SettingsSettings for this calculationUS
The U-value the element has to reach, from the regulation or specification for this job.
Take it from the document in force for this address — Approved Document L in England, the IECC or IRC table for the climate zone in the US — rather than from an article. The same wall is 5.678 times smaller in BTU/hr·ft²·°F than in W/m²K, and the box follows the unit switch.
Sets the internal surface resistance, which depends on which way the heat is flowing.
Still air resists heat least when it rises and most when it would have to sink, so the internal surface resistance is 0.10 m²K/W (0.57 hr·ft²·°F/BTU) under a ceiling, 0.13 (0.74) on a wall and 0.17 (0.97) on top of a floor. A floor on the ground is a different calculation — see the limitations.
A well-ventilated cavity counts for nothing, and nor does anything outside it.
BS EN ISO 6946 leaves out a well-ventilated air layer and every layer between it and the outside, and takes the external surface resistance as equal to the internal one — the air in a vented cavity is treated as still air, not wind. Enter only the layers on the warm side of the cavity when this is chosen.
Everything in the construction except the new insulation and the two surface films, added together.
Each layer is its thickness divided by its conductivity: 100 mm (4 in) of dense block is about 0.09 m²K/W (R-0.5) and 12.5 mm (1/2 in) of plasterboard about 0.06 (R-0.34). A layer bridged by timber or steel is worth less than its full figure — see the limitations.
The declared conductivity on the product's certificate, at the thickness you are buying.
PIR board is commonly declared around 0.022 W/m·K (a k of 0.15 BTU·in/hr·ft²·°F, R-6.5 per inch), EPS nearer 0.038 (0.26) and mineral wool between about 0.032 and 0.044 (0.22 to 0.31). Some products declare a different λ for different thicknesses.
The thickness you can actually buy, to see the U-value it reaches with the rest of the construction.
Merchants stock insulation in steps, so the thickness the arithmetic asks for is rarely on the shelf. Enter the stock size above it and the page gives the U-value the element reaches with that board.
Insulation thickness needed
4.232 in
Thickness = (1 ÷ U − the two surface resistances − the other layers) × λ: the layer sum of BS EN ISO 6946 run backwards.
- Total resistance the target asks for (1 ÷ U)
- 31.55 hr·ft²·°F/BTU
- Internal surface resistance
- 0.74 hr·ft²·°F/BTU
- External surface resistance
- 0.23 hr·ft²·°F/BTU
- The other layers
- 2.84 hr·ft²·°F/BTU
- Resistance the insulation has to supply
- 27.74 hr·ft²·°F/BTU
They open the calculator with your figures already in it
Insulation Thickness for a Target U-Value Calculator (BS EN ISO 6946): 4.23 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- BS EN ISO 6946:2017, Building components and building elements — Thermal resistance and thermal transmittance: U = 1 ÷ (Rsi + ΣR + Rse), each layer's resistance its thickness ÷ λ; surface resistances in 6.8, a well-ventilated air layer in 6.9.4
- Surface resistances as tabulated for heat flow upwards 0.10, horizontal 0.13 and downwards 0.17 m²K/W, and 0.04 m²K/W outside (BS EN ISO 13788:2012 Table 2 and 4.4.1 carry the same values)
- Declared thermal conductivity λD from the product standards — BS EN 13165 (PIR and PUR), BS EN 13163 (EPS), BS EN 13162 (mineral wool)
Inputs used
- Target U-Value
- 0.03 BTU/(hr·ft²·°F)
- Element and Direction of Heat Flow
- Wall — heat flows sideways
- What the Outside of the Construction Faces
- The outside air
- Thermal Resistance of the Other Layers
- 2.84 hr·ft²·°F/BTU
- Thermal Conductivity of the Insulation (λ)
- 0.15 BTU·in/hr·ft²·°F
- A Board Thickness to Check (0 to skip)
- 0 in
Intermediate steps
- Total resistance the target asks for (1 ÷ U)
- 31.55 hr·ft²·°F/BTU
- Internal surface resistance
- 0.74 hr·ft²·°F/BTU
- External surface resistance
- 0.23 hr·ft²·°F/BTU
- The other layers
- 2.84 hr·ft²·°F/BTU
- Resistance the insulation has to supply
- 27.74 hr·ft²·°F/BTU
Confidence note: Thickness = (1 ÷ U − the two surface resistances − the other layers) × λ: the layer sum of BS EN ISO 6946 run backwards.
What this calculation does not cover
- A layer bridged by timber or steel — insulation between rafters, studs or joists — is worth less than its full resistance, because the framing carries heat around it. BS EN ISO 6946 averages the two paths with its upper and lower limit method; enter the bridged layer's reduced figure, or use the thermal bridging calculator for it.
- A floor on the ground loses heat in a way that depends on its exposed perimeter as well as its layers, calculated to BS EN ISO 13370. This page gives the thickness the layers need, not the ground floor's U-value.
- The corrections of BS EN ISO 6946 Annex F — for air gaps in the insulation layer, mechanical fixings through it and rain on an inverted roof — are not applied.
- The target comes from the regulation or specification for the job, and the conductivity from the product's own certificate; neither is supplied here.
Add the equipment this sizes
This result is a specification — 4.232 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-22 · v1.0.0
Regulatory standards & verification citations3
- BS EN ISO 6946:2017, Building components and building elements — Thermal resistance and thermal transmittance: U = 1 ÷ (Rsi + ΣR + Rse), each layer's resistance its thickness ÷ λ; surface resistances in 6.8, a well-ventilated air layer in 6.9.4
- Surface resistances as tabulated for heat flow upwards 0.10, horizontal 0.13 and downwards 0.17 m²K/W, and 0.04 m²K/W outside (BS EN ISO 13788:2012 Table 2 and 4.4.1 carry the same values)
- Declared thermal conductivity λD from the product standards — BS EN 13165 (PIR and PUR), BS EN 13163 (EPS), BS EN 13162 (mineral wool)
Which documents these citations point at
Standards referenced: ISO 6946, ISO 13788 (International Organization for Standardization, International); BS EN 13165, BS EN 13163, BS EN 13162 (European Committee for Standardization, as published in the UK by BSI, European (EN)).
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