What’s the difference between U-value and R-value?

R-value (m²·K/W) is the thermal resistance of a layer or full assembly, higher is better. U-value (W/m²·K) is the transmittance, the reciprocal of total R (U = 1/R total), lower is better. Norway and most European countries use U-value in code. US and Canada often use R-value.

What are Rsi and Rse?

Rsi and Rse are the surface resistances on the inside (i) and external (e) of the assembly respectively. They represent the still air film on each side that reduces heat flow. Standard values from NS-EN ISO 6946: wall Rsi 0.13 and Rse 0.04, roof Rsi 0.10 and Rse 0.04, floor Rsi 0.17. Added automatically in the tool.

How do I find the lambda for a specific product?

Check the manufacturer’s data sheet or CE Declaration of Performance (DoP). All insulation products must state lambda-D (declared lambda, W/m·K). Glass wool typically 0.032-0.040, mineral wool 0.035-0.040, EPS 0.033-0.038, XPS 0.032-0.038, PIR 0.022-0.025. The tool has a built-in list of common materials.

What about thermal bridges and moisture?

The tool does one-dimensional U-value (heat straight through). Thermal bridges (studs, beams, corners, junctions), moisture transport and air leakage aren’t computed and need separate analysis (e.g. SIMIEN, Wufi or expert review). For screening and teaching, U-value still gives a good indication of insulation quality.

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U-value Calculator - Thermal Transmittance of Walls and Roofs

Calculate the U-value (thermal transmittance) of a building element by entering layers with thickness and thermal conductivity (lambda).

Thickness (mm)	λ (W/m·K)

Rsi (inside)

Rse (outside)

Total thermal resistance R
U-value

How it works

The U-value tells you how much heat escapes through a building element, in watts per square metre per degree (W/m²K). Lower is better. It is calculated as U = 1 / R, where the total thermal resistance R = Rsi + the sum of (thickness / lambda) for each layer + Rse. Thickness is in metres and lambda in W/m·K.

Surface resistances depend on direction: wall Rsi 0.13 and Rse 0.04, roof 0.10/0.04, floor 0.17/0.04. Typical lambda values: mineral and glass wool about 0.037, wood about 0.13, plaster about 0.20, lightweight concrete about 0.15, concrete about 2.0, brick about 0.7. For comparison, recommended values in the Norwegian TEK17 building code are roughly 0.18 for an external wall, 0.13 for a roof and 0.10 for a floor. Everything is calculated locally in your browser.

About this tool

The U-value calculator computes the thermal transmittance (U-value) of a building assembly like a wall, roof or floor. You add each material layer with thickness in mm and thermal conductivity (lambda, λ) in W/m·K, and the tool sums the thermal resistance and gives the U-value in W/m²·K. Lower U-value means better insulation. Norwegian building code TEK17 requires U-values 0.18 for external wall, 0.13 for roof and 0.10 for ground floor. Handy for checking whether an assembly meets code and for experimenting with thickness and materials.

How to use it

Add each material layer from inside to outside with thickness in mm and lambda value.
Look up lambda values in the preset list or in product data sheets.
See computed R-value (total thermal resistance) and U-value.
Compare U-value against the TEK17 requirements shown in the UI.

Examples

Standard TEK17 external wall

Input

Interior plasterboard 13 mm (λ 0.25), vapour barrier, 200 mm mineral wool between studs (λ 0.036), 50 mm exterior insulation (λ 0.036), wind barrier, cladding 22 mm (λ 0.13). Rsi = 0.13, Rse = 0.04.

OutputTotal R ≈ 7.32 m²·K/W. U ≈ 0.137 W/m²·K. Meets the TEK17 external wall requirement (0.18).

Wooden studs weaken the insulation (thermal bridges). A fully detailed calc should apply a lambda correction for the stud fraction. The tool does a flat-average estimate and provides guidance, not stamped documentation.

Roof with 400 mm insulation

InputInterior plasterboard 13 mm, 400 mm blown mineral wool (λ 0.04), roofing. Rsi = 0.10, Rse = 0.04.

OutputTotal R ≈ 10.3 m²·K/W. U ≈ 0.097 W/m²·K. Well below the TEK17 roof requirement of 0.13.

Blown attic insulation is cost-effective and gives a low U-value. 400 mm has become standard in new builds to hit passive-house level.

Common use cases

Design of new houses or extensions for TEK17 compliance.
Retrofit-insulation projects on older homes to check if the upgrade lifts U enough.
Comparing insulation products with different lambda values.
Energy calculations for Enova grants and energy labelling.
Teaching building or HVAC courses about heat transfer.

Frequently asked questions

What’s the difference between U-value and R-value?: R-value (m²·K/W) is the thermal resistance of a layer or full assembly, higher is better. U-value (W/m²·K) is the transmittance, the reciprocal of total R (U = 1/R total), lower is better. Norway and most European countries use U-value in code. US and Canada often use R-value.
What are Rsi and Rse?: Rsi and Rse are the surface resistances on the inside (i) and external (e) of the assembly respectively. They represent the still air film on each side that reduces heat flow. Standard values from NS-EN ISO 6946: wall Rsi 0.13 and Rse 0.04, roof Rsi 0.10 and Rse 0.04, floor Rsi 0.17. Added automatically in the tool.
How do I find the lambda for a specific product?: Check the manufacturer’s data sheet or CE Declaration of Performance (DoP). All insulation products must state lambda-D (declared lambda, W/m·K). Glass wool typically 0.032-0.040, mineral wool 0.035-0.040, EPS 0.033-0.038, XPS 0.032-0.038, PIR 0.022-0.025. The tool has a built-in list of common materials.
What about thermal bridges and moisture?: The tool does one-dimensional U-value (heat straight through). Thermal bridges (studs, beams, corners, junctions), moisture transport and air leakage aren’t computed and need separate analysis (e.g. SIMIEN, Wufi or expert review). For screening and teaching, U-value still gives a good indication of insulation quality.

Technical background

Formula: R_layer = thickness[m] / λ[W/m·K]. R_total = Rsi + ΣR_layer + Rse. U = 1 / R_total (W/m²·K). Standard Rsi/Rse values from NS-EN ISO 6946. Runs in JavaScript, no server calls. TEK17 requirements shown: external wall 0.18, roof 0.13, ground floor 0.10, window 1.2. For passive-house standard (NS 3700/3701): wall 0.12, roof 0.09. Note TEK17 § 14 allows energy-measure or energy-frame approaches; U-value is only one of three paths to approval.