You've probably heard someone say their walls are R-19 or R-21. That number is useful, but it usually describes the insulation in the cavity, not the performance of the entire wall. And the whole wall is what your house actually has to live with. The studs did not politely disappear when the insulation truck arrived. Thermal bridging happens when heat moves through a part of the building envelope that conducts heat more readily than the insulation around it. In typical wood framed walls the studs, headers, corners, and other framing members create paths for heat to move through. Wood is not as conductive as steel, but it still transfers heat much more readily than many insulation materials.
Imagine a wall with insulation between the studs. Heat moves through the insulated cavities, but it can also move through the wood framing through thermal conduction. So the wall's overall thermal performance is lower than the cavity insulation's R-value alone would suggest. The amount of thermal bridging depends on the framing layout, material, wall design, and other details. But usually the weighted R value is down about 20% after taking framing into account. Steel framing can create especially significant thermal bridges because steel conducts heat so well. Windows, doors, rim joists, and transitions can also create weak spots in the thermal envelope. This doesn't mean cavity insulation is pointless. It means you shouldn't confuse the R-value of one material with the performance of the complete assembly. It's the difference between judging a car by the engine specs and judging whether the entire car actually gets you where you're going.
One common approach to fixing thermal bridging is continuous insulation, that’s a layer of insulation that runs across the framing rather than only between the studs. Depending on the design it can be installed on the exterior or interior. Because it crosses over the framing, it reduces the direct heat flow paths created by studs and other structural members. Other strategies include advanced framing, careful detailing around openings, and simply paying attention to rim joists and other transitions. The right solution depends on the climate, wall assembly, moisture strategy, and construction details. Adding insulation without considering moisture and drying can create problems, so thermal performance is only one part of the design. Thermal bridging is a good example of why high-performance homes need more than a big R-value printed on a specification sheet. A good assembly needs continuous insulation, air control, moisture management, and details that work together.