Multi-Layer Wall R-Value Calculator
Total thermal resistance and U-value of a three-layer wall, and how much of it the insulation contributes.
Results
What this tool does
Resistances in series simply add, which is why one good insulation layer dominates a wall and why the render and the blockwork barely register beside it. The defaults are plaster, 150 mm of mineral wool and a brick outer leaf. Typical conductivities: mineral wool 0.035 to 0.040, EPS 0.038, PIR 0.022, brick 0.5 to 0.8, dense concrete 1.5, timber 0.13.
Formula
R total = Rsi + Σ(thickness ÷ λ) + Rse ; U = 1 ÷ R
Variables
| Symbol | Meaning | Unit |
|---|---|---|
t1 | Layer thickness | mm |
l1 | Thermal conductivity λ | W/(m·K) |
t2 | Layer thickness | mm |
l2 | Thermal conductivity λ | W/(m·K) |
t3 | Layer thickness | mm |
l3 | Thermal conductivity λ | W/(m·K) |
ri | Inside surface resistance | m²·K/W |
re | Outside surface resistance | m²·K/W |
RT | Thermal resistance R | m²·K/W |
UU | U-value | W/(m²·K) |
R2 | Thermal resistance R | m²·K/W |
PC | Percentage | % |
QQ | Heat flux | W/m² |
Worked example
- Layer thickness20 mm
- Thermal conductivity λ0.7 W/(m·K)
- Layer thickness150 mm
- Thermal conductivity λ0.04 W/(m·K)
- Layer thickness110 mm
- Thermal conductivity λ0.5 W/(m·K)
- Inside surface resistance0.13 m²·K/W
- Outside surface resistance0.04 m²·K/W
- Thermal resistance R4.16857 m²·K/W
- U-value0.23989 W/(m²·K)
- Thermal resistance R3.75000 m²·K/W
- Percentage89.96 %
- Heat flux4.798 W/m²
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
Why do the surface resistances matter?
Because heat has to get into the wall and out of it again, and the thin layer of still air clinging to each face resists that. The standard values are 0.13 m²·K/W inside and 0.04 outside for a wall with horizontal heat flow, the outside being smaller because wind strips the boundary layer away. On a well insulated wall they are a rounding error; on a single sheet of glass they are most of the resistance there is.