Layer resistance
For each uniform layer, thermal resistance is R = thickness in metres ÷ thermal conductivity λ.
HOME · BUILDING PERFORMANCE
U-Value Calculator
A simple calculator when you want speed, with professional controls when you need them. Build a wall, roof or floor layer by layer and keep every assumption visible.
Automatic surface resistances: Rsi 0.13 + Rse 0.04 m²K/W. Change the element and the calculator applies the appropriate heat-flow direction automatically.
MF build-up100 mm insulationConcrete soffit · insulated MF ceiling zone · plasterboard
The suspended void depth is not used as a separate thermal layer. This models the insulation within the MF ceiling zone and the plasterboard lining below. MF5/MF7 sections, hangers and perimeter channels are not treated as a zero-bridge layer; the displayed U-value is the plane insulated path and should be checked against system-specific thermal-bridge data where compliance accuracy is required.
Material / layerThicknessλ or RR-value
TARGET SOLVER
What insulation thickness do I need?
Choose an insulation layer and target U-value. We’ll calculate the minimum theoretical thickness and suggest the next practical board/slab thickness.
Choose an insulation layer and target U-value.
For soffit build-ups, the solver will use the selected soffit insulation product and suggest the next stored practical slab thickness where available.
Calculated U-value—
Add one or more material layers to calculate.
Professional, transparent estimate. Based on simplified methods in BS EN ISO 6946 and UK BR 443 conventions. Complex steel frames, significant point/linear thermal bridges, ground floors and constructions requiring 2D/3D numerical analysis need an appropriate specialist method. Always verify product design values and project-specific compliance requirements.
CALCULATION REPORT
Print / Save professional PDF
Add optional project details, then create a clean A4 calculation sheet using your browser’s Save as PDF option.
How the U-value calculation works
Total resistance
Add the resistance of every layer, plus the selected internal and external surface resistances.
U-value
For uniform layers, U = 1 ÷ total thermal resistance. In Pro mode, repeating timber bridges can be included using upper and lower resistance limits before corrections are applied.
Material values and sources
The material picker now separates generic reference values from named UK products. Named product λ-values are taken from current manufacturer technical information; use the source link shown beneath the selected layer to verify it. Product values can change, and the appropriate design value may depend on application and calculation convention.
Starter verified products: Kingspan Thermawall TW50 (λ 0.022 W/mK), Knauf DriTherm Cavity Slab 32 (λ 0.032 W/mK), Knauf FrameTherm Slab 32 (λ 0.032 W/mK), and ROCKWOOL Thermal Insulation Cavity Slab 32 (λ 0.032 W/mK).
Concrete soffit insulation
Use the Concrete floor + soffit insulation quick build-up for floors over car parks, undercrofts and other unheated spaces. ROCKWOOL Soffit Slab is included as a sourced product with declared thermal conductivity of 0.034 W/mK and common thicknesses of 50, 130, 145, 160, 180, 200 and 220 mm.
Design note: soffit constructions can also require checks for condensation, fixings, fire performance and junction thermal bridging. The U-value calculation should therefore be treated as the plane-element calculation, with project-specific corrections added where required.
Expanded verified UK insulation library
The verified product library now includes current UK loft, partition and soffit products from ROCKWOOL, Knauf Insulation, Isover and Superglass. Each named product shows its manufacturer source and stored practical thicknesses where published.
Isover APR 1200: its declared conductivity varies by thickness, so the calculator automatically uses λ 0.039 W/mK at 25/50 mm, 0.040 at 65 mm, and 0.043 at 75/100 mm.
MF ceiling with insulation below a soffit
The Concrete soffit + insulated MF ceiling build-up is intended for suspended drylining ceilings beneath concrete slabs where mineral wool is installed within the ceiling zone rather than using a dedicated soffit board. Set the insulation thickness and plasterboard layers, and the calculator updates the build-up automatically.
Thermal-bridge note: the insulation path is calculated as a plane element. MF5 ceiling sections, MF7 primary channels, hangers and perimeter channels can create local thermal bridges, particularly where insulation is interrupted or compressed. For compliance-critical design, use project/system-specific thermal bridge information where available.
Frequently asked questions
What is a U-value?
A U-value describes the rate of heat transfer through a building element. Lower values generally indicate better thermal performance.
What does λ mean?
Thermal conductivity, λ, is a material property measured in W/mK. A lower λ means the material conducts heat less readily.
Can I use manufacturer λ-values?
Yes. For a useful estimate, use the declared or design thermal conductivity appropriate to the product and calculation method you are working with.
Is this suitable for Building Regulations sign-off?
The calculator supports transparent simplified ISO 6946 calculations, including repeating timber bridges in Pro mode. Some constructions still require specialist numerical methods, so project-specific compliance should be checked by a competent person.
Built for quick, transparent estimates. See how our calculators work and check page-specific assumptions before relying on a result.
CALCULATOR GUIDE
How the U-value calculation works
A U-value measures heat flow through a building element. The calculator converts each material layer into thermal resistance using its thickness and thermal conductivity (λ-value), adds the internal and external surface resistances, then calculates U = 1 ÷ total R.
For each layer: R = thickness in metres ÷ λ. Add all layer R-values and surface resistances, then divide 1 by the total resistance to obtain W/m²K.