What U-Value Calculator does

Lay up an assembly layer by layer (insulation, masonry, air gap, etc.), and the tool reports the U-value, R-value, thermal mass, and whether it meets climate-aware targets.

Standard:

ISO 6946

Input:

Material library

Output:

U / R-value

How it works

Straight ISO 6946 layer arithmetic — no approximation and nothing proprietary. The standard is public, the material conductivities are cited, and the arithmetic is simple enough to check by hand, which is the point: you should be able to reproduce any number this tool gives you.

• ISO 6946 thermal resistance: R = thickness / conductivity per layer, plus standard internal and external surface resistances.
• Bridged layers handled by the standard's combined upper/lower resistance limits method.
• Interstitial condensation and dew-point checks follow ISO 13788.
• Compliance targets are climate-aware: load an EPW and the target follows the site's climate zone rather than a national default.
• ASHRAE 90.1 envelope targets available for comparison.

How to use it

1. Pick your workflow. Quick Check for a single element; Project Analysis for a full 3D envelope.
2. Add layers. Stack materials in the order they appear in your wall section — exterior to interior.
3. Set thicknesses. Adjust each layer's thickness; U/R-values recalculate live.
4. Load climate (optional). Add an EPW to see climate-aware compliance targets.
5. Read the results. U-value, R-total, thermal mass, and pass/fail verdict appear on the right.

Best used for

• Sizing insulation early, when the build-up is still a decision rather than a drawing.
• Comparing two wall build-ups on equal terms before specifying.
• Catching a condensation risk while the section can still be changed.
• Teaching the relationship between layer thickness, conductivity and U-value.

Not a substitute for

• Building-control or energy-code submission — the figure is right, but the submission needs a stamped calculation.
• Detailed thermal-bridge analysis at junctions (that needs 2D or 3D heat-flow modelling).
• Certifying a proprietary product's performance.

Known limits

• One-dimensional steady-state heat flow. Junctions, fixings and repeating bridges are the standard's correction terms, not a full 2D solution — a heavily bridged build-up deserves a specialist.
• Material conductivities come from a library of published values; a real product's declared lambda may differ, and the declared value is what a submission needs.
• Air-gap resistances follow the standard's tabulated values and assume the cavity is built as described — an unintended ventilated cavity behaves very differently.
• Thermal mass is reported as a property of the build-up, not as a dynamic simulation of how it behaves over a day.
• Moisture checks are steady-state per ISO 13788, so they flag risk rather than predict accumulation over seasons.

Read the full U-Value Calculator guide →