Knauf U-Value Calculator
Calculate thermal transmittance (U-value) for walls, roofs, or floors by entering each construction layer. Add Knauf insulation products using typical conductivity values, then calculate total resistance and U-value instantly.
Material Layers (Inside to Outside)
Formula used: U = 1 / (Rsi + Σ(d/λ) + Rse), where thickness d is in meters and conductivity λ is W/m·K.
What is a U-Value?
The U-value tells you how much heat passes through a building element (wall, roof, floor, or partition). It is measured in W/m²K. A lower number means better insulation and less heat loss.
When people search for a Knauf U-value calculator, they usually want a quick way to test different insulation thicknesses and check compliance targets for a project. This page gives you that exact workflow with editable layers and instant results.
How this calculator works
1) Add each layer of construction
Enter layers in order from inside to outside. For each layer, you need:
- Thickness in millimeters (mm)
- Thermal conductivity (λ) in W/m·K
2) Include surface resistances
The calculator uses internal and external surface resistances (Rsi and Rse). Default values are commonly used for many vertical assemblies, but you can adjust them if your design standard requires different values.
3) Get U-value and optional heat loss
The tool returns total thermal resistance, U-value, and (if area and temperature difference are entered) a simple heat loss estimate in watts.
Typical Knauf insulation conductivity values (guide only)
Knauf ranges vary by product and market region. Always verify exact declared lambda values from the latest product datasheet and local certification documents.
- Knauf Earthwool 32: λ ≈ 0.032 W/m·K
- Knauf Omnifit Slab 34: λ ≈ 0.034 W/m·K
- Knauf Earthwool 35: λ ≈ 0.035 W/m·K
- Knauf Dritherm 37: λ ≈ 0.037 W/m·K
Practical example
Suppose you build a cavity wall with plasterboard, mineral wool, and brickwork. If you increase insulation thickness from 100 mm to 150 mm while keeping lambda constant, the insulation layer resistance increases significantly, and the overall U-value drops. This is why insulation thickness is such a powerful design lever for envelope performance.
Common mistakes to avoid
- Mixing units (mm vs m) when calculating layer resistance.
- Using outdated or generic lambda values instead of product-specific data.
- Forgetting surface resistances.
- Ignoring repeating thermal bridges (studs, fixings, junctions) in final compliance calculations.
- Assuming one build-up works for every climate zone and regulation set.
Design note
This calculator is excellent for concept design, options appraisal, and quick checks. For final building regulation submissions or Passive House workflows, include full condensation risk analysis, thermal bridge modelling, and project-specific detailing.