Bridged U-Value Calculator (Timber Stud or Joist Wall, ISO 6946 Combined Method)
U-value of a wall, roof or floor where timber studs or joists interrupt the insulation, by the ISO 6946 combined method, with the heat loss the studs add — drawn against the stud share.
Results
The dashed line marks the value you entered.
What this tool does
Insulation between studs is not the whole story: the timber conducts heat about four times better, so a quoted U-value of the insulation alone is too optimistic. ISO 6946 brackets the answer between two limits — one treating each path (through studs, through insulation) separately, the other assuming the heat spreads out sideways at each layer — and takes their average. A frame with 15 % studs, 140 mm of 0.035 insulation and some boards loses roughly 30 % more heat than the insulation alone suggests, and continuous insulation over the frame is how that is reduced. The method suits the cases where the two limits are within a factor of 1.5.
Formula
R′T = 1 ÷ [f ÷ R_stud path + (1 − f) ÷ R_insulation path] · R″T = Rsi + 1 ÷ [f λ_t/d + (1 − f) λ_i/d] + R_other + Rse · RT = (R′T + R″T) ÷ 2 · U = 1 ÷ RT (ISO 6946)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
fr | Share of the area taken by studs or joists | % |
dd | Thickness of the layer with studs and insulation | mm |
li | Thermal conductivity of the insulation | W/(m·K) |
lt | Thermal conductivity of the studs | W/(m·K) |
ro | Resistance of all the other layers together | m²·K/W |
rsi | Inside surface resistance Rsi | m²·K/W |
rse | Outside surface resistance Rse | m²·K/W |
U | U-value with the studs counted | W/(m²·K) |
U0 | U-value if there were insulation only | W/(m²·K) |
UP | How much the studs raise the heat loss | % |
RU | Upper limit of the total resistance R′T | m²·K/W |
RL | Lower limit of the total resistance R″T | m²·K/W |
RT | Total resistance RT | m²·K/W |
OK | Is the combined method suitable? | — |
Worked example
- Share of the area taken by studs or joists15 %
- Thickness of the layer with studs and insulation140 mm
- Thermal conductivity of the insulation0.035 W/(m·K)
- Thermal conductivity of the studs0.13 W/(m·K)
- Resistance of all the other layers together0.3 m²·K/W
- Inside surface resistance Rsi0.13 m²·K/W
- Outside surface resistance Rse0.04 m²·K/W
- U-value with the studs counted0.294 W/(m²·K)
- U-value if there were insulation only0.224 W/(m²·K)
- How much the studs raise the heat loss31.6 %
- Upper limit of the total resistance R′T3.483 m²·K/W
- Lower limit of the total resistance R″T3.313 m²·K/W
- Total resistance RT3.398 m²·K/W
- Is the combined method suitable?Yes — the two limits are within 1.5 of each other
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- 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.