Where fins is the insulation fraction and fstud is the timber fraction.
Timber has a thermal conductivity of 0.13 W/mK, much higher than mineral wool (0.035 W/mK). Where studs replace insulation, heat flows more easily, increasing overall U-value by 15-25% depending on stud spacing and size.
The combined method calculates separate U-values for the insulated sections and bridged sections, then combines them based on area fractions. This accounts for the parallel heat flow paths through different parts of the construction.
For 600mm stud centres with 38mm studs, use 6.3% (38/600). For 400mm centres, use 9.5%. Add extra for additional timbers at openings, corners, and sole/head plates - typically adding 5-10% to the basic calculation.
Yes. A typical 140mm timber frame with mineral wool and 50mm external insulation can achieve around 0.18 W/m²K. Using I-joists or increasing stud spacing reduces bridging and improves performance.