and plasterboard. It is roughly 1 ÷ (the total of every layer’s R-value), with corrections for timber and fixings. Lower is better, and it’s the figure Building Regulations (Part L) set limits for.
So the R-value tells you how good the insulation is on its own, but to check a construction meets Part L you need its U-value. Use our PIR U-value calculator for floors, walls and roofs, or the loft roll calculator to work out how many rolls you need for a loft.
Where our λ values come from
R-value questions
How do I work out the R-value of insulation?
Divide the thickness in metres by the thermal conductivity (λ). For example, 150mm of PIR at λ 0.022 W/mK is 0.150 ÷ 0.022 = 6.82 m²K/W. For two or more layers, add the R-values together.
Is a higher R-value better?
Yes. A higher R-value means more resistance to heat flow. A lower λ (thermal conductivity) is better, because the same thickness gives a higher R-value.
Why does PIR need less thickness than mineral wool?
Foil-faced PIR has a declared λ of about 0.022 W/mK, while mineral wool is typically 0.032 to 0.044 W/mK. To reach the same R-value, mineral wool needs roughly 1.5 to 2 times the thickness. Mineral wool is non-combustible and helps with sound, so the right choice depends on the job.
Can I add the R-values of two layers together?
Yes. Thermal resistances in series add up, so 100mm plus 50mm of the same PIR board gives the same R-value as 150mm. That’s why loft insulation is often laid as one layer between the joists and a second across them.
Calculator only: figures are typical values for general information. Always check the manufacturer's current data sheets and confirm Building Regulations compliance with your designer, SAP assessor or building control. Insulation Warehouse Direct accepts no liability for decisions made using this calculator.