Every wall, roof and floor in a building is a stack of materials fighting to hold heat in — and one number sums up how well they do it: the U-value. Architects, HVAC engineers and energy assessors live by it, because it sets heat loss, heating-system size, and whether a design passes building regulations. But you can't just add U-values — you have to work in R-values (thermal resistance), layer by layer, then invert. This free U-value & R-value Calculator does exactly that: build up your wall from inside to outside, add the surface resistances, and get the total R-value and U-value instantly — the proper ISO 6946 way.
Table of Contents
The U-value / R-value Calculator
Add each material layer (thickness + conductivity), choose the element type for the right surface resistances, and the tool returns the total R-value, U-value, and a layer-by-layer resistance breakdown — in both metric and imperial units.
U-value & R-value Calculator (Multi-Layer)
U-value vs R-value: What's the Difference?
These two numbers describe the same physics from opposite ends:
- R-value (thermal resistance) — how well a material or layer resists heat flow. Higher is better. Units m²K/W (metric) or ft²·°F·h/Btu (imperial).
- U-value (thermal transmittance) — how fast heat passes through the whole assembly. Lower is better. Units W/m²K.
The ISO 6946 Method
The international standard ISO 6946 (also underpinning UK Part L and ASHRAE work) defines a clean three-step process:
where d is each layer's thickness (m) and λ (lambda) is its thermal conductivity (W/mK). This same resistance-in-series logic drives the building heat loss calculation — the U-value you get here is exactly the input that method needs.
Surface Resistances (Rsi & Rse)
A thin, still film of air clings to each face of the element — and because still air insulates well, it adds free thermal resistance. ISO 6946 gives standard values:
| Element | Rsi (internal) | Rse (external) |
|---|---|---|
| Wall | 0.13 | 0.04 |
| Roof / ceiling (heat flow up) | 0.10 | 0.04 |
| Floor (heat flow down) | 0.17 | 0.04 |
Thermal Conductivity (Lambda) of Common Materials
The lambda (λ) value is the material property that matters — lower λ means better insulation. Typical values (W/mK):
| Material | λ (W/mK) | Note |
|---|---|---|
| PIR / PUR insulation board | 0.022 – 0.024 | Best common insulant |
| Mineral wool / fibreglass | 0.035 – 0.040 | Widespread, cheap |
| EPS / XPS foam | 0.030 – 0.038 | Rigid boards |
| Timber (softwood) | 0.13 | Also a thermal bridge |
| Brick | 0.77 | Structural, poor insulator |
| Dense concrete | 1.5 – 2.0 | Very poor insulator |
| Plasterboard | 0.21 | Internal lining |
R-value vs RSI: Imperial vs Metric
The US uses imperial R-value (ft²·°F·h/Btu); the rest of the world uses metric RSI (m²K/W). They measure the same thing:
Rule of thumb: 1 RSI ≈ 5.68 US R-value. So a US R-13 batt is roughly RSI 2.29. The calculator shows both automatically — but mixing the two systems is a classic, costly mistake.
What's a Good U-value?
| U-value (W/m²K) | Performance |
|---|---|
| ≤ 0.15 | Passive house / high performance |
| 0.18 – 0.30 | Modern building-regs walls |
| 0.30 – 0.70 | Older / partially insulated |
| 1.5 – 2.0 | Uninsulated solid wall (poor) |
Because U-value directly sets heat loss, it feeds straight into heating-system sizing and the room-by-room HVAC load calculation — a lower U-value means a smaller, cheaper heating and cooling system.
Worked Example
An insulated cavity wall, inside to outside, on a wall element (Rsi = 0.13, Rse = 0.04), with a 0.18 cavity:
| Layer | d (mm) | λ | R = d/λ |
|---|---|---|---|
| Internal surface (Rsi) | – | – | 0.130 |
| Plaster / block (inner leaf) | 100 | 0.51 | 0.196 |
| PIR insulation | 100 | 0.022 | 4.545 |
| Air cavity | – | – | 0.180 |
| Brick (outer leaf) | 102.5 | 0.77 | 0.133 |
| External surface (Rse) | – | – | 0.040 |
| Total | R ≈ 5.22 |
- Rtotal ≈ 5.22 m²K/W
- U = 1 / 5.22 ≈ 0.19 W/m²K — passes the typical 0.18–0.30 regs band
Common Mistakes
- Adding U-values. Never — add R-values, then invert once at the end.
- Forgetting surface resistances. Rsi and Rse are required by ISO 6946, not optional.
- Mixing imperial and metric. Keep R-value (US) and RSI (metric) separate; convert deliberately.
- Using thickness in mm inside R = d/λ. d must be in metres — a 1000× error otherwise.
- Ignoring thermal bridging. Studs, ties and slabs can cut real performance by 10–50%; a plain series calc misses this.
- Wrong element type. Roofs and floors use different surface resistances than walls.
- Trusting nominal insulation R. Compression, gaps and moisture reduce real-world λ performance.
Frequently Asked Questions
What is the difference between U-value and R-value?
R-value is thermal resistance (higher is better) and adds in series; U-value is the whole-assembly transmittance (lower is better) and equals 1 divided by the total R-value.
How do you calculate the U-value of a multi-layer wall?
Compute each layer's R = thickness/conductivity, add Rsi, all layer R's, any cavity R, and Rse for the total R, then U = 1/R_total (ISO 6946).
What are Rsi and Rse surface resistances?
The internal and external still-air film resistances. For walls: Rsi = 0.13, Rse = 0.04 m²K/W. Rse is lower because wind strips the outer film.
What is a good U-value?
Lower is better. Modern walls target ~0.18–0.30 W/m²K; passive standards go below 0.15; an uninsulated solid wall is ~1.5–2.0.
How do I convert between R-value and RSI?
RSI = imperial R × 0.17611 (and 1 RSI ≈ 5.68 R-value). A US R-13 batt ≈ RSI 2.29.
What is thermal bridging and why does it matter?
Heat bypassing insulation through studs, ties or slabs. It can cut real performance 10–50% and risks condensation; a plain series calc ignores it.
Does this calculator work for roofs and floors?
Yes — select the element type so the correct surface resistances (roof 0.10/0.04, floor 0.17/0.04) are applied, then build up the layers as normal.
Conclusion
The U-value is the single figure that decides whether a wall, roof or floor keeps heat in — and getting it right is refreshingly logical: convert each layer to an R-value with R = d/λ, add the surface resistances, sum everything, and invert once to get U = 1/Rtotal. Never add U-values, always include Rsi and Rse, keep your units straight, and remember that thermal bridging makes real assemblies perform worse than the ideal series calculation suggests.
Use the calculator above to build up any construction and see its U-value and R-value instantly — then feed that U-value straight into your heat-loss and HVAC-load calculations.
For more HVAC, building-energy and heat-transfer tutorials plus free engineering calculators, explore Free CFD Tutorial. If this tool helped you, please share it with your fellow engineers and students.